RTI Course Notes 2008
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The Welding Institute
TWI TRAINING AND EXAMINATION SERVICES Course Title
RADIOGRAPHIC INTERPRETATION (RTI) Course Notes
Course Reference NDT 2/20
REVISION 3 May 2008
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The Welding Institute
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The Welding Institute
Contents
Subject Index: pages i to vii
Training notes & glossary: pages 1 to 153 Appendix 1: pages 154 to 158
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Section 1.0 2.0
Subject Index Title INTRODUCTION TO NDT METHODS
Pages 1-5 6-7 8-11
INTRODUCTION & HISTORICAL BACKGROUND
2.1
PROPERTIES OF IONISING RADIATION
3.0
THE ELECTROMAGNETIC SPECTRUM
12-13
4.0
SIMPLE ATOMIC THEORY
14-15
5.0
IONISING RADIATION
6.0
X-RAYS OR BREMSTRAHLUNG 6.1 X-RAY EQUIPMENT 6.1.1 THE CATHODE 6.1.2 THE ANODE 6.1.3 X-RAY TUBES 6.1.4 X-RAY TUBE POWER SUPPLY 6.1.5 X-RAY TUBE CONTROLS 6.1.5.1 PENETRATING POWER OR RADIATION QUALITY (kV) 6.1.5.2 QUANTITY OF RADIATION (mA) 6.1.6 HIGH ENERGY X-RAY SOURCES 6.1.6.1 BETATRONS 6.1.6.2 LINEAR ACCELERATORS 6.1.6.3 VAN der GRAAF GENERATORS 6.1.7 SPECIAL TYPES OF X-RAY UNIT 6.1.7.1 MICROFOCUS X-RAY SOURCES 6.1.7.2 ROD ANODE X-RAY TUBES 6.1.7.3 ROTATING ANODE X-RAY EQUIPMENT
16-18 18 19 20-21 22 22-24 25 25-26
GAMMA RAYS 7.1 ALPHA AND BETA EMISSION 7.1.1 ALPHA PARTICLES 7.1.2 BETA PARTICLES 7.2 SEALED SOURCES 7.3 PENETRATING POWER OF GAMMA RADIATION 7.4 QUANTITY OF GAMMA RADIATION 7.5 RADIOISOTOPE CONTAINERS FOR INDUSTRIAL RADIOGRAPHY 7.6 COMPARISON OF X-RAYS AND GAMMA RAYS 7.6.1 ENERGY AND OUTPUT OF RADIATION 7.6.2 RADIOGRAPHIC CONTRAST 7.6.3 FOCAL SPOT SIZE VERSUS SOURCE SIZE 7.6.4 EXPOSURE TIME (FILM RADIOGRAPHY) 7.6.5 POWER SUPPLY 7.6.6 PHYSICAL SIZE AND WEIGHT 7.6.7 EQUIPMENT COST
29 30 30 30 31-32 32 32-34 34-36
7.0
15
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i
26-27 27 27 27 28 28 28 28 29
37 37 37-38 38-39 39 39 39 39-40
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Section 8.0
9.0
Subject Index Title METHODS OF PRODUCING A RADIOGRAPHIC IMAGE 8.1 RADIOGRAPHIC FILM 8.1.1 LATENT IMAGE FORMATION 8.1.2 FILM CASSETTES 8.1.3 INTENSIFYING SCREENS 8.1.3.1 METALLIC FOIL INTENSIFYING SCREENS 8.1.3.2 SALT SCREENS 8.1.3.3 FLUOROMETALLIC SCREENS 8.1.4 FILM PROCESSING 8.1.4.1 DEVELOPMENT 8.1.4.2 STOP BATH 8.1.4.3 FIXING AND HARDENING 8.1.4.4 WASHING 8.1.4.5 DRYING 8.2 ADVANCED IMAGING TECHNIQUES
Pages 40 40-41 42-43 43 44 44 44 45 45 45-46 46 46-47 47 47 47-48
PRODUCTION OF A RADIOGRAPH (FILM RADIOGRAPHY) 9.1 RADIOGRAPHIC QUALITY 9.1.1 CONTRAST 9.1.1.1 FILM TYPE (AFFECTS FILM CONTRAST) 9.1.1.2 FILM DENSITY (AFFECTS FILM CONTRAST) 9.1.1.3 BASE FOG LEVEL (AFFECTS FILM CONTRAST) 9.1.1.4 FILM PROCESSING (AFFECTS FILM CONTRAST) 9.1.1.5 RADIATION QUALITY (AFFECTS SUBJECT CONTRAST) 9.1.1.6 SCATTER (AFFECTS FILM AND SUBJECT CONTRAST) 9.1.2 DEFINITION 9.1.2.1 GEOMETRIC UNSHARPNESS 9.1.2.2 INHERENT UNSHARPNESS 9.1.2.2.1 FILM (EFFECT ON INHERENT UNSHARPNESS) 9.1.2.2.2 QUALITY OF RADIATION (EFFECT ON INHERENT UNSHARPNESS) 9.1.2.2.3 INTENSIFYING SCREENS (EFFECT ON INHERENT UNSHARPNESS) 9.1.2.3 RELATIVE MOVEMENT DURING EXPOSURE
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48 48-49 49-50 50-51 51-52 52 53 53 54 54-55 55 56 56 56 57 57
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Subject Index Section
Title 9.2
9.4
RADIATION SCATTER AND SCATTER CONTROL 9.2.1 SCATTERING MECHANISMS – THE CAUSES OF SCATTER 9.2.1.1 THE PHOTOELECTRIC EFFECT 9.2.1.2 COMPTON SCATTERING (INCOHERENT SCATTERING) 9.2.1.3 PAIR PRODUCTION 9.2.1.4 TOTAL SCATTER AT DIFFERENT PRIMARY BEAM ENERGIES 9.2.2 TYPES OF SCATTER 9.2.2.1 SIDE SCATTER 9.2.2.2 BACK SCATTER 9.2.2.3 SELF-SCATTER 9.2.3 SCATTER CONTROL 9.2.3.1 COLLIMATION 9.2.3.2 DIAPHRAGMS 9.2.3.3 MASKING OR BLOCKING 9.2.3.4 GRIDS 9.2.3.5 FILTERS 9.2.3.6 METALLIC FOIL SCREENS 9.2.3.7 HIGHER RADIATION ENERGY 9.2.3.8 CHANGE FROM X-RAY TO GAMMA RAY RADIOGRAPHY 9.2.3.9 REDUCING THE FOCUS OR SOURCE TO FILM DISTANCE DETERMINING THE CORRECT EXPOSURE: EXPOSURE CHARTS 9.4.1 EXPOSURE CHARTS 9.4.1.1 USING EXPOSURE CHARTS (X-RAY) 9.4.1.1.1 FOCUS TO FILM DISTANCE 9.4.1.1.2 TUBE VOLTAGE 9.4.1.1.3 CHANGING THE FILM DENSITY 9.4.1.1.4 CHANGING THE FILM TYPE 9.4.1.1.5 RADIOGRAPHY OF OTHER MATERIALS 9.4.1.1.6 COMPENSATING FOR THE USE OF A FILTER 9.4.1.1.7 OTHER POSSIBLE CHANGES 9.4.1.2 GAMMA RAY EXPOSURES
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iii
Pages 57 58 58 58-59 59 60 60 60-61 61 61 62 62 62 62-63 63 63-64 64 64 65 65 65-66 66-71 71 71-73 73-74 75 75-76 76-77 77 77 77-78
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Section 10.0
11.0
Subject Index Title SENSITIVITY 10.1 RADIOGRAPHIC SENSITIVITY 10.2 CONTROLLING RADIOGRAPHIC QUALITY 10.3 BS EN 462-1 WIRE TYPE IQIs 10.4 OTHER WIRE TYPE IQIs 10.5 BS EN 462-2 STEP-HOLE TYPE IQIs 10.6 ASTM E 1025 PLAQUE TYPE PENETRAMETERS 10.7 IQI SENSITIVITY RADIOGRAPHIC TECHNIQUES (FOR WELDS IN PLATE AND PIPE) 11.1 11.2 11.3 11.4 11.5 11.6
11.7 11.8
12.0
Pages 78 78 78 78-79 80 80-81 82 82-83
IQI TYPE AND PLACEMENT LOCATION MARKERS IDENTIFICATION OF RADIOGRAPHS RADIATION ENERGY SOURCE TO FILM DISTANCE SWSI TECHNIQUES 11.6.1 SINGLE WALL SINGLE IMAGE TECHNIQUE FOR PLATE 11.6.2 SINGLE WALL SINGLE IMAGE TECHNIQUE: SOURCE INTERNAL, PLACED CENTRALLY (PANORAMIC TECHNIQUE) 11.6.3 SINGLE WALL SINGLE IMAGE TECHNIQUE: SOURCE INTERNAL, OFFSET 11.6.4 SINGLE WALL SINGLE IMAGE TECHNIQUE: FILM INSIDE, SOURCE OUTSIDE DWSI TECHNIQUE DWDI TECHNIQUES 11.8.1 DOUBLE WALL DOUBLE IMAGE (ELLIPTICAL)
84 85 86 86 86-87 87 87
11.8.1
95-96
DOUBLE WALL DOUBLE IMAGE (SUPERIMPOSED)
12.4.4 12.4.5
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RADIOGRAPHIC SENSITIVITY ARTEFACTS AND OTHER UNWANTED IMAGES
INTERPRETATION OF RADIOGRAPHIC IMAGES ARTEFACTS 12.6.1 PRESSURE MARKS (CRIMP MARKS) 12.6.2 SCRATCHES: ON THE FILM
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88
89 89-91 91-93 94-95 95-95
97 97 97-98 98-99 99 99 99 99-100
INTERPRETATION OF RADIOGRAPHS 12.1 INTRODUCTION 12.2 VIEWING CONDITIONS 12.3 REPORTING 12.4 FILM QUALITY 12.4.1 COMPONENT IDENTIFICATION 12.4.2 LOCATION MARKERS 12.4.3 FILM DENSITY
12.5 12.6
84
100 100-101 101 101 101 101
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Subject Index Section
Title 12.6.3
12.7
12.8
SCRATCHES: ON LEAD INTENSIFYING SCREENS 12.6.4 DIRT: ON THE FILM OR SCREENS 12.6.5 STREAKINESS OR MOTTLING: POOR DEVELOPMENT 12.6.6 DEVELOPER SPLASHES 12.6.7 FIXER SPLASHES 12.6.8 WATER SPLASHES 12.6.9 WATER MARKS 12.6.10 AIR BELLS 12.6.11 DIFFRACTION MOTTLING 12.6.12 STATIC MARKS 12.6.13 DICHROIC FOGGING 12.6.14 RETICULATION 12.6.15 FILM FOGGING BY X OR GAMMA RAYS 12.6.16 LIGHT FOGGING 12.6.17 FILM FOGGING DUE TO INADEQUATE STORAGE CONDITIONS 12.6.18 SOLARISATION 12.6.19 A FINAL WORD ON ARTEFACTS INTERPRETATION OF WELD RADIOGRAPHS 12.7.1 RADIOGRAPHIC INDICATIONS DUE TO SURFACE GEOMETRY 12.7.1.1 EXCESSIVE ROOT PENETRATION 12.7.1.2 ROOT CONCAVITY 12.7.1.3 INCOMPLETELY FILLED GROOVE 12.7.1.4 LACK OF REINFORCEMENT 12.7.1.5 UNDERCUT 12.7.1.6 SPATTER 12.7.1.7 EXCESSIVE DRESSING (GRINDING MARKS) 12.7.1.8 HAMMER MARKS (TOOL MARKS) 12.7.1.9 TORN SURFACE 12.7.1.10 SURFACE PITTING 12.7.2 INTERNAL DEFECTS 12.7.2.1 CRACKS 12.7.2.2 LACK OF FUSION 12.7.2.3 INCOMPLETE ROOT PENETRATION 12.7.2.4 NON-METALLIC INCLUSIONS 12.7.2.5 METALLIC INCLUSIONS 12.7.2.6 GAS PORES: POROSITY 12.7.2.7 ELONGATED CAVITIES (HOLLOW BEAD) 12.7.2.8 WORMHOLES 12.7.2.9 CRATER PIPES & CRATER CRACKS INTERPRETATION OF CASTING RADIOGRAPHS 12.8.1 VOIDS 12.8.1.1 MACROSHRINKAGE 12.8.2.2 FILAMENTARY SHRINKAGE (ALSO CALLED SPONGINESS) 12.8.2.3 MICROPOROSITY / MICROSHRINKAGE 12.8.2.4 PINHOLE POROSITY 12.8.2.5 GASHOLES
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v
Pages 101 102 102 102 102 102 102 102 103 103 103 103 103 104 104 104 104 104 104 105 105 106 106 107 107 108 108 108 108 109 109-111 111-113 114 114 114-115 115-116 116 116 116-117 117 117 117 117 117-118 118 118
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Subject Index Section
Title
Pages 12.8.2.6 12.8.2 12.8.2.1 12.8.2.2 12.8.3 12.8.4 12.8.5
118 119 119 119 119 120 120
AIRLOCKS (ENTRAPPED AIR) CRACKS HOT TEARS STRESS CRACKS COLD SHUTS INCLUSIONS SEGREGATIONS
120 120 120-123 123-125
13.0
LOCALISATION 13.1 90° METHOD 13.2 TUBE (SOURCE) SHIFT METHOD 13.3 TUBE (SOURCE) SHIFT METHOD WITH LEAD MARKERS
14.0
UNITS USED IN RADIOGRAPHY 14.1 IONISATION (EXPOSURE) 14.2 ABSORBED DOSE 14.3 MAN MAMMAL EQUIVALENT or RADIOBIOLOGICAL EQUIVALENT 14.4 DOSE RATE 14.5 SOURCE STRENGTH OR ACTIVITY 14.6 SPECIFIC ACTIVITY 14.7 OUTPUT
15.0
RADIATION MONITORING DEVICES 15.1 SURVEY METERS 15.1.1 IONISATION CHAMBERS 15.1.2 PROPORTIONAL COUNTERS 15.1.3 GEIGER COUNTERS 15.1.4 SOLID STATE RADIATION DETECTORS 15.1.5 SCINTILLATION COUNTERS 15.2 PERSONAL MONITORS 15.2.1 FILM BADGES 15.2.2 THERMOLUMINESCENT DOSIMETERS (TLD) 15.2.3 QUARTZ FIBRE ELECTROMETER (PERSONAL DOSIMETER) 15.2.4 SOLID STATE INTEGRATING DOSIMETERS
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126 126 126 127 128 128 128 128-129 129 129 129-130 130-131 131 131 131-132 132 132-133 133-134 134 134
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Section 15.0
16.0
Subject Index Title RADIATION SAFETY 15.1 PRECAUTIONS 15.1.1 EXPOSURE BOOTHS 15.1.2 SITE WORK 15.1.3 SCATTER 15.2 EXPOSURE LIMITS FOR RADIATION WORKERS 15.2.1 DOSIMETERS 15.3 PERMITTED LEVELS 15.3.1 CLASSIFIED WORKERS 15.3.2 UNCLASSIFIED PERSONNEL, CONTROLLED & SUPERVISED AREAS 15.4 ‘SAFE’ WORKING DISTANCES 15.4.1 SHIELDING
137-138 138
A GLOSSARY OF TERMS (USED IN RADIOGRAPHIC TESTING)
139-151
APPENDIX 1: IONISING RADIATION REGULATIONS
152-156
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The Welding Institute 1.0
INTRODUCTION Non destructive testing is the ability to examine a material (usually for discontinuities) without degrading it. The five principal methods, other than visual inspection, are: • • • • •
Penetrant testing Magnetic particle inspection Eddy current testing Radiography Ultrasonic testing
In all the NDT methods, interpretation of results is critical. Much depends on the skill and experience of the technician, although properly formulated test techniques and procedures will improve accuracy and consistency. 1.1
Penetrant Testing Penetrant testing locates surface breaking discontinuities by covering the item with a penetrating liquid, which is drawn into the discontinuity by capillary action. After removal of the excess surface penetrant the indication is made visible by application of a developer. Colour contrast or fluorescent systems may be used. Advantages • • • • • •
Applicable to non-ferromagnetics Able to test large parts with a portable kit Batch testing Applicable to small parts with complex geometry Simple, cheap easy to interpret Sensitivity
Disadvantages • • • • • •
Will only detect defects open to the surface Careful surface preparation required Not applicable to porous materials Temperature dependant Cannot retest indefinitely Compatibility of chemicals
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The Welding Institute 1.2
Magnetic Particle Inspection Magnetic particle inspection is used to locate surface and slightly subsurface discontinuities in ferromagnetic materials by introducing a magnetic flux into the material. Advantages • • • • • •
Will detect some sub-surface defects Rapid and simple to understand Pre-cleaning not as critical as with DPI Will work through thin coatings Cheap rugged equipment Direct test method
Disadvantages • • • • • • 1.3
Ferromagnetic materials only Requirement to test in 2 directions Demagnetisation may be required Odd shaped parts difficult to test Not suited to batch testing Can damage the component under test
Eddy Current Inspection Eddy current inspection is based on inducing electrical currents in the material being inspected and observing the interaction between those currents and the material. Eddy currents are generated by coils in the test probe and monitored simultaneously by measuring the coils' electrical impedance. As it is an electromagnetic induction process, direct electrical contact with the sample is not required; however, the material must be an electrical conductor. Advantages • • • • • • •
2
Sensitive to surface defects Can detect through several layers Can detect through surface coatings Accurate conductivity measurements Can be automated Little pre-cleaning required Portability
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The Welding Institute Disadvantages • • • • • • 1.4
Very susceptible to permeability changes Only on conductive materials Will not detect defects parallel to surface Not suitable for large areas and/or complex geometry's Signal interpretation required No permanent record (unless automated)
Radiography Radiography monitors the varying transmission of ionising radiation through a material with the aid of photographic film or fluorescent screens to detect changes in density and thickness. It will locate internal and surface breaking defects. Advantages • • • • • • •
Gives a permanent record, the radiograph Detects internal flaws Detects volumetric flaws readily Can be used on most materials Can check for correct assembly Gives a direct image of flaws Fluoroscopy can give real time imaging
Disadvantages • • • • • • • • • 1.5
There is a radiation health hazard Can be sensitive to defect orientation and so can miss planar flaws Has limited ability to detect fine cracks Access is required to both sides of the object Limited thickness of materials can be penetrated Skilled radiographic interpretation is required Is a relatively slow method of inspection Has a high capital cost Has a high running cost
Ultrasonic Testing Ultrasonic Testing measures the time for high frequency (0.5MHz - 50MHz) pulses of ultrasound to travel through the inspection material. If a discontinuity is present, the ultrasound reflects back to the probe in a time other than that appropriate to good material.
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The Welding Institute Advantages • • • • •
Sensitive to cracks at various orientations Portability Safety Able to penetrate thick sections Measures depth and through wall extent
Disadvantages • • • • 1.6
No permanent record (unless automated) Not easily applied to complex geometries and rough surfaces. Unsuited to coarse grained materials Reliant upon defect orientation
Choice of Method Before deciding on a particular NDT inspection method it is advantageous to have certain information. •
Reason for inspection. (To detect cracks, to sort between materials, to check assembly, etc.) • Likely orientation of planar discontinuities, if they are the answer to the above question. Type of material. • Likely position of discontinuities. • Geometry and thickness of object to be tested. • Accessibility
•
This information can be derived from: • •
Product knowledge Previous failures
Accuracy of critical sizing of indications varies from method to method. 1.6.1
Liquid Penetrant Inspection •
1.6.2
Magnetic Particle Inspection •
4
The length of a surface breaking discontinuity can be determined readily, but the depth dimensions can only be assessed subjectively by observing the amount of 'bleed out'.
The length of a discontinuity can be determined from the indication, but no assessment of discontinuity depth can be made.
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1.6.3
Eddy Current Inspection •
1.6.4
The length of a discontinuity can be determined. The depth of a discontinuity or material thinning can be determined by amplitude measurement, phase measurement or both, but the techniques for critical sizing are somewhat subjective. Ultrasonic Testing
•
1.6.5
The length and position of a discontinuity can be determined. Depth measurements are more difficult but crack tip diffraction or time of flight techniques can give good results. Radiography
•
The length and plan view position can be determined. Through thickness positioning requires additional angulated exposures to be taken. The through thickness dimension of discontinuities cannot readily be determined.
2.0 INTRODUCTION & HISTORICAL BACKGROUND TWI Training & Examination Services – NDT 2/20 (Rev 3 05/08)
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The discovery of x-rays was made in 1895 by Wilhelm Conrad Roentgen (1845-1923) who was a Professor at Wuerzburg University in Germany. Whilst doing some experiments in which he passed an electric current through Crookes tubes, an evacuated glass tube with an anode and a cathode. When a high voltage was applied, the tube produced a fluorescent glow. Roentgen noticed that some photographic plates located nearby became fogged. This caused Roentgen to conclude that a new type of ray was being emitted from the tube. He believed that unknown rays were passing from the tube and through the plates. He found that the new ray could pass through most substances casting shadows of solid objects. Roentgen also discovered that the ray could pass through the tissue of humans, but not bones and metal objects. One of Roentgen's first experiments late in 1895 was a film of the hand of his wife, Bertha. Shortly after the discovery of X-rays, another form of penetrating rays was discovered. In 1896, French scientist Henri Becquerel discovered natural radioactivity. Many scientists of the period were working with cathode rays, and other scientists were gathering evidence on the theory that the atom could be subdivided. Some of the new research showed that certain types of atoms disintegrate by themselves. It was Henri Becquerel who discovered this phenomenon while investigating the properties of fluorescent minerals. One of the minerals Becquerel worked with was a uranium compound. On a day when it was too cloudy to expose his samples to direct sunlight, Becquerel stored some of the compound in a drawer with his photographic plates. Later when he developed these plates, he discovered that they were fogged (exhibited exposure to light). Becquerel questioned what would have caused this fogging. He knew he had wrapped the plates tightly before using them, so the fogging was not due to stray light. In addition, he noticed that only the plates that were in the drawer with the uranium compound were fogged. Becquerel concluded that the uranium compound gave off a type of radiation that could penetrate heavy paper and expose photographic film. Becquerel continued to test samples of uranium compounds and determined that the source of radiation was the element uranium. One of the minerals Becquerel worked with was a uranium compound. On a day when it was too cloudy to expose his samples to direct sunlight, Becquerel stored some of the compound in a drawer with his photographic plates. Later when he developed these plates, he discovered that they were fogged (exhibited exposure to light). Becquerel questioned what would have caused this fogging. He knew he had wrapped the plates tightly before using them, so the fogging was not due to stray light. In addition, he noticed that only the plates that were in the
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The Welding Institute drawer with the uranium compound were fogged. Becquerel concluded that the uranium compound gave off a type of radiation that could penetrate heavy paper and expose photographic film. Becquerel continued to test samples of uranium compounds and determined that the source of radiation was the element uranium. While working in France at the time of Becquerel's discovery, Polish scientist Marie Curie became very interested in his work. She suspected that a uranium ore known as pitchblende contained other radioactive elements. Marie and her husband, French scientist Pierre Curie, started looking for these other elements. In 1898, the Curies discovered another radioactive element in pitchblende, and named it 'polonium' in honor of Marie Curie's native homeland. Later that year, the Curies discovered another radioactive element which they named radium, or shining element. Both polonium and radium were more radioactive than uranium. Since these discoveries, many other radioactive elements have been discovered or produced. Radium became the initial industrial gamma ray source. The material allowed castings up to 10 to 12 inches thick to be radiographed. While working in France at the time of Becquerel's discovery, Polish scientist Marie Curie became very interested in his work. She suspected that a uranium ore known as pitchblende contained other radioactive elements. Marie and her husband, French scientist Pierre Curie, started looking for these other elements. In 1898, the Curies discovered another radioactive element in pitchblende, and named it 'polonium' in honor of Marie Curie's native homeland. Later that year, the Curies discovered another radioactive element which they named radium, or shining element. Both polonium and radium were more radioactive than uranium. Since these discoveries, many other radioactive elements have been discovered or produced. In 1946, man-made gamma ray sources such as cobalt and iridium became available. These new sources were far stronger than radium and were much less expensive. The manmade sources rapidly replaced radium, and use of gamma rays grew quickly in industrial radiography. Radium became the initial industrial gamma ray source. The material allowed castings up to 10 to 12 inches thick to be radiographed. During World War II, industrial radiography grew tremendously as part of the Navy's shipbuilding program. In 1946, man-made gamma ray sources such as cobalt and iridium became available. These new sources were far stronger than radium and were much less expensive. The manmade sources rapidly replaced radium, and use of gamma rays grew quickly in industrial radiography. William D. Coolidge's name is inseparably linked with the X-ray tube-popularly called the 'Coolidge tube.' This invention completely revolutionized the generation of X-rays and remains to this day the model upon which all X-ray tubes for medical applications are patterned. He invented ductile tungsten, the filament material still used in such lamps. He was awarded 83 patents during his lifetime.
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The Welding Institute 2.1 Properties of Penetrating radiation Penetrating radiation can be used in non-destructive examination because: (1) Penetrating radiation travels in a straight line. (2) Penetrating radiation is absorbed as it passes through matter. The extent to which it is absorbed depends upon three factors: (i)
The thickness of the absorber.
(ii)
The physical characteristics of the absorber (in particular its density and atomic number).
(iii)
The wavelength or “photon energy” of the radiation itself.
(3) Penetrating radiation can be detected using a photographic emulsion or by other means. The system used to detect the radiation must be capable of differentiating between different intensities of radiation.
Figure 1. Penetrating radiation passing through an object
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Figure 1A. The radiograph that would result from the set-up in figure 1 above (Note that in film radiography thin sections appear darker while thicker sections appear lighter. The opposite is true if a fluorescent screen rather than a photographic film is used as a radiation detector)
Figure 2. Radiation passing through an object containing 2 voids at different depths
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Figure 2A. The radiograph resulting from the set-up in figure 2. (Note that the radiograph cannot be used to determine the through thickness position of the voids.) Two important things to keep in mind when viewing a radiographic image are: (1)
A radiograph is a two-dimensional image of a three-dimensional object: The through thickness position and size of an object producing a radiographic image cannot be determined solely from the information given by a single radiograph (this is demonstrated in figures 2 and 2A).
(2)
A defect will only appear as an image in a radiograph if: (a) the defect causes a local difference in radiation absorption and (b) the method used for detecting the radiation is capable of detecting the difference in radiation intensity so caused.
For example, suppose that a chosen radiographic technique is capable of detecting a thickness difference of say 0.5 mm in 50 mm of steel. If we use this technique to radiograph the weld shown in figure 3 then: (1) The gas pore will readily be detected because A - (B + C) = 3 mm. (2) The lack of side fusion will not appear as an image on the radiograph because A - (D + E) = 0.01 mm which is much too small to be detected by the technique used.
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Figure 3. Radiography of a weld
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The Welding Institute 3.0 THE ELECTROMAGNETIC SPECTRUM From the early part of the nineteenth century it was well understood that light was a waveform. Light, however, was well known to be capable of passing through a vacuum. The scientists of the day puzzled long and hard as to how it could be possible that a wave form could travel without some form of matter to support it. Therefore the concept of a substance called ‘ether’, which filled otherwise apparently empty space, was postulated and a lot of research time was expended in trying to isolate this mysterious substance. The search continued until around 1865 when a scientist called James Maxwell predicted the existence of electromagnetic waves. Such waves, he said, would be capable of passing through a vacuum, since they were supported by oscillating magnetic and electrical fields mutually at right angles to each other and to the direction of propagation. Moreover, using mathematics, Maxwell predicted a speed of travel for such waves that was equal to the then known speed of light. It soon became clear that light was in itself a form of electromagnetic radiation. All types of electromagnetic radiation travel at the same velocity (v), the velocity of light, which is about 2.998 x 108 ms-1 (186,000 miles per second in old money), but differ in terms of their wavelength (λ) and frequency (f). Wavelength can be defined as the distance travelled during one complete field oscillation while frequency is the total number of oscillations occurring in one second.
v = fλ As scientific knowledge advanced it became clear that in some circumstances light behaved not so much like a waveform, but more like a particle. Considering such behaviour in 1900 a scientist called Max Planck first put forward the theory that light had, what he called, a ‘quantum’ nature. Planck postulated that electromagnetic energy could not exist in amounts (‘quantum’ being Latin for ‘amount’) smaller than a given very small amount of energy and that all larger amounts of electromagnetic energy were exact multiples of this amount to which he gave the name “photon”. Planck believed that the photon energy of any form of electromagnetic radiation would be equal to a constant multiplied by its frequency. In later years Planck’s hypothesis was proved to be true and the constant in question became known as ‘Planck’s Constant’, usually abbreviated as ‘h’.
E = hf Where ‘h’ is Planck’s constant ( = 6.63 x 10 –23 Js) and ‘E’ is the photon energy of electromagnetic radiation of frequency ‘f’.
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The Welding Institute The properties of electromagnetic radiation, especially in the way it interacts with matter are largely determined by its wavelength. Figure 6 is a schematic of the electromagnetic spectrum.
Figure 6. The electromagnetic spectrum Notes on figure 5: 1. The electron volt (eV) is a unit of energy which is equal to the kinetic energy that an electron obtains when it accelerates through an electric field of 1 volt. A Mega electron volt (MeV) is equal to the kinetic energy of an electron that has accelerated through an electric field of 1 million volts. On electron volt is equal to 1.6 x 10-19 Joules. 2. The relationship between wavelength and photon energy on which the diagram above has been based is approximate.
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The Welding Institute 4.0 SIMPLE ATOMIC THEORY In order to understand how x and gamma rays are produced it is necessary to have a basic understanding of atomic theory. An atom is the smallest part of any chemical element. Atoms are known to consist of 3 basic types of particle, these being the positively charged proton, the neutron (which has no electrical charge) and the negatively charged electron. The electrical charge on the proton and electron are equal in magnitude but opposite in polarity. The atomic mass of a proton is, by definition equal to 1 atomic mass unit (Abbreviation: amu. 1 amu = 1.6725 x 10-27 kg). The electron has a tiny mass, around 1/1836 that of a proton (0.000545 amu or about 9.11 x 10-31 kg), while that of a neutron is very slightly greater than that of a proton at 1.0014 amu (or 1.6748 x 10-27 kg). The atom is thought to consist of a positively charged nucleus (which consists of protons and neutrons) surrounded by a cloud of orbiting negatively charged electrons.
Figure 4. Simple model of atomic structure In the equilibrium state the number of orbital electrons is equal to the number of protons and there is no net electrical charge. When there is inequality between the numbers of protons and electrons then there is a net electrical charge and the atom is said to be ionised. Ions may be negatively charged if the number of electrons exceeds the number of protons or positively charged if the converse is true. So called electropositive elements, a group which includes all metals, ‘like’ to form positive ions while the electronegative elements such as oxygen, phosphorous, chlorine and sulphur ‘like’ to form negative ions. The orbital electrons exist in fixed energy levels or shells. Each shell can contain a fixed maximum number of electrons. The shells are identified by letters – K, L, M, N and so on. The lowest energy level is represented by the K-shell; this is the innermost of the electron shells and it can contain a maximum of 2 electrons. The L-shell can contain up to 8 electrons while the M-shell contains a maximum of 18 and the N-shell contains a maximum of 32. The maximum total number of electrons in each shell is equal to 2n2 where ‘n’ is the shell number counting the K-shell as 1, L-shell as 2 etc. Within the M, N and other shells certain groupings of electrons produce greater stability, elements having an even number of electrons tend to less chemically reactive than those which have an odd number. A group of 8 electrons in the M or N shells produces an element which is the most chemically inert of all elements – an inert gas. In electropositive elements the orbital electrons are relatively loosely bound and there is a tendency to form positive ions. In electronegative elements the
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The Welding Institute orbital electrons are relatively tightly bound and there is a tendency to form negative ions. The inert gasses such as neon, argon, xenon and krypton either have an outer shell that is completely full or one which contains a very stable grouping of electrons. Based on their chemical properties the elements can be organised into a ‘periodic table’ as shown below. Elements falling in the same vertical column share very similar chemical properties. 1
2
H
1.008 3
He
4
5
6
7
8
9
4.003 10
Li
Be
B
C
N
O
F
6.940 11
9.012 12
10.81 13
12.01 14
14.01 15
16.00 16
19.00 17
20.17 18
22.99 19
24.30 20
Ca
30
26.98 31
28.09 32
30.97 33
32.06 34
35.45 35
39.95 36
39.10 37
40.08 38
85.47 55
Na
Mg
Al
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
50.94 41
52.00 42
54.94 43
55.85 44
58.93 45
58.71 46
63.55 47
65.38 48
69.74 49
72.59 50
74.92 51
78.96 52
79.90 53
83.80 54
87.62 56
88.91 57
91.22 72
92.91 73
95.94 74
98.91 75
101.1 76
102.9 77
106.4 78
107.9 79
112.4 80
114.8 81
118.7 82
121.8 83
127.6 84
126.9 85
131.3 86
132.9 87
137.3 88
138.9 89
178.5
181.0
183.9
186.2
190.2
192.2
195.1
197.0
200.6
204.4
207.2
209.0
(209)
(210)
(222)
(223)
226.0
(227)
Cs Fr
Ba Ra
La
Hf
Nb Ta
Mo W
Tc
Re
Ru
Os
Rh Ir
28
Pd Pt
29
Ar
47.90 40
Zr
27
Cl
Sc Y
26
S
44.96 39
Sr
25
P
23
Rb
24
Si
22
K
21
Ne
Ag
Au
Cd
Hg
Ga In
Tl
Ge Sn
Pb
As
Sb Bi
Se Te
Po
Br I
At
Kr
Xe
Rn
Ac
Lanthanide Series Actinide Series
58
59
60
61
62
63
64
65
66
67
68
69
70
71
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
140.1
140.9
144.2
(145)
150.4
152.0
157.3
158.9
162.5
164.9
167.3
168.9
173.0
175.0
Th
90
Pa
91
92
U
Np
93
Pu
94
Am
95
Cm
96
Bk
97
Cf
98
Es
99
Fm
100
Md
101
No
102
Lw
103
232.0
231.0
238.0
237.1
(244)
(243)
(249)
(247)
(251)
(254)
(257)
(258)
(259)
(260)
(The numbers above and below each chemical symbol are the atomic number and the atomic weight of each element. Note that the atomic weight differs slightly from the atomic mass number)
Figure 5. Periodic table of elements X-rays result from ionisation and de-ionisation events: when a positively charged ion captures a free electron the atom descends into a lower energy state and the left over energy may be released in the form of an x-ray ‘photon’. X-rays can also result as a negative ion loses a captured electron, because again there is a reduction in the energy stored in the atom as it returns to a state of zero electrical charge. Each element has its own characteristic number of protons in the nucleus. This number is the ‘atomic number’, usually abbreviated as ‘Z’. It is the atomic number that determines the chemical properties of a given substance. However, each element can exist as any one of a number of ‘nuclides’ or ‘isotopes.’ Each isotope of a given element has the same atomic number, the same number of protons and the same chemical properties, but each isotope has a different ‘atomic mass number’. The difference in atomic mass number is due to a difference in the number of neutrons in the nucleus. The atomic mass number is equal to the total of protons + neutrons in the nucleus. Most elements can exist in nature as any one of several stable isotopes. Some isotopes, however, are not stable – these are the so called ‘radioactive isotopes’. The following notation is typically used:
Where 59 is the number of protons + neutrons (the atomic mass number), 27 is the number of protons (the atomic number) and Co is the chemical symbol, in this case cobalt. The example shown, if in a non-ionised state, would have 27 protons, 27 electrons and 32 neutrons in each TWI Training & Examination Services – NDT 2/20 (Rev 3 05/08)
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The Welding Institute atom. Radioactive isotopes undergo fission as they ‘decay’ towards a more stable atomic structure. Some isotopes achieve stability in a single fission event while others may undergo a series of fission events before the result is a stable isotope or combination of stable isotopes. Some radioactive isotopes can decay along any one of several decay paths. Each individual fission event is random, however, each unstable atom of a given isotope has the same probability of decay. One atom of an isotope may undergo fission with a life of a few microseconds while its neighbour may not decay until several centuries have passed, but both radioactive atoms still have the same probability of decay. Applied to a very large number of active atoms, and a typical Iridium 192 gamma ray source [as used in industrial radiography], despite its small physical size, may contain around 1020 radioactive atoms (that’s one hundred million million million), the constant probability of decay gives rise to a constant ‘half-life.’ With such a large number the random nature of decay just ‘averages out’. Gamma rays are an occasional by product of this process of nuclear fission. Fission means ‘splitting’. There are several routes by which nuclear fission can take place and two of these are of importance in the production of gamma rays. These will be discussed in greater detail in the section on gamma ray sources. 5.0 IONISING RADIATION The 2 types of penetrating radiation most used in industrial radiography, x-rays and gamma rays, are often referred to as “ionising radiation”. This is because the nature of their interaction with matter is to cause ionisation. Ionisation is caused by loss of an orbiting electron which leaves the atom in a electrically positively charged state (+ Ion). Alpha particles and beta particles, which are products of radioactive fission also cause ionisation and are therefore included within the term ‘ionising radiation’. Neutron radiation is a hazard in the nuclear power industry, it can [indirectly] cause ionisation, and it is therefore often included within this group of types of radiation referred to as ionising. Alpha and beta particle radiation are covered in greater detail in the sections below. 6.0 X-RAYS or BREMSTRAHLUNG The term ‘x-ray’ is applied to ionising radiation produced when a beam of high velocity (i.e. high kinetic energy) electrons collides with the atoms of a ‘target’ material. The ‘photon energy’ of the x-radiation thereby produced depends two factors: (i) the kinetic energy of the electron at the point of collision and (ii) the relative efficiency of the process of stopping the incident electron – does this occur in a single large event or in a series of events of varying magnitude?
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Figure 7. X-ray production The maximum energy of the x-ray photons produced is determined by the maximum kinetic energy of the high velocity electrons impacting upon the target material. There is no minimum to the energy of the x-ray photons produced. This is because there is wide variation in the amount of energy which the electron loses on collision with an atom. Some of the electrons will score only a glancing hit on the atom, in so doing interacting with the loosely bound electrons in the outermost electron shells. This causes the impacting electron to be deflected and it loses part of its velocity. The reduced energy electron may then interact with another atom and in so doing produce another photon of x-rays of variable although reduced energy. In addition to this the x-ray photons produced by electron collisions can themselves interact with adjacent atoms and thereby produce reduced energy photons. X-ray spectra are commonly termed to be “continuous, white or polychromatic” (this is because there is no minimum energy). Figure 8 shows the form of a typical x-ray spectrum. Note that higher energy x-rays have shorter wavelengths.
Figure 8. X-ray spectrum
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The Welding Institute The value of the minimum wavelength which will be produced by an x-ray tube having an accelerating voltage V can be estimated using the following formula:
λmin = 1240 / V nanometres The minimum wavelength, λmin, is sometimes called the threshold wavelength. The maximum intensity (the number of photons per square metre) in the continuous spectrum produced occurs at approximately 2 x λmin . The ability of the x-rays to penetrate matter depends on their photon energy. The shorter the wavelength, the higher the photon energy, the more penetrating the radiation. The penetrating power of the x-rays can be controlled by increasing or decreasing the accelerating voltage. The greater the accelerating voltage, the more penetrating the radiation. In an x-ray set the accelerating voltage is the tube voltage. The total number of photons produced at all wavelengths is directly related to the number of high velocity electrons arriving at the target. The total number of electrons is directly proportional to the magnitude of the electric current passing through the accelerating field. This current in an x-ray set is referred to as tube current. Radiation intensity is directly proportional to tube current. The two characteristic peaks shown in figure 8 are caused by target material inner shell electrons jumping to a higher energy level, then falling back to their equilibrium state. Characteristic radiation generally occurs at relatively low energy, long wavelength and is of no great importance in the industrial radiography of metallic components although it can cause a problem known as diffraction mottling (see the section on artefacts). As the name suggests, each element produces its own specific characteristic peaks, and measurement of these can be used to perform chemical analysis (x-ray fluoroscopy). Low energy x-rays can be ‘diffracted’ by crystalline materials such as metals. In the diffraction process radiation is deflected from its original path at an angle that is determined by its wavelength and the spacing of the atoms in the crystalline material. This effect can be used to produce the monowavelength x-rays that are used in “x-ray crystallography”. 6.1 X-RAY EQUIPMENT In order to produce x-rays three things are required: 1. A source of electrons. 2. A target, constructed from a suitable high melting point material. 3. A means of accelerating electrons toward the target. High velocity electrons cannot travel far in air, therefore the process of acceleration must take place in a high vacuum. 6.1.1 THE CATHODE
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The Welding Institute The source of electrons is called the cathode. In a conventional x-ray tube it consists of a tungsten filament which is heated by passing a small current through it. Heating the filament produces a cloud of loosely bound, low kinetic energy electrons in close proximity to the filament. This process is known as “thermionic emission.” Electrons are negatively charged and can be accelerated toward the target by making it positively charged with respect to the source of electrons.
Figure 9. Section through a typical x-ray tube cathode
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The Welding Institute 6.1.2 THE ANODE (TARGET) The anode consists of a heavy section of high conductivity copper with a small tungsten (or other high melting point high atomic number metal) insert which is called the target. The anode has a positive electrical potential with respect to the cathode. The body of the anode is always copper because copper is an efficient conductor of heat. This property is necessary because approximately 95% of the kinetic energy of the impacting electrons is converted to heat at the anode. The target material is usually tungsten because this has a very high melting point (3370°C). This reduces the chances that it will be vaporised by the large amount of heat generated. Tungsten has a high atomic number and therefore a large number of electrons. This makes it a relatively efficient material for converting kinetic energy to x-ray energy which in turn helps to reduce the amount heat produced as a proportion of the total output of energy. Sometimes the target is constructed from Tantalum (melting point 2996°C) and less frequently from other refractory metals. Nearly all anodes are ‘hooded’ (see figures 10 and 11) the hood is a high conductivity copper shroud which is designed to intercept stray electrons and to prevent them from hitting the tube walls. The hood has a ‘window’ in the form of a beryllium insert or a thinned section of copper which permits x-rays to exit without unduly increasing ‘inherent filtration’. Inherent filtration is the term used to describe removal of x-rays from the primary beam due to absorption by the materials used in x-ray head construction. The reason that a beryllium window is used in many x-ray heads is that beryllium has a very low absorption factor and this minimises inherent filtration whilst still affording the tube walls protection from stray electrons. Anodes maybe directional as in figure 10 or panoramic as in figure 11. In either case anode design is such that the effective focus size in the direction of the useful beam is much smaller than the actual focus size. This arrangement is called a “line” or “Benson” focus and it serves to maximise anode life without unduly compromising image quality.
Figure 10. Directional anode
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Figure 11. Panoramic anode The target is generally set at an angle of about 70° to the electron beam as shown in figures 10 and 11. This produces a small effective focus size whilst maintaining a large actual focal spot size. The large actual focus size helps to dissipate the heat generated more efficiently. Therefore higher tube currents can be sustained without the risk of damaging the target. This design feature is known as “Benson” or “Line” focusing. See figure 12 below.
Figure 12. Line or Benson focus
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The Welding Institute 6.1.3 X-RAY TUBES The cathode and anode are mounted in an evacuated glass (or in modern tubes metal-ceramic) envelope as shown in figure 13. The tube may be provided with shielding to absorb any unwanted radiation that is not already shielded out by the natural geometry of the anode. Directional type tubes produce a useful beam of radiation that is usually in the form of a cone with a dihedral angle of around 40°. X-ray tubes fitted with a panoramic anode produce a useful beam of radiation through an angle of 360° about the tube axis.
Figure 13. Directional x-ray tube (metal – ceramic type) The x-ray beam produced is filtered by the wall of the glass (or metal-ceramic) envelope. This reduces the useful quantity of x-rays produced, with the low energy components of the spectrum being particularly effected. Therefore it is common in glass tubes that the tube wall will be ground thinner in the region of the useful beam in order to minimise the x-ray energy lost due to self-filtration. Metal-ceramic x-ray tubes (and low kilovoltage glass tubes) may have Beryllium inserts (usually called windows) in order to minimise the filtration effect of the tube wall. Beryllium is used for this purpose because it has a very low x-ray absorption coefficient and because it is mechanically strong enough to contain the necessary vacuum. X-ray tubes are invariably mounted inside some form of ‘tank’. This is usually a metal cylinder that may be fitted with a beryllium or plastic window in order to minimise self-filtration of the x-rays produced. The tank contains a coolant which may be oil or some type of gas. It provides high voltage insulation and mechanical protection. In portable equipment the high voltage transformer is mounted inside the tank. 6.1.4 X-RAY TUBE POWER SUPPLY In order to produce a beam of electrons from the filament in the tube it is necessary to make the anode positive with respect to the cathode. If an AC supply is connected across the tube then the beam of electrons will pass only when the anode is positive and the tube will act as a half wave rectifier.
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Figure 14. Current flow across a half-wave self-rectified x-ray tube Most older type portable x-ray sets were half-wave self-rectified. This produced a considerable weight saving compared with the earlier types of constant potential unit. Most modern portable units are constant potential and use lightweight solid state rectifiers to produce what is effectively DC current. The metal ceramic tubes used in modern equipment are able to safely withstand a greater potential difference between the anode / cathode and the tube wall. This permits the use of “grounded anode” type circuitry which in turn permits direct water cooling of the anode. In an older type unit operating at say 200 kV the cathode voltage would have been minus100 kV while the anode voltage would have been plus 100 kV, giving a maximum potential difference between the electrodes and the glass tube wall of 100 kV. With modern grounded anode circuitry it is safe to hold the cathode at minus 200 kV with the anode at zero volts to produce the same 200 kV potential difference. An anode held at zero volts can be safely cooled by water. Water is a very efficient coolant and direct water cooling of the anode permits operation at greatly increased tube currents. For example the maximum tube current for an older type 200 kV oil cooled head was typically 5 mA self-rectified. With modern portable equipment maximum constant potential tube currents of 15 or 20 mA are not unusual for a 200 kV head. Older type constant potential industrial x-ray units were extremely heavy and bulky and were suitable for use only in fixed installations. Much of the weight and bulk involved came from the rectification circuitry used, the so called “Greinacher Circuit” and the large external oil cooling system that was necessary in order to dissipate the large amount of heat generated.
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Figure 15. Greinacher circuit voltage The total quantity of x-rays produced by an x-ray unit is directly proportional to the area under the line showing voltage against time. Thus for the same tube current and peak voltage, an x-ray tube using a smoothed and fully rectified supply (i.e. a constant potential unit) will produce more x-rays than a self rectified tube. In fact the output of x-rays is more than doubled for the same tube current. In a self-rectified unit the tube voltage varies from zero to the peak voltage and back again with each cycle. In a constant potential unit the tube voltage is close to constant. Thus, looking at the spectrum of x-rays produced, a self-rectified unit produces proportionally more low energy radiation than does a similar constant potential unit.
Figure 16. X-ray spectra of constant potential and self rectified tubes
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The Welding Institute 6.1.5 X-RAY TUBE CONTROLS The radiation produced by the x-ray tube can be varied in quantity and penetrating power (or quality) by controlling the electrical supplies to the tube. 6.1.5.1 PENETRATING POWER OR RADIATION QUALITY (kV) The penetrating power of x-rays depends on the magnitude of the accelerating voltage which is applied between the cathode and the anode. The higher the voltage - the higher the kinetic energy of the accelerated electrons - the higher the photon energy of x-rays produced. The higher the photon energy - the shorter the wavelength - the greater the penetrating power. Thus the penetrating power or quality of x-rays is controlled by the tube voltage. Conventional x-ray tubes, as used in industrial radiography, are capable of being operated in the range from below 50 to 400 kV. If greater penetrating power is required high energy x-ray sources such as betatrons, linear accelerators or Van der Graaf generators can be used to provide x-ray energies of up to 30 or even 40 MeV. National codes and standards such as ASME V (pre-1996 revisions) and BS EN 1435 relate the maximum kilovoltage which may be used to the material thickness which is to be examined. Table 2 gives the approximate limiting maximum economically penetrable thicknesses of steel for various kilovoltages. The figures given are typical for film radiography using lead intensifying screens and portable self rectified equipment . Constant potential units can be used economically on greater thicknesses than can self-rectified units.
Figure 17. BS EN 1435 maximum permissible x-ray tube voltage for various materials
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The Welding Institute Occasionally penetrating radiation is referred to as being either ‘hard’ or ‘soft’. These terms are relative, but hard radiation is produced by high tube voltages (above 150 kV say) whilst soft radiation is produced at lower tube voltages. Kilovolts
Penetrating power: mm of steel 6 20 30 45 60
120 160 200 250 300
Table 2. Approximate penetrating power in mm of steel for various kilovoltages. 6.1.5.2 QUANTITY OF RADIATION (mA) The quantity of radiation produced by the x-ray tube per unit time depends on the number of electrons released by the cathode filament. The number of electrons per second reaching the anode multiplied by the charge on the electron is equal to the tube current. The tube current is not controlled directly. It is increased or decreased by controlling the size of the heating current supplied to the cathode filament. The higher the heating current, the hotter the filament, the greater the thermionic emission of electrons and hence the greater the tube current. Tube current will also be increased for the same heating current if the tube voltage is increased. This is because higher voltages can draw more electrons from the filament even though the filament temperature does not change. So if the tube voltage is altered it will be necessary to adjust the heating current if the same value of tube current is to be maintained. The use of too high a tube current would cause damage to the anode due to overheating, therefore x-ray equipment always incorporates a safety cut-out switch in order to prevent the use of a too-high value of tube current. The total quantity of radiation produced by the x-ray set is directly proportional to the product of the exposure time (i.e. the time for which the x-ray tube is energised) and the tube current; therefore x-ray exposures are usually given in milliampere minutes (mAmin) at a given tube voltage. The standard controls on the x-ray set are: (1) Voltage control: This alters the tube voltage (kV) by varying the low voltage supply to the high voltage transformer. Note that high voltage is not generated in the control panel. This minimises the hazard to personnel. (2) Milliampere control: An ammeter incorporated into the control panel measures (albeit indirectly) the current flowing across the tube. This is proportional to the number of electrons flowing from the cathode to the anode per unit time. In order to increase the supply of electrons the heating current to the filament is increased using the milliampere control. Note that the ammeter measures the current flowing between the anode and the cathode, not the current flowing in the filament (i.e. the heating current).
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The Welding Institute (3) Timer: Since the quantity of x-rays produced is proportional to the length of time during which the tube is energised it is convenient to incorporate a time-switch into the control panel of the x-ray set which automatically terminates the exposure at a preset time. As explained above it is convenient to refer to x-ray exposures in terms of milliampere minutes. For example an exposure which produces an acceptable radiograph may have been determined to be, 36 mAmins at 200 kV. If this was the case then any of the following exposures should give an identical acceptable result: (a) 9 mA for 4 mins at 200 kV. (b) 18 mA for 2 mins at 200 kV. (c) 2 mA for 18 mins at 200 kV. This is because the amount of radiation produced is the same in each case. Obviously it would be desirable to use a high value of mA, in order to reduce the exposure time, but as explained above the use of high tube currents can severely damage the anode of the x-ray tube and thus reduce its service life. Therefore it is usual to operate at a value of mA which is well within the tube’s specified capabilities. The reciprocal relationship between time and tube current is sometimes referred to as the reciprocity law or the Bunsen Roscoe reciprocity law. 6.1.6 HIGH ENERGY X-RAY SOURCES 6.1.6.1 BETATRONS Betatrons are used to produce ultra-hard extremely penetrating radiation with photon energies in the range 1-30 MeV. The efficiency with which the kinetic energy of the accelerated electrons is converted to x-rays is much better at these high voltages than at those experienced in conventional x-ray tubes. Consequently betatrons usually benefit from quite small focal spots. In betatrons electrons are accelerated in a spiral path of perhaps 1,000,000 revolutions by means of alternating magnetic fields before being deflected towards the target. The radiation produced by betatrons can penetrate 300 mm or more of steel. They are primarily used for the radiography of castings or large section welds in fixed installations but portable units are available. These are sometimes used on site for the inspection of reinforcing bars in heavy concrete sections. Up to around 10 MeV betatrons are usually preferred to linear accelerators because they are more compact and less expensive to manufacture. 6.1.6.2 LINEAR ACCELERATORS Linear accelerators (often called linacs) accelerate electrons to very high velocities along a straight path by means of an electromagnetic waveform generated by a device called a magnetotron. The particle velocities are similar to those achieved in betatrons but a much higher output of radiation is achievable. For radiation energies above 10 MeV linear accelerators are generally the preferred solution.
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The Welding Institute 6.1.6.3 VAN der GRAAF GENERATORS Van der Graaf generators generate a high voltage charge of static electricity by mechanical means - friction. The static charge produced can be used to accelerate electrons for x-ray production. Van der Graaf generators can produce short intense pulses of x-ray energy. They have therefore found some application in the field of ‘ballistic radiography.’ 6.1.7 SPECIAL TYPES OF X-RAY UNIT 6.1.7.1 MICROFOCUS X-RAY SOURCES Standard x-ray equipment has an effective focus size usually in the range from 1 to 4 mm. This is small enough to provide adequate image quality for most standard techniques. Microfocus x-ray equipment may have an effective focus size as small as 0.1 mm. Using such a small focus size geometric enlargement techniques are possible whilst still producing an adequately sharp image.
Figure 18. Image enlargement using standard and microfocus x-ray equipment 6.1.7.2 ROD ANODE X-RAY TUBES In a rod anode tube the target is at the end of a copper or aluminium tube which is usually less than 20 mm outside diameter and may be up to a metre long. The target is invariably of the panoramic variety. Grounded anode circuitry is essential for this type of tube. The anode can be positioned inside small diameter pipes in order to carry out panoramic radiography of girth welds; it can also be positioned in many other otherwise inaccessible locations. Rod anode tubes are most often used in aerospace applications.
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Figure 19. Rod-anode x-ray tube 6.1.7.3 ROTATING ANODE X-RAY EQUIPMENT In medical radiography a very large tube current is generally desirable – a high tube current permits a very short exposure time which in turn helps to eliminate or reduce unsharpness caused by relative movement during exposure. In order to maximise tube current some medical equipment is fitted with a rotating anode. In a rotating anode x-ray tube the anode rotates at high speed and the focus area of the target is therefore constantly changing. Each section of the tungsten target is ‘in use’ for a short time followed by a slightly longer period of ‘resting’. This helps to prevent overheating so the tube current can be greatly increased. 7.0 GAMMA RAYS ‘Gamma (γ) ray’ is the term applied to the electromagnetic radiation which is sometimes produced when the atomic nuclei of a radioactive isotope disintegrate in the process known as atomic fission. Alpha (α) and beta (β) particles may also be produced during the disintegration process; in fact gamma emission is always a by-product of alpha or beta emission. Of the three main types of radiation produced by fission alpha is by far the most hazardous to health; alpha and beta radiation must be taken into consideration when assessing safety. Except as a health hazard, alpha and beta particle radiation have no significance for industrial radiography since they are easily absorbed by very thin materials. The disintegration process is fixed for each radioactive isotope and as a result the gamma ray energies produced are also fixed.
Figure 20. Decay path for cobalt 60
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The Welding Institute The spectra produced are ‘line’ or ‘discrete’ spectra as opposed to the ‘continuous’ spectra produced by x-ray equipment. Table 3 lists the principal gamma emissions for various commonly used isotopes. Figure 21 shows the line spectrum for Iridium 192. 7.1 ALPHA AND BETA EMISSION 7.1.1 ALPHA PARTICLES Alpha particles are emitted during the decay of heavy nuclides such as Uranium (U) 238 and Plutonium (Pu) 239. An alpha particle consist of 2 protons and 2 neutrons – basically a Helium (He) nucleus, emitted from the nucleus at very high velocity.
Thus in alpha emission there is a loss of 4 amu from the nucleus and a reduction in atomic number of 2 (see the example above). Alpha particle radiation cannot penetrate more than a thin sheet of paper or a few centimetres of air, it is, however, very strongly ionising. The great danger to health with alpha emitters is that they may be ingested – radioactive contamination. Once within the human body they will in most cases cause cancer. 7.1.2 BETA PARTICLES Beta particles may be emitted during radioactive decay. A beta particle consists of a very high velocity electron emitted from the nucleus of a radioactive atom when a neutron converts to a proton. It is important to note that although the beta particle is an electron it has very much higher kinetic energy than a free electron which has resulted from an ionisation event.
Thus in beta emission there is no loss from the atomic mass number whilst the atomic number increases by 1 (see the example above). Beta radiation is more penetrating than alpha. It can penetrate the outer layers of the skin and lead to fatal skin burns. The damage caused is very similar to sunburn, but much more severe. Many of the early victims of the Chernobyl disaster died as a result of skin burns caused by exposure to high intensities of beta radiation. If beta emitters are ingested they will often lead to cancer.
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The Welding Institute 7.2 SEALED SOURCES The first gamma ray emitting radioisotopes to be used in industrial radiography were naturally occurring radioactive materials such as Radium. Such sources were not sealed and therefore there was a danger of exposure to alpha (α) and beta (β) particles, both of which are extremely damaging to human tissue. Coupled with this was the even greater hazard of radioactive contamination by which radioactive materials might find their way into the human body. All gamma sources in use today are man-made. They are manufactured by neutron bombardment of non-radioactive raw materials in the core of a small nuclear reactor. The sources in use are all beta emitters, gamma rays being produced as a by-product of beta emission. In order to prevent beta emission or contamination hazard the sources used in industrial radiography are invariably sealed sources. The fissile material is encapsulated in a high integrity titanium or stainless steel shell. Beta radiation is not capable of penetrating the walls of the capsule, and the capsule further precludes any possible contamination hazard so long as it remains intact. Isotope
Half-Life
Iridium (Ir) 192
74.4 days
Cobalt (Co) 60 Thulium (Tm) 170 Ytterbium (Yb) 169
5.3 years 127 days 32 days
Selenium (Se) 75
118.5 days
Principle Emissions (MeV) 0.31,0.47, 0.60 1.17,1.33 0.052,0.084 0.17,0.20 0.121, 0.136, 0.265, 0.28, 0.401
Equivalent x-ray Kilovoltage (kV)
Penetrating Power in mm of Steel
400
75
1200 80 145 217 (low energy beam components improve sensitivity)
200 4 10 30
Table 3. Gamma emissions for commonly used isotopes.
Figure 21. Iridium 192 - principal gamma emissions
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Figure 22. Sealed source Figure 22 shows the typical encapsulation arrangement for Iridium 192 and Cobalt 60. Some isotopes such as Caesium 137 are double encapsulated. In the case of Caesium 137 this is because the caesium is in the form of caesium chloride which is highly corrosive and highly water soluble (but this is still an improvement on caesium metal which causes an explosion on contact with water). 7.3 PENETRATING POWER OF GAMMA RADIATION The penetrating power is fixed for each isotope because the spectrum of gamma radiation emitted is fixed. If a material thickness is too great to produce a radiograph using, say, Ir192 then an isotope which produces higher energy gamma radiation such as Co60 must be used. 7.4 QUANTITY OF GAMMA RADIATION The amount of gamma radiation – the number of photons, produced by an isotope is controlled by the number of disintegrations (atomic fissions) per unit time. The “source strength” of an isotope is usually expressed in curies (Ci) or becquerels (Bq). “Source strength” may also be referred to as “source activity.” 1 Ci = 3.7 x 1010 disintegrations per second 1 Bq = 1 disintegration per second The becquerel, which is the SI unit of radioactivity, is a very small unit in terms of what is required for industrial radiography. The curie is therefore generally preferred. If the becquerel is used at all then it is usually in the form of gigabecquerels (GBq). One gigabecquerel is equal to one thousand million (109) becquerels. One curie is equal to 37 gigabecquerels (37 GBq). In the great majority of cases gamma ray exposures are expressed in curie-hours, curie-minutes or curie-seconds; this in each case being the product of source strength measured in curies multiplied by exposure time measured in hours minutes or seconds.
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The Welding Institute Example: A steel section 50 mm thick requires an exposure of 700 curie-minutes using Iridium 192 with a source to film distance of 1 metre using Kodak CX film and lead intensifying screens. All other factors being equal the exposure time would therefore be: (a) 1 hour 10 minutes with a source strength of 10 Curies. or (b) 20 minutes with a source strength of 35 Curies. or (c) 7 minutes with a source strength of 100 Curies. Gamma rays are produced by a disintegration process. Atoms having unstable nuclei decay with a fixed probability to form other atoms having stable nuclei. Therefore the source strength of the radioactive isotope will reduce with time. The probability decay for a large number of unstable atoms is fixed and proportional to the number of unstable atoms present. This means that the strength of a radioactive source will always reduce exponentially: i.e. the strength of a given source will reduce by 50% in a fixed time. This fixed time is referred to as ‘HALF LIFE’. The half life of various commonly encountered isotopes is given in table 3. If the half life of an isotope is known then the source activity at a given time can be calculated if at some point previously the source activity was measured. Suppose that an isotope having a half life, h had an activity, S0 at time, t = 0. Then at time, t the source strength or activity, St can be calculated using :
St = S0 2-(t/h) Alternatively the activity of a source can be estimated using a decay chart. Figure 23 shows the decay chart for Iridium 192.
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Figure 23. Iridium 192 decay 7.5 RADIOISOTOPE CONTAINERS FOR INDUSTRIAL RADIOGRAPHY Radioactive isotopes emit gamma rays continuously. The decay process cannot be switched off or in any way slowed down. Gamma radiation is extremely harmful to human body tissues so radioactive isotopes must be shielded when not in use. The shielding materials used in isotope containers are always dense materials such as lead, tungsten or (more commonly) depleted uranium. Most modern containers use depleted uranium shielding because uranium is an extremely efficient absorber of gamma radiation. Uranium shielded isotope containers are much lighter and more portable than their lead shielded counterparts. A uranium shielded container having a weight of about 20 kg can safely store 100 Ci of Iridium 192. A lead shielded container of the same weight would be capable of safely containing only 20 Ci of Iridium 192. Radioactive isotope containers are designed to fulfil two important functions : (1) To contain the radioactive isotope and reduce the emitted intensity of radiation to a level which allows for safe transportation and storage. (2) To allow the radioactive isotope to be safely exposed in order that it may be used for radiography. In addition, radioactive isotope containers have to be capable of withstanding possible accidents involving impact or fire.
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The Welding Institute All modern isotope containers are designed to be operated by cable (see figure 26). They are of two basic types (see figures 24 & 25). Of the two types depicted the “S” tube type is intrinsically safer but around 30% heavier than the equivalent shutter type. Older types of isotope container did not provide for remote operation.
Figure 24. S-tube type radioactive source container
Figure 25. Shutter type radioactive source container (schematic only)
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Figure 26. Remote control isotope delivery system
Figure 27. Sealed source with flexible cable (pigtail) attached
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The Welding Institute 7.6 COMPARISON OF X-RAYS AND GAMMA RAYS 7.6.1 ENERGY AND OUTPUT OF RADIATION X-ray equipment produces a continuous range of photon energies up to a threshold level dependent upon the tube voltage setting. The threshold photon energy level can be adjusted from 50 keV or less up to a maximum (for high energy equipment) of perhaps 30 MeV. The photon energy of gamma ray sources is fixed. The output of radiation per unit time is variable for x-ray equipment up to the maximum mA rating of the tube. The output of radiation from a radioactive isotope is fixed by the source activity. The output of radiation produced by x-ray equipment is generally much greater than that produced by radioactive isotopes. The penetrating power of ionising radiation is controlled by its maximum photon energy and the photon energy distribution. Table 4 gives an indication of the maximum steel thickness that can practically be radiographed using conventional x-ray equipment and the commonly encountered isotopes. The penetrating power of x-rays produce by self rectified equipment is less than that of x-rays produced by constant potential equipment operating at the same tube voltage. This is because the constant potential equipment produces a larger proportion of high energy radiation than does the self rectified. Source of Radiation x-ray SelfRectified x-ray Constant Potential
Gamma Ray
Useful Thickness Range/mm of Steel Maximum 6 mm Maximum 20 mm Maximum 30 mm Maximum 60 mm Maximum 10 mm Maximum 32 mm Maximum 45 mm Maximum 100 mm Maximum 4 mm 4-30 mm 2-8 mm 10-75 mm 40-200
100 kV (peak) 150 kV (peak) 200 kV (peak) 300 kV (peak) 100 kV 150 kV 200 kV 300 kV Thulium 170 Selenium 75 Ytterbium 169 Iridium 192 Cobalt 60
Table 4. Useful thickness range for various sources of radiation. Note: Steel sections of 500 or 600 mm can be radiographed using x-rays generated by linear accelerators or betatrons. 7.6.2 RADIOGRAPHIC CONTRAST Low energy radiation is more easily absorbed by than high energy radiation. Therefore low energy radiation will show a bigger change in radiation intensity for the same change of penetrated section thickness than will high energy radiation. Thus radiographs
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The Welding Institute made with low energy radiation will usually show better contrast than those made using high energy radiation. The contrast of x-radiographs is generally better than gamma-radiographs since it is possible to optimise the x-ray energy for the thickness of the material which is to be examined; in so doing obtaining the best possible contrast. With a gamma ray source the radiation energy is fixed and is optimum for a narrow range of thickness only. Contrast is better for x-rays of the same maximum radiation energy than it is for gamma rays because xray tubes produce a continuous range of energies as opposed to the line spectrum which is obtained from a gamma ray source. 7.6.3 FOCAL SPOT SIZE VERSUS SOURCE SIZE A radiograph produced using small effective source size will usually be of higher quality than one produced with a larger effective source size. The average focal spot size an x-ray tube is similar to the average physical size of the gamma ray sources which are commonly used. Most x-ray tubes have a fixed effective focal spot of between 1 and 4 mm. With some x-ray equipment the focal spot size can be varied. Microfocus x-ray tubes may have an effective focal spot of less than 0.1 mm. The size of the focal spot in an x-ray tube tends to be larger for the higher maximum kilovoltage tubes. This is due to the need to dissipate the increased amount of heat generated at high kV. The practical source size for a radioactive isotope is determined by the maximum economically achievable specific activity. Specific activity is usually expressed in curies (or becquerels) per gram. Table 5 below gives typical practical achievable maximum specific activity for 4 common isotopes.
Isotope
Cobalt 60 Iridium 192 Caesium 137 Thulium 170
Practically achievable maximum specific activity (Curies per gram)
Density (g/cm3)
50 350 25 1,000
8.9 22.4 3.5 (note 1) 4 (note 2)
Maximum practically achievable activity for 3 mm diameter, 3 mm long cylindrical pellet (Curies) 10 166 2 85
Note 1. Density is for compressed caesium chloride (CsCl) Note 2. Density is for thulium oxide (Tm2O3)
Table 5. Specific activity for common radioisotopes Note that the maximum activity of a gamma ray source is limited by it’s physical size. The most useful isotopes are those which have a high value of practically achievable specific activity. In an iridium 192 source at the maximum achievable activity about 2.5 atoms per 100 million are radioactive. In a cobalt 60 source the figure is only about 1 atom in every 10,000 million. The output of radiation from a typical x-ray machine is much greater than the output of radiation from a typical gamma source. This means that in x-radiography the use of long 38
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The Welding Institute and therefore more prone to damage in site conditions. Gamma ray sources have to be replaced on a regular basis due to radioactive decay. This can become costly if the source is not used regularly. Gamma ray sources have, by law in most countries, to be stored in very secure conditions. This factor also adds to costs when compared to x-ray equipment. X-ray exposures tend to be shorter so there can be a cost saving with x-ray equipment if the setting up time between successive exposures is minimised. Overall gamma radiography tends to be the most cost effective solution for construction site work but xradiography may provide the cheapest option where are large number of similar radiographs are required (such as may be the case in pipeline or mass production environments). 8.0 METHODS OF PRODUCING A RADIOGRAPHIC IMAGE 8.1 RADIOGRAPHIC FILM Radiographic film is essentially the same as that used in photography in that it consists of a suspension of silver halide grains in a gelatine binder on an acetate or polyester base. Radiographic film, however, differs from photographic film in the following respects: (i) The acetate or polyester base material is considerably thicker than is the case with photographic film. (ii) The emulsion is applied to both sides of the film. This effectively doubles the film density (i.e. degree of darkness) for the same exposure to radiation and thereby doubles the film speed. (iii) The emulsion tends to be thicker (usually around 0.025 mm) than that used in photographic films, in order to further increase the film speed. Two types of radiographic film are used for industrial radiography, these being: Direct type film, where the principal cause of image formation is the ionising radiation itself. This may be coupled with the effect of “secondary electrons” emitted from metallic foil intensifying screens. and Screen type film, where the principal cause of image formation is light emitted from fluorescent image intensifying screens under the action of ionising radiation.
Figure 28. Cross-section through a radiographic film
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The Welding Institute Some radiographic film will produce good results either as a direct type or as a screen type film. The film emulsion in screen type films usually has a matt finish so as to avoid reflecting the light produced by fluorescence. Radiographic film is generally further divided into three categories, these being: (BS EN 584-1 lists a total of 6 film classifications, C1 to C6. BS EN 1435, however, refers to only 3 of these 6, these being C3, C4 and C5. Class C3 corresponds to ultrafine grain (e.g. Agfa D4). Class C5 corresponds to fine grain (e.g. Agfa D7 or Fuji 100) while C4 is intermediate between the two (e.g. Agfa D5, Kodak MX or Fuji 80).
ultrafine grain (slow film) e.g. Agfa D4 or D5, Fuji 80 or 50 and Kodak MX fine grain (medium speed film) e.g. Agfa D7, Fuji 100 or Kodak CX and coarse grain (high speed film). Coarse grained film requires a shorter exposure time than does fine grain film because each grain of silver halide needs only to receive as few a single photon of radiation or single ‘secondary electron’ in order to become ‘sensitised.’ When a sensitised grain contacts the developer solution the entire grain, regardless of its size is converted to image forming metallic silver. Large grains of silver will block out more light than small grains so a coarse grained film will appear darker after processing than will a fine grained film even though the exposure conditions were exactly the same. Table 6 lists the direct type radiographic films produced by various manufacturers in order of their relative film speed. For the most part, industrial radiography is carried out using fine grain film such as Kodak CX, Agfa D7, Cronex NDT 70 or Fuji 100. Ultrafine grained film is used where adequate sensitivity cannot be achieved using fine grained film. Coarse grained film is rarely used where except very rapid results have to be obtained. Film manufacturer Film type
Kodak
Ultrafine grain
MX
Fine grain
AA CX
Coarse grain
Agfa Gevaert
D4 D5 D7 D10
Dupont (Cronex) NDT 45 NDT 55 NDT 65 NDT 70 NDT 75 NDT 91
Fuji
50 80 100 150 400
Table 6. Radiographic film
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The Welding Institute 8.1.1 LATENT IMAGE FORMATION When film is exposed to x-rays, gamma rays, or light, an invisible change called a latent image is produced in the film emulsion. The areas so exposed become dark when the film is immersed in a developing solution, the degree of darkening depending on the amount of exposure. Photographic emulsion consists of myriads of tiny crystals of silver halide (usually silver bromide with a small quantity of silver iodide) dispersed in gelatine. Each tiny crystal responds as an individual unit to during exposure to radiation and subsequent development. A latent image can be defined as that radiation induced change in a silver halide crystal that renders the crystal susceptible to the chemical action of the developer. Photography utilising film emulsion similar to that which is in use today has been with us since around 1839 but the mechanisms involved in latent image formation remained a mystery until 1938 when the ‘Gurney – Mott Theory’ was first put forward. Although this theory is now generally accepted there remain areas of speculation. Formation of a latent image involves very subtle change in the silver halide grain. It is known to involve the absorption of only one or a few photons of radiation. Because of the small amount of energy involved it is obvious that only a few atoms, out of the ten thousand million or so atoms in a typical silver halide grain, can actually be affected. To date it has proved impossible to detect either the physical or the chemical nature of the tiny changes involved. Against this, however, much can be deduced about what the physical nature of these changes must be. For one thing we know that the substance which forms the radiographic image must be metallic silver. We also know that the latent image is localised at certain discrete sites within the silver halide grain. The evidence for this is shown in figure 29 below. Figure 29 is an electron micrograph of a section of film emulsion that has been exposed to light followed by a brief contact with developer. Note how tiny amounts of silver have appeared (the dark areas) within each grain of silver halide. Further it is known that prolonged exposure to light will darken the film emulsion even without development. Therefore the mechanism of latent image formation will by itself cause the release of silver from a silver halide grain under extreme conditions (see figure 30 below).
Figure 29. Electron micrograph of exposed, partially developed, partially fixed grains of silver halide, showing initiation of development at localised sites in the grains (1µ = 1 micron = 0.001 mm).
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Figure 30. Electron micrograph of the release of silver in a grain of silver halide caused by very intense exposure to light. Based on such evidence it is fairly safe to assume that the mechanism of latent image formation releases a few atoms of silver within each exposed silver halide grain. The chemicals contained in the developer then preferentially attack such exposed grains. An excellent explanation of the Gurney – Mott Theory can be found “Radiography in Modern Industry” which can be found on the Kodak website. 8.1.2 FILM CASSETTES Radiographic film is highly sensitive to light, in particular to light at the blue end of the spectrum. It has to be protected from exposure to light (except for the light from darkroom safe lamps) at all times up to the point where the fixing process has commenced. Prior to use the film is stored in light proof boxes. In order that the film can be used for radiography the film has first to be inserted into a suitable light proof container. Such containers are called ‘cassettes’. Film cassettes are of two types: rigid and flexible. Film cassettes serve three important functions: firstly they protect the film from unwanted exposure to light, secondly they help to maintain good film - screen contact and thirdly they protect the film against environmental or handling damage. Film cassettes must be manufactured in such a way that they do not produce any unwanted image on the radiograph. Some radiographic film is available pre-packed in a protective light proof envelope complete with lead intensifying screens. For the most part however, flexible cassettes manufactured from opaque PVC or other plastic are used because they are cheap, durable and versatile. Film cassettes must be handled with care, flexible cassettes are particularly easy to rupture during loading and unloading of the film. Cassettes which leak light can add considerably to the cost of radiography if they lead to a radiograph having to be retaken. Therefore it is good practice to inspect cassettes prior to use. Leaky cassettes can often be satisfactorily repaired using opaque adhesive tape.
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The Welding Institute 8.1.3 INTENSIFYING SCREENS In industrial radiography intensifying screens of one form or another tend to be used for most exposures. An intensifying screen amplifies the effect that the primary radiation beam has upon the radiographic film emulsion, thus shortening the required exposure time. 8.1.3.1 METALLIC FOIL INTENSIFYING SCREENS At first it may seem to be a little paradoxical that metallic (nearly always lead but occasionally copper, steel or tantalum) foil screens can produce an intensifying effect. All metals are good absorbers of ionising radiation so one would naturally expect that the film density would be reduced rather than increased. In practice however, lead or copper foil screens brought into close contact with direct type radiographic film reduce the required exposure for radiation energies in excess of 120 keV by a factor of about two. The reason for this is that under the action of ionising radiation of energy 120 keV or greater metals produce ‘secondary electrons’ which have kinetic energy sufficient to cause the sensitisation of the grains of silver halide which they strike. Metallic foil screens further add to the quality of the radiograph by filtering out a large proportion of the scattered radiation which is of lower energy (and therefore more easily absorbed) than the primary beam. For most purposes lead foil screens 0.125 mm thick are used but thicker screens are used for high energy radiography. Copper screens tend to be used only for extremely high energy techniques (above 1 MeV). The lead screens found in pre-packed film are only a few microns thick, they produce a strong intensifying effect but have a much reduced effect on the scattered radiation as compared with standard re-useable lead screens. Pre-packed film is available either in individual disposable cassettes or as ‘rollpack’ where a long narrow length of film is supplied complete with lead screens in a protective light proof sheath. Rollpack film can be cut to any desired length. The cut ends have to be light sealed with suitable adhesive tape. Rollpack is commonly used on pipelines in conjunction with the panoramic technique. BS EN 1435 tables 2 and 3 specify metallic foil screens of lead, copper, steel and tantalum. The specified thickness range and screen material change for different x-ray tube voltages and for different isotopes. 8.1.3.2 SALT SCREENS Salt screens consist of a layer of calcium tungstate (or other fluorescent material), attached, using a suitable binding material, to a sheet of cardboard. While salt screens can produce a dramatic reduction in exposure time when used with screen type film they are seldom used in industrial radiography because they produce an image of inferior quality. They are expensive and they are very easily damaged. Salt screens produce an image intensifying effect by fluorescing, usually the blue part of the spectrum, under the action of ionising radiation. They are capable of cutting the exposure time required by a factor of up to 500.
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The Welding Institute 8.1.3.3 FLUOROMETALLIC SCREENS These screens, which attempt to combine the advantages of lead screens with those of salt screens are occasionally used in industrial radiography when there are strong financial pressures for a reduction in exposure time. One such application is on offshore pipe laying barges. They are even more expensive than salt screens at around £70 for a pair of 10 x 40 cm screens and they are just as easily damaged. They do not provide quite the same reduction in exposure time as do salt screens but the image quality is considerably improved (although still inferior to that produced using lead screens). Fluorometallic screens consist of a cardboard backing material to which a layer of lead foil is attached, attached to the lead foil is a layer of calcium tungstate or other fluorescent crystalline material suspended in a suitable binding material. 8.1.4 FILM PROCESSING Radiographic film forms a ‘latent image’ during exposure to ionising radiation, light or secondary electrons. By a process which is not fully understood silver halide grains become sensitised during exposure (see section 7.1.1 above). In order to make the latent image formed by the sensitised grains visible it is necessary to chemically process the film. Films can be processed either manually or automatically but the chemical processes involved are the same. Radiographic film must not be exposed to light except that from darkroom safelamps; even exposure to safe-lamps must be minimised as prolonged exposure can result in film fogging. Extreme care must be exercised during film processing because the wet film emulsion is extremely fragile.
Figure 31. Film / lead screens / flexible cassette 8.1.4.1 DEVELOPMENT The first stage in film processing is development. During this stage a reducing agent such as hydroquinone or metol reduces the sensitised silver halide grains in the film emulsion to metallic silver. Development whether manual or automatic must be carried out within the temperature range recommended by the developer manufacturer otherwise image quality will be severely impaired.
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Developers for manual processing are usually designed for use at 20°C, for automatic processing this will usually be increased to around 27°C. Films should always be developed for the optimum processing time of about 5 minutes for manual development and under 2 minutes for automatic development. Film developed for a time which is not within the developer manufacturer’s recommendations will have impaired image quality. Films must be agitated thoroughly in the first 20 - 30 seconds of development when the reaction with the developer is rapid and at regular intervals (usually 10 seconds per minute) throughout the remainder of allotted development time. Failure to properly agitate the film will result in a streaky radiograph and inferior image contrast. The developer solution undergoes chemical changes during film processing and it must be replenished regularly in order to maintain its effectiveness. Exposure to air must be minimised because developer is readily oxidised. Contamination of the developer with foreign material, especially metal particles, is likely to lead to unwanted images being produced on the film. 8.1.4.2 STOP BATH The stop bath serves two purposes: (1) to curtail the action of the developer and (2) to protect the fixer by reducing carry over of developer solution. It is not essential to use a stop bath but it is desirable because it will considerably extend the life of the fixer. It will also help to avoid possible film artefacts, ‘dichroic fogging’ in particular. Two types of stop bath may be used: (1) an acid stop bath or (2) a running water stop bath. In either case the stop bath must be maintained at a temperature which is comparable to that of the developer otherwise ‘reticulation’ may result. The developer operates in an alkaline buffer solution. Acid stop baths neutralise the alkalinity of the developer and stop the development process almost immediately. It is normal to allow the film to remain in such a stop bath for 10 to 30 seconds. Running water stop baths quickly dilute the developer solution thus rapidly slowing the development reaction and minimising the damage to the fixer. It is normal to allow a time of 2 to 3 minutes when using running water stop baths. 8.1.4.3 FIXING AND HARDENING The chemicals which are used to ‘fix’ the image and ‘harden’ the emulsion are normally combined together in a single chemical bath. Both types of chemical have to be protected by an acid buffer solution. In acid solution the active ingredient in the fixer will dissolve only silver halide from the film; if the solution becomes alkaline it will, in addition, begin to dissolve any metallic silver present. The silver halides which remain intact in the film emulsion after development must be removed in order to preserve the image which has been formed and in order that it can be viewed using transmitted light. Any silver halide which remains in the emulsion of a fully processed film will quickly deteriorate on contact with air under the influence of light to form brown stains which severely degrade the quality of the image.
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The Welding Institute The process of removing excess silver halides is called “fixing”. The chemical used to achieve this is sodium (or ammonium) thiosulphate (sometimes called hypo). The gelatine binder which holds the silver and silver halides becomes soft and spongy in the developer. ‘Hardening’ the film serves to get rid of some of this sponginess and gives the film better resistance to the formation of water marks during drying. Hardening of the film emulsion, although always desirable, is not absolutely necessary if the films are subsequently washed at a temperature of less than 25°C and dried manually If automatic dryers are to be used, however, the film emulsion will be badly damaged by the rollers if the film has not been properly hardened.
For manual processing the fixer-hardener bath should be maintained at the same temperature as the developer, although in this case the temperature is not so critical. Films should be fixed for twice the ‘clearing time’ where ‘clearing time’ is the time taken for the fixer to strip out the remaining silver halide. The clearing time for a fixer bath maintained in good condition will generally be less than 2.5 minutes at 20°C. While fixing times of 1 hour or more generally have no ill effects, over fixing should be avoided because in some circumstances it can lead to ‘frilling’. Frilling is a film artefact whereby the film emulsion becomes detached from the base. Frilling can be caused by allowing film to remain in the fixer for an extended period at high temperature, particularly where hardener has not been added to the fixer. 8.1.4.4 WASHING After fixing the film must be thoroughly washed so as to remove all traces of the fixer chemicals from the emulsion. Insufficient washing will result in the formation of brownish yellow stains while over-washing can cause water marks or even frilling (see above). Adequate wash times in a running water wash vary from 10 minutes at 30°C to 30 minutes at 10°C. Most film manufacturers recommend that the wash temperature should not be more than 25°C. Film can be washed successfully in a still water bath provided that the water is changed regularly. 8.1.4.5 DRYING The application of a wetting agent to the film prior to drying will help the film to dry quickly / evenly and without watermarks. If the films are to be dried using a warm air draught then care must be taken to ensure that dust is not blown onto the wet films. Warm air dryers with a downward draught dry the film more quickly. 8.2 ADVANCED IMAGING TECHNIQUES Computed Radiograhy Computed Radiography (CR) uses very similar equipment to conventional radiography except that in place of a film to create the image, an imaging plate is used. The imaging plate contains photostimulable storage phosphors, which store the radiation level received at each point in local electron energies. When the plate is put through the scanner, a scanning laser beam causes the electrons to relax to lower energy levels, emitting light that is measured to compute the digital image. Hence, instead of taking a film into a darkroom for developing in chemical trays, the imaging plate is run through a computer scanner to read and digitize the image. The image can then be viewed and enhanced using software that has functions very similar to conventional image-processing software, such as contrast, brightness, and zoom.
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Computed radiography is commonly distinguished from Direct Radiography (DR). In the same way that a CR system requires a short burst of radiation, so does a DR system. The difference is that on exposure a DR system will almost instantly display the image on the screen in front of the radiographer, therefore removing any need for processing. Post production can of course be performed on DR images in the same way that CR images can. Computed Tomography The technique of tomography involves passing a series of X-rays through an object, and measuring the change in intensity or attenuation of these X-rays by placing a series of detectors on the opposite side of the object from the X-ray source. The measurements of Xray attenuation are called projections and are collected at a variety of angles. Computer Tomography (CT) is a powerful NDT technique that uses a computer to produce 2-D crosssectional and 3-D images of an object from X-radiographs. Characteristics of the internal structure of an object such as dimensions, shape, internal defects, and density are readily available from CT images. Real-time Radiography (Fluoroscopy) Real-time radiography (RTR), or real-time radioscopy, is a nondestructive test (NDT) method whereby an image is produced electronically, rather than on film, so that very little lag time occurs between the item being exposed to radiation and the resulting image. In most instances, the electronic image that is viewed results from the radiation passing through the object being inspected and interacting with a screen of material that fluoresces or gives off light when the interaction occurs. The fluorescent elements of the screen form the image much as the grains of silver form the image in film radiography. The image formed is a "positive image" since brighter areas on the image indicate where higher levels of transmitted radiation reached the screen. This image is the opposite of the negative image produced in film radiography. In other words, with RTR, the lighter, brighter areas represent thinner sections or less dense sections of the test object. Real-time radiography is a well-established method of NDT having applications in automotive, aerospace, pressure vessel, electronic, and munition industries, among others. The use of RTR is increasing due to a reduction in the cost of the equipment and resolution of issues such as the protecting and storing digital images. 9.0 PRODUCTION OF A RADIOGRAPH (FILM RADIOGRAPHY) A radiograph is a record of the way in which a beam of radiation has been differentially absorbed by an object stored on photographic film. In order to produce good quality radiographs in an economical time numerous factors have to taken into account. First it is necessary to understand what is meant by ‘radiographic quality’. 9.1 RADIOGRAPHIC QUALITY The quality or sensitivity of a radiograph is a measure of the ability of the radiograph to detect small changes in radiation intensity caused by variations in object thickness or composition. In order to detect small imperfections in an object an adequate level of sensitivity must be achieved. The standard methods of measuring or estimating sensitivity are described in the next chapter. Achieving adequate sensitivity is the crucial factor which determines the level of success of radiography as an NDT technique.
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Figure 32 summarises the factors which must be considered if radiographs of adequate quality are to be produced. Radiographic quality or sensitivity depends on achieving good ‘contrast’ and good ‘definition’.
Figure 32. Factors affecting radiographic sensitivity 9.1.1 CONTRAST Contrast can be defined as the ease with which it is possible to distinguish between two adjacent areas of different film density. The chief factor which determines whether or not the two areas will be clearly defined is the degree of difference in film density. Radiographic contrast comes from two sources: 1. The object being radiographed: ‘subject contrast’. and 2. The film used to produce the radiograph: ‘film contrast’. Subject contrast can be defined as the degree of difference in transmitted radiation intensity produced by a given change in subject thickness. This is primarily a function of the type of material from which the subject is made. For instance a 1 mm step in a 10 mm section of lead will produce a much greater change in transmitted radiation intensity than would the same step in a similar section of aluminium (assuming that the energy of the incident radiation was the same in both cases). Film contrast can be defined as the degree of difference in film density produced by a given change in radiation intensity or exposure time. This is primarily controlled by the type of film used. The factors affecting film and subject contrast are discussed below: 9.1.1.1 FILM TYPE (AFFECTS FILM CONTRAST)
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In considering the effect that film type has upon film contrast (the type of film has no effect on subject contrast) it is useful to refer to a type of graph called a ‘film characteristic curve’. Such graphs relate the logarithm of the relative exposure time to the achieved film density. A few examples are given in figure 33. The gradient of a film characteristic curve represents the change in film density produced by a small change in subject thickness. Figure 34 shows how the gradient of the film characteristic curve varies with film density. Note that the curve for Kodak MX, an ultrafine grain film, has the steepest gradient. The Agfa D7 curve is in turn steeper than that of Kodak CX. D7 and CX are both class C5 fine grain film, but CX is a faster film than D7. Thus MX will provide the best film contrast, whilst D7 should produce contrast better than that of CX.
Figure 33. Film characteristic curves CX-D7-MX (direct type film / lead screens)
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Figure 34. Gradient of the film characteristic curve versus film density for Kodak MX, Agfa D7 and Kodak CX
9.1.1.2 FILM DENSITY (AFFECTS FILM CONTRAST) Film density can be defined as the degree of darkening of the film or more properly the degree to which the film prevents light from passing through it. Mathematically, film density is defined as the logarithm to the base 10 of the ratio of the incident to transmitted light intensity. It can be calculated using the following formula:
The logarithm to the base 10 of a number is just the power of 10 that will produce the number itself. For example: 102 = 100 and the logarithm of 100 = 2; 103 = 1,000 and the logarithm of 1,000 =3; 100.301 = 2 and the logarithm of 2 = 0.301
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The Welding Institute Thus a film having a density of 2.0 transmits 1% of the incident light intensity while a film with a density of 3.0 transmits only 0.1%. A film with a density of 0.3 would transmit about 50% of the incident light intensity. Figures 33 and 34 above show how film density affects film contrast. Film density does not affect subject contrast. The gradient of the film characteristic curve is a good measure of film contrast. The gradient for all films increases with increasing film density. If the gradient is steep then a small change in radiation intensity or exposure time will produce a large change in film density. The gradient of all of the film characteristic curves becomes shallow at film densities of less than 1.5, indicating that film contrast will be poor at low film densities. In view of this all relevant national standards stipulate a minimum film density of about 2.0 for industrial radiography. 9.1.1.3 BASE FOG LEVEL (AFFECTS FILM CONTRAST) National standards generally limit the base fog level of unexposed radiographic film to 0.3. If the base fog level exceeds this value film contrast can be quite severely affected. Fog level can be checked by processing a sample of the unexposed film. Figure 35 demonstrates how the base fog level affects film contrast.
Figure 35. Effect of film fogging on the film characteristic curve (The dotted lines show the average gradient between a film density of 1.5 and a film density of 2.5 for film having a base fog level of 0.1 and 0.5 respectively. The average gradient with a base fog level of 0.1 is about 3.6 while that for a base fog level of 0.5 is about 2.7. This decrease in average gradient is indicative of a reduction in film contrast.)
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The Welding Institute 9.1.1.4 FILM PROCESSING (AFFECTS FILM CONTRAST) Radiographic film should always be processed in accordance with the manufacturer’s recommendations. Any deviation from these will result in a lowering of film contrast and hence sensitivity. The film attains about 80% of it’s final density in the first 30 seconds of development. During the remaining 3½ to 5½ minutes of the standard development time radiographic developers are designed to increase film contrast. The developer works more vigorously in areas where a lot of metallic silver has already been released than in areas where the converse is true. Thus film contrast gradually improves during the final minutes of the development process. This is why radiographs which have been ‘pulled’ – intentionally underdeveloped in an effort to produce acceptable film density, invariably show poor film contrast. If the film is allowed to remain in the developer for too long, however, the developer will begin to attack all areas of the film and contrast will begin to suffer. All radiographic developers are designed to for use at the processing temperature specified by the developer manufacturer. Development time can to some extent be increased to compensate for a lower developer temperature or reduced if the temperature is above the optimum, but this will invariably be at the cost of reduced film contrast. In order to maintain the developer in good condition it must be replenished. Developer which has not been properly replenished quickly leads to low contrast low quality radiographs. 9.1.1.5 RADIATION QUALITY (AFFECTS SUBJECT CONTRAST) As the photon energy of the incident beam of radiation increases the subject contrast produced by the same change in component thickness of decreases. This is because higher energy radiation is less absorbed as it passes through a given thickness of the same material than is lower energy radiation. It is useful to talk about different radiation energy in terms of its ‘half value layer’. The half value layer can be defined as the thickness of any particular material which will reduce the intensity of the incident radiation by a factor of 2. The thickness of the half value layer for any material increases with increasing radiation energy. Examples of half value layers for various materials and radiation energies are given in table 7. Nature of incident radiation Iridium 192 γ-rays Cobalt 60 γ-rays 100 kV x-rays 150 kV x-rays 200 kV x-rays 300 kV x-rays
Half value layer (steel) / mm 13 22 2.4 4.5 6 8
Half value layer (aluminium) / mm 35 70 15 18.5 21 25
Half value layer (lead) / mm 4.8 12.5 0.1 0.3 0.65 1.6
Table 7. Half value layers (Note that as half value layer decreases subject contrast increases.)
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The Welding Institute 9.1.1.6 SCATTER (AFFECTS FILM AND SUBJECT CONTRAST) As ionising radiation passes through any material it undergoes a process known as scattering. Scattering occurs due to various mechanisms (the principal cause of scattering varies with the radiation energy) all of which occur due to the way in which radiation photons interact with atoms. When an x-ray photon strikes an atom it will cause the atom to lose one or more electrons, so that the affected atom becomes positively charged. Such electrically charged atoms are normally referred to as ions. Ions , by their nature are not stable, they will try to attract electrons into their empty energy shells in order to achieve a zero electrical charge. As electrons are captured from free space by ions they give up part of their kinetic energy as a photon of radiation. These photons will radiate in all directions from the affected atoms. Such radiation is known as scattered radiation. Scattered radiation can lead to an overall ‘fogging’ of the film emulsion. This reduces film contrast by effectively shifting the film characteristic curve upwards on the y-axis of the graph (the affect is the same as excessive base fog level, see figure 35 above). Scatter can severely reduce subject contrast by reducing the differences in radiation intensity reaching the film from various parts of the component under test. Scattering mechanisms and methods of controlling scattered radiation are discussed in a later section. 9.1.2 DEFINITION ‘Definition’ is a measure of the ‘sharpness’ of the images on the radiograph. Definition can be defined as the width of the boundary between two areas of different density on a film. The opposite of definition is ‘unsharpness’. A graphic illustration of unsharpness is given by figure 36. The total unsharpness on a radiograph is due to three factors: Geometric unsharpness (also called penumbra or penumbral shadow) inherent unsharpness (also called film unsharpness) and relative movement during exposure.
These are described and discussed below.
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Figure 36. Unsharpness (The unsharpness of the boundary between light and dark increases from left to right)
9.1.2.1 GEOMETRIC UNSHARPNESS A major factor affecting definition on a radiograph is geometric unsharpness. This can be defined as the width of the ‘penumbra’. Penumbra is the word used in physics to describe the lack of sharpness at the edge of a shadow. ‘Umbra’ means full shadow and penumbra means half shadow. So long as the source of radiation is not a true point source (and it never is a true point source) there will be a penumbral area at the edge of any shadow. A radiographic image is basically just a shadow. The focal spot of an x-ray tube and a radioactive isotope always have finite physical dimensions so a penumbra is always produced. Once the achieved penumbra falls below about 0.2 mm the unaided human eye ceases to perceive any further improvement in definition. For very high quality radiographic techniques geometric unsharpness is therefore generally kept to a value of less than 0.2 mm. To achieve this the object to film distance is kept short and the radiation source to film distance is made as long as necessary depending upon the radiation source dimensions.
Figure 37. Geometric Unsharpness 9.1.2.2 INHERENT UNSHARPNESS TWI Training & Examination Services – NDT 2/20 (Rev 3 05/08)
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Inherent unsharpness depends on three factors: the type film, the type of intensifying screen and the quality (or photon energy) of the radiation. 9.1.2.2.1 FILM (EFFECT ON INHERENT UNSHARPNESS) The emulsion of any radiographic film is made up of silver halide grains. As already mentioned the size of these grains is varied in order to control the film speed. Each grain of silver halide needs to interact with as few as a single photon of ionising radiation in order to become ‘sensitised’. Grains of silver halide which have become sensitised are very rapidly reduced to silver metal on contact with the reducing agents contained in film developer. Obviously if the grains of silver halide are large they will tend to cause a blurring effect on the radiographic image. A similar effect occurs on a computer screen. At a resolution of 640x480 the image quality is quite poor. This is like a coarse grain film. At a resolution of 1024x768 the image quality is considerably better because the pixel size is much smaller. This is like a fine grain film. Each silver halide grain in a fine grain film is around 1 μm in size. To give an idea of just how small this is, the pixel size on a computer screen at a resolution of 1024X768 is in excess of 40 μm. This is one of the reasons why digital imaging systems have found it so hard to replace film radiography. 9.1.2.2.2 QUALITY OF RADIATION (EFFECT ON INHERENT UNSHARPNESS) When ionising radiation passes through a material (including radiographic film) it is scattered. Scattering processes involve the emission of electrons, so called ‘secondary electrons’. As the radiation energy of the primary beam increases the kinetic energy of the secondary electrons released also increases. As the velocity of the secondary electrons increases they become capable of penetrating an ever increasing thickness of the surrounding material. The reason why lead screens produce no intensification effect below 120 keV is that the below this primary beam energy the secondary electrons released in the screens have insufficient energy to penetrate the supercoat of the film. It is now known that most silver halide grains in a direct type radiographic film are not sensitised directly by the penetrating radiation itself. They are for the most part sensitised by the secondary electrons released by the intensifying screens and by secondary electrons generated within the film emulsion itself. The greater the distance the secondary electrons are able to travel within the emulsion the greater the resulting unsharpness. At very high radiation energies (exceeding 1.02 MeV) a scattering process called pair production begins to predominate (pair production is more fully described in section 8.2.1.3). Pair production releases high energy electron positron pairs. The positron released is annihilated as it meets with a free electron. This produces a burst of x-rays with a characteristic photon energy of 0.51 MeV. This so-called annihilation radiation greatly reduces the sharpness of the image. The bremstrahlung released as the electron half of the pair collides with neighbouring shell electrons further reduces image sharpness.
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Figure 38. Scattered radiation
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9.2.1 SCATTERING MECHANISMS – THE CAUSES OF SCATTER There are three scattering mechanisms which are of particular importance in radiography. These are the photoelectric effect, Compton or incoherent scattering and pair production. The predominant scattering mechanism depends on the photon energy of the primary beam. Several other scattering mechanisms are possible, including Rayleigh or coherent scattering, but these are of little importance in industrial radiography. 9.2.1.1 THE PHOTOELECTRIC EFFECT The predominant scattering mechanism below 0.6 MeV is the photoelectric effect. In the photoelectric effect all of the energy of the incident x or gamma ray photon is transferred to an orbital electron. The absorbing electron is in most cases ejected from the atom and ionisation occurs. In a few cases where the energy of the incident photon is correct the absorbing electron merely jumps from an inner energy shell to an outer energy shell. As this electron at some later stage falls back into its original energy state a photon of x-rays is emitted. The energy of this photon is a characteristic of the scattering atom. Characteristic radiation emission can be used to perform chemical analysis. Atoms which have been ionised emit x-rays as they capture an electron from free space and the ejected electrons emit x-rays as they collide with and interact with the atoms in their path.
Figure 39. Photoelectric effect ( An incident X-ray photon, energy E0 collides with an outer shell electron which is ejected from the atom with energy E0 – Eb where Eb is the binding energy)
9.2.1.2 COMPTON SCATTERING (INCOHERENT SCATTERING) The predominant scattering mechanism above 0.6 MeV and up to 6 MeV is Compton scattering. In Compton scattering only part of the energy of the incident x or gamma ray photon is transferred to an orbital electron. The absorbing electron is ejected from the atom and ionisation occurs. The remaining photon energy continues as a lower energy x-ray although slightly deflected from its original path. Atoms which have been ionised emit x-rays as they capture an electron from free space and the ejected electrons emit x-rays as they collide with and interact with the atoms in their path.
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Figure 40. Compton scattering
9.2.1.3 PAIR PRODUCTION The predominant scattering mechanism above 6 MeV is pair production. In pair production the incident x or gamma ray photon collides either with the nucleus or with an inner shell electron. The incident photon then converts to an electron - positron pair. A positron is a particle having the same size and mass as an electron but opposite electrical charge. Pair production cannot occur below a threshold photon energy of 1.02 MeV. The electron – positron pair is ejected at high velocity but the positron has a very short life. It quickly meets a free electron and annihilation occurs – the positron and electron cease to exist and 2 photons of 0.51 MeV radiation are emitted. The ejected electron emits x-rays as it collides with and interacts with the atoms in its path.
Figure 41. Pair production
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Figure 42. Total scatter versus radiation energy (schematic only)
Figure 42 shows how scatter is a greater problem at low incident radiation energy. Scatter as a percentage of the total radiation is at a minimum at around 2 MeV, however, as radiation energy increases through the threshold photon energy of 1.02 MeV pair production within the film emulsion begins to increase inherent unsharpness resulting in poor image quality. Usually a decrease in incident radiation energy would be expected to produce improved subject contrast and reduced inherent unsharpness. There is a limit to this, however, where the increased scatter associated with low incident radiation energy begins to counteract the beneficial effect of reducing the incident radiation energy and a decrease (rather than an increase) in image contrast may be the overall result. 9.2.2 TYPES OF SCATTER Several types of scatter cause problems in radiography, these being side scatter, back scatter and internal scatter (self scatter). The angle formed between the direction of travel of the primary beam and the direction of travel of the scattered radiation (reaching the film) is called the scattering angle or angle of scatter. Side scatter and internal scatter have an angle of scatter which is less than or equal to 90° while for back scatter the angle exceeds 90°. 9.2.2.1 SIDE SCATTER Radiation may be scattered by parts of the object that are not within the diagnostic area of the radiograph or by the walls of the exposure room. This is termed ‘side scatter’. This type of scatter can be reduced by collimating the beam such that only the area to be examined is subjected to the primary beam and by the use of lead masking, diaphragms or grids. In x-radiography the use of a filter may help to reduce side scatter.
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9.2.2.2 BACK SCATTER Back scatter is caused by the primary beam striking an object behind the film and scattering back. This type of scatter can easily be reduced by shielding the back of the film cassette with a sheet of lead. A sheet of lead approximately 2 mm thick is adequate for most applications. In x-radiography the use of a filter may help to reduce back scatter. The presence of excessive back scatter may be detected by placing a lead letter ‘B’ on the back surface of the cassette (i.e. the cassette surface furthest from the source of radiation). If there is excessive back scatter then a light image of the letter ‘B’ will be seen on the developed film. The use of a lead letter ‘B’ is mandatory when working in accordance with the ASME code and is required for each new technique by BS EN 1435 (i.e. not for production radiography). In accordance with BS EN 1435 the lead letter ‘B’ shall be a minimum of 10 mm high and 1.5 mm thick. Note: If a dark image of the letter ‘B’ appears this is not an indication of excessive back scatter. It merely indicates scatter caused by the letter ‘B’ itself. Should back scatter be detected then the thickness of the lead sheet shielding the back of the film cassette must be increased. 9.2.2.3 SELF-SCATTER Self-scatter is scattered radiation originating from within the test component itself. The detrimental effect of self-scatter on film quality can be reduced by the use of lead intensifying screens placed in contact with the film and, in x-radiography, by the use of filters. If the radiation source is an x-ray tube then the use of a copper filter can help to reduce the effects of this type of scatter. A copper filter significantly reduces the proportion of low energy radiation within the primary beam. Since it is the low energy radiation which is chiefly responsible for scatter the use of such a filter can reduce the overall amount of scatter occurring and in this way improve image quality. Filters made from lead, steel or other metals may be used in a similar way. Metallic foil intensifying screens made from lead or other metals reduce the effects of self scatter for both x-ray and gamma ray radiography. They filter out the low energy scattered radiation and prevent it from reaching the film.
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Figure 43. Using a diaphragm
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Figure 44. Radiographs produced with & without blocking 9.2.3.4 GRIDS The use of a grid is generally limited to medical radiography. A grid consists of a matrix of parallel metal bars which is set in oscillation during exposure such that the grid itself does not produce a radiographic image. The use grid is a very effective method of reducing the effects of side scatter, but grids are very rarely a practical option for industrial situations. In order to be effective the grid must be placed as close as possible to the film. In microfocus x-radiography it may be placed between the film and the object. 9.2.3.5 FILTERS Figure 42 above shows how the percentage of scattered radiation is high when the radiation energy is low. Placing a thin sheet (typically 1 to 2 mm) of copper or other metal in the primary beam, close to the source of radiation, greatly reduces the amount of low energy radiation while permitting most of the higher energy radiation to pass through. If there is less low energy radiation there will be less scatter, although it is possible that film contrast will be reduced. The use of a filter to reduce scatter is limited to x-radiography because gamma ray sources do not produce long wavelength low energy radiation.
Figure 45. Oscillating grid
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Figure 46. The effect of filtration on a typical x-ray spectrum 9.2.3.6 METALLIC FOIL SCREENS Lead screens, or screens made from other metals such as steel or copper are a very effective means of reducing scatter, for both x and gamma radiography, particularly when the energy of the primary beam exceeds 120 keV. Such screens are placed in contact with the film. The front screen works like a filter, greatly reducing the proportion of low energy radiation which reaches the film. Scattered radiation is always lower energy than the primary beam, so the scatter is more affected by the filtration effect than is the primary beam. The back screen reduces back scattered radiation which reaches the film. In addition to this both screens intensify the effect of radiation, the energy of which, exceeds 120 keV. The screens do this by producing secondary electrons to which the film emulsion is sensitive. Most of the radiation exceeding 120 keV will be part of the primary beam. Thus the effect of the primary beam is amplified at the expense of the unwanted scattered radiation.
Figure 47. The effect of lead screens 9.2.3.7 HIGHER RADIATION ENERGY Up to a radiation energy of around 1.5 MeV increasing the maximum radiation energy of the primary beam will reduce scatter (in proportion to the primary beam) and improve image quality. At higher radiation energy there is a continued decrease in the proportion of scattered radiation, but pair production within the film emulsion begins to have an increasingly detrimental effect on film unsharpness. See figure 42 above.
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9.2.3.8 CHANGE FROM X-RAY TO GAMMA RAY RADIOGRAPHY The absence of low energy components in radiation obtained from radioactive materials such as Iridium 192 or Cobalt 60 is the reason why gamma ray radiography is much less affected by scatter than is x-ray radiography. 9.2.3.9 REDUCING THE FOCUS OR SOURCE TO FILM DISTANCE Reducing focus or source to film distance reduces the amount of matter through which the penetrating radiation is passing, in so doing reducing the amount of scatter. The use of a source to film distance that will produce a value of geometric unsharpness which is less than the inherent unsharpness for the film – screen combination in use is therefore not recommended. 9.4 DETERMINING THE CORRECT EXPOSURE: EXPOSURE CHARTS ‘Exposure’ in film radiography can be defined as the amount of radiation striking or passing through the film. The amount of radiation striking the film is equal to the product of the radiation intensity* at the film and the exposure time. Exposure is critical, because underexposed films will show low film density and therefore reduced contrast, while overexposed films which exceed a film density of about 4.0 cannot usually be satisfactorily viewed using the unaided human eye and standard film illuminators. (* Note that in film radiography it is better to think of intensity as photons per square metre rather than energy per square metre. This is because radiographic film has a fairly flat response to changes in photon energy right across the full spectrum of x and gamma rays. In terms of film density 1 photon of 150 keV radiation has much the same effect as 1 photon of 20 MeV radiation.)
In x-radiography the number of photons produced per unit time is directly proportional to the tube current. Therefore it is usual to express x-ray exposures in milliampere-minutes (mA-min); the product of the exposure time and the tube current. In gamma radiography the number of photons produced per unit time is directly proportional to the source activity. Source activity is usually measured in curies (Ci) or, less commonly in gigabecquerels (GBq). Therefore gamma ray exposures are usually expressed in curie-minutes (Ci-min) or curie-hours (Ci-h) but may also be expressed in gigabecquerelminutes (GBq-min) or gigabecquerel-hours (GBq-h). The factors listed in table 8 will affect either the film speed or the amount of radiation reaching the film and have to be taken into account when determining the correct exposure for film radiography. In addition to these factors the required film density obviously has an impact upon the required exposure time (see table 8).
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Factor Film type Intensifying screen type Film processing Radiation energy
Factors affecting film speed (table 8 part 1) Comments Coarse grained films are fast & require a short exposure while fine grained films are slow & require a long exposure. The affect of intensifying screens varies with the incident radiation energy. Metallic foil intensifying screens reduce the required exposure time by a factor of about 2 or 3 at radiation energies of 120 keV or more. Fluorometallic screens reduce the exposure time by a factor of about 50 with films designed for use with such screens. Salt screens can reduce the required exposure time by a factor of 500 but are seldom used for industrial radiography. Developer type and concentration together with the development temperature can affect the film speed. Automatic processing usually gives a slight increase in film speed when compared with manual processing. Fairly minor compared with other factors. Can affect the efficiency of the intensifying screens, and to a lesser extent the film speed.
Factors affecting the intensity of radiation reaching the film (table 8 part 2) Factor Comments The amount of radiation absorbed by a material increases with increasing density and atomic number. Material type The amount of radiation absorbed by an object rises exponentially with increasing material thickness. Material thickness - μt I = I0e
Radiation energy Radiation energy distribution
Source or focus to film distance Filters Source strength or tube current
Factor Film Density
Where I0 = the intensity of the incident radiation; I = the intensity of the transmitted radiation; μ = the coefficient of linear absorption; t = the penetrated thickness The amount of radiation absorbed by a material decreases with increasing radiation energy. For x-ray techniques the amount of radiation which will be absorbed for a given radiation energy can not be calculated - this has to be measured experimentally. Exposure times for self-rectified x-ray equipment are always longer than those for constant potential equipment operating at the same peak tube voltage. The radiation produced by constant potential equipment is said to be ‘harder’ because it contains proportionally more high energy radiation. In fact radiation energy distribution can vary quite markedly even between different types self-rectified or constant potential unit. X-ray exposure charts therefore are applicable to only a single equipment type. The amount of radiation reaching the film is proportional to the reciprocal of the square of the source to film distance. As the required exposure time is inversely proportional to the radiation intensity: E
∝
al
D2
Where E is the exposure time at distance D. If the half value layer of the material from which the filter is made is known and its thickness is known then it is possible to compensate the exposure for the insertion of or removal of a filter. Radiation intensity is proportional to the tube current for x-rays and to the source activity for gamma rays.
Film Density (table 8 part 3) Comments The required exposure is strongly dependent upon the required film density. Film characteristic curves can be used to compensate for a change in the required film density.
Table 8. Factors affecting radiographic exposure (film radiography) 9.4.1 EXPOSURE CHARTS ‘Exposure charts’ provide a convenient means of estimating radiographic exposures for both x-ray and gamma ray techniques. All exposure charts are correct only for a fixed set of conditions - all of the factors mentioned in table 8 are fixed for any particular chart. Exposure charts for x-ray equipment are usually applicable only to a single type of
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Figure 48. Exposure chart for Andrex 140 kV SR directional
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Figure 49. Exposure chart for Philips 300 kV SR directional
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Figure 50. Exposure chart for Pantak 200 kV CP directional
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Figure 51. Exposure chart for Iridium 192
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Figure 52. Butt weld in 15 mm plate 9.4.1.1 USING EXPOSURE CHARTS (X-RAY) 9.4.1.1.1 FOCUS TO FILM DISTANCE Two factors affect the choice of focus to film distance, geometric unsharpness (Ug) requirements and the desired diagnostic film length (DFL). BS EN 1435 does not directly specify geometric unsharpness, but controls it by specifying a minimum value of focus to film distance for a given effective focus size and object to film distance. See figures 53 and 54 below. Taking the example given in figure 52 above a let’s suppose that we wish to achieve a geometric unsharpness of 0.25 mm or better. This would be sufficient to satisfy the requirements of most national codes or standards including BS EN 1435 class A.
but the OFD can be taken as being equal to the material thickness, in this case 18 mm, and the FFD is equal to the FOD plus the material thickness so:
If we choose to use the Pantak 200 CP the effective focus size on the broad focus setting is about 4 mm, so:
+ 18
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Note that this diagram is for an effective source size of 3mm. The minimum SFD for other source sizes can be found as follows: 1.
2.
Find the minimum SFD for the desired image class from the figure, let this distance = d Minimum SFD for source size = f is then equal to:
Figure 53. BS EN 1435 minimum SFD / FFD Note that in BS EN 1435 ‘standard techniques’ are ‘class A’, while ‘enhanced techniques’ are ‘class B’.
Figure 54. BS EN 1435 geometric unsharpness at minimum SFD / FFD So a focus to film distance of 306 mm will achieve the required value of geometric unsharpness. However if we use this minimum FFD the diagnostic film length (DFL) will be rather short due to fade off. BS EN 1435 has a requirement for class B techniques that the penetrated thickness (based on the nominal thickness) at the ends of the DFL shall not exceed 110% of the nominal thickness. For class A the requirement is that the penetrated thickness shall not exceed 120% of the nominal thickness at the end of the DFL. This translates to a DFL that is approximately 0.9 x FFD for class B and 1.3 x FFD for class A. In this case let’s 72
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The Welding Institute apply the BS EN 1435 class A requirement. If we want to achieve a DFL of 450 mm the minimum FFD based on this will be 450/1.3 = 346. For convenience we can round this up to say 400 mm. 9.4.1.1.2 TUBE VOLTAGE After deciding on an appropriate value for focus to film distance the next thing to consider when determining an exposure time for an x-ray technique is what tube voltage will be appropriate. Some national codes and standards specify the maximum tube voltage which may be used for a given thickness of material. See figure 17 above for BS EN 1435 requirements, the maximum tube voltage in our case would be about 250 kV. However, common good practice, which is to choose a tube voltage which will produce an exposure time of between 1 and 5 minutes at around 75-100% of the maximum tube current, will in nearly all cases satisfy such codes and standards. The exposure chart for the Pantak 200 CP is drawn for a source to film distance of 914 mm. The maximum tube current is 14 mA. Let’s use 10 mA. We would like to achieve an exposure time of between 1 and 5 minutes, giving an exposure of between 10 and 50 mAmins. The focus to film distance that we wish to use is 400 mm. The exposure chart has been constructed using FFD = 914. Using the inverse square law (see figure 55) we can see that an exposure of 10 to 50 mA-mins at 400 mm FFD is equivalent to an exposure at 914 mm FFD of between 10 x 9142 / 4002 = 52.2 mA-mins and 50 x 9142 / 4002 = 261 mA-mins. Looking at the exposure chart (see figure 50) a density of 2.2 will be achieved using Kodak CX with an exposure of about 150 mA-mins at 120 kV or with an exposure of about 40 mA-mins at 140 kV. These values are for an FFD of 914 mm. The equivalent exposures at an FFD of 400 mm will be: 150 x 4002 / 9142 = 29 mA-mins at 120 kV or 40 x 4002 / 9142 = 7.66 mA-mins at 140 kV. Thus for Kodak CX and a film density of 2.2 these exposures should work: 120 kV , 10 mA, 2 minutes 54 seconds or 140 kV, 6 mA, 1 minute 16 seconds The 120 kV exposure should produce the best film contrast while the 140 kV exposure will be more economic.
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Figure 55. The inverse square law Intensity = number of photons per square metre. At 2D the same number of photons that passed through one square at D now passes through 4 squares. Thus the intensity at 2D is one quarter of what it was at D. Intensity is proportional to 1/(distance)2. Radiographic exposure time is proportional to 1/(intensity), thus exposure time is proportional to (distance)2. The inverse square law can be stated as follows:
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The Welding Institute 9.4.1.1.3 CHANGING THE FILM DENSITY If for example a film density of 2.2 was thought to be too light, then the film characteristic curve (see figure 33) can be used to find the correct amount of exposure compensation. For example, let’s say that we wanted to achieve a film density of 2.5. Using figure 33 logarithm (relative exposure) for a film density of 2.2 on CX = 1.3 while for a film density of 2.5 logarithm (relative exposure) = 1.38 1.38 – 1.3 = 0.08 antilogarithm (0.08) = 100.08 = 1.2 So at 120 kV the exposure required = 1.2 x 29 = 35 mA-mins while at 140 kV it would be 1.2 x 7.66 = 9.2 mA-mins Thus if we use Kodak CX film the following exposures should achieve a film density of 2.5: 120 kV , 10 mA, 3 minutes 30 seconds or 140 kV, 6 mA, 1 minute 32 seconds 9.4.1.1.4 CHANGING THE FILM TYPE Sometimes we may wish to change to a different type of film. For example if the required radiographic sensitivity could not be achieved using Kodak CX film we might consider using MX film which should produce better contrast and consequently better sensitivity. This can be achieved using the film characteristic curves, but it is more convenient to use ‘film factors.’ Table 9 lists film factors for some common direct type x-ray films. FILM MANUFACTURER Kodak
Agfa
Fuji
Dupont (Cronex)
FILM TYPE CX AX MX D7 D5 D4 150 100 80 75 65 55
FILM FACTOR 2.50 3.75 10.00 3.50 5.50 13.00 1.90 3.10 6.50 2.00 4.00 8.00
Table 9. Film factors Note that the film factors given in table 7 are approximately correct for radiation energy in the range 0.1 to 1.0 MeV. Film factors can vary with radiation energy.
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The Welding Institute Suppose that we wish to change from film ‘A’ to film ‘B’
Thus to change from CX to MX we divide by the film factor for CX (= 2.50) and multiply the result by the film factor for MX (= 10.0). Therefore using MX film, for a film density of 2.5 our exposures become 140 mA-mins at 120 kV or 37 mA-mins at 140 kV. The exposure at 120 kV is now rather long (14 minutes at 10mA) so an exposure using 140 kV of around 3 minutes at 12.1 mA would be preferred. 9.4.1.1.5 RADIOGRAPHY OF OTHER MATERIALS All of the exposure charts in the figures above have been constructed for steel. If radiography has to be carried out on materials other than steel then the exposure time will have to be adjusted to compensate for the difference in radiation absorption. This can be done using ‘half value layers.’ The half value layer of a given material for a given incident radiation energy being the thickness of the material which reduces the intensity of the incident radiation by a factor of 2. However the simplest way is to use ‘equivalence factors’, some examples of which are listed in table 10. Material Steel Copper Aluminium Al alloy 4.5% Cu
Titanium
100 keV 1.0 1.5 0.08 0.13 0.5
Radiation Energy / Isotope 150 keV 220 keV 400 keV 1.0 1.0 1.0 1.6 1.4 1.4 0.12 0.18 0.16 0.22 0.45 0.35 -
Ir192 1.0 1.1 0.35 0.35 -
Table 10. Equivalence factors Going back to the example, if we needed to radiograph a weld dimensionally similar but made in a copper based alloy rather than steel then the first thing to do would be to work out the ‘steel equivalent thickness.’ This is equal to the actual thickness of copper divided by the copper equivalence factor and multiplied by the equivalence factor for steel. Stated generally this is:
From the table it can be seen that at 140 kV the copper equivalence factor is probably about 1.55, while that for steel is 1.0. So 18 mm copper is radiographically equivalent to 18 ÷ 1.0 x 1.55 = 28 mm steel.
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The Welding Institute Looking at figure 50 it can be seen that an exposure of about 205 mA-mins would be required using the Pantak 200 CP to achieve a film density of 2.2 at a FFD of 914 mm using CX film on 18 mm thick copper (28 mm steel equivalent). 9.4.1.1.6 COMPENSATING FOR THE USE OF A FILTER If problems arise with scattered radiation one possibility in x-radiography is to use a filter. Typically filters made from 1 or 2 mm thick copper sheet are used. Filters made from other metals such as lead may also be used. The difference which the use of a filter will make to the exposure time for the steel weld in the example can be calculated using the equivalence factors in table 10. Suppose that we wish to radiograph our 18 mm thick steel weld using the Pantak 200 at 140 kV with a focus to film distance of 400 mm. The exposure required for a film density of 2.5 without a filter was calculated above as 9.2 mA-mins if using CX film. The equivalence factor for copper at 140 kV is about 0.64. Therefore 1.0 mm of copper will be radiographically equivalent to 1.0 ÷ 0.64 x 1.0 = 1.6 mm of steel. To find the correct exposure (for a copper filter thickness of 1 mm) we simply need to add this amount to the steel thickness which is being radiographed : 18 + 1.6 = 19.6 mm Figure 50 gives an exposure of about 50 mA-mins for 19.6 mm of steel at a focus to film distance of 914 mm, which is an increase of 25% compared to the exposure which was required under the same conditions without the filter. Thus the exposure required with a FFD of 400 mm using CX film for a film density of 2.5 increases by 25% from 9.2 to 11.5 mAmins. 9.4.1.1.7 OTHER POSSIBLE CHANGES The exposure charts given in figures 48 to 50 were made using lead screens, 0.125 mm thick and automatic film processing. If either or both of these two factors is changed then the only way to establish the correct exposure will be by experimentation, although the charts will still act as a guide. The exposure charts in figures 48 to 50 are fixed for a particular type of x-ray equipment. They cannot be used to predict exposures for any other type of x-ray equipment. 9.4.1.2 GAMMA RAY EXPOSURES The method used to establish gamma ray exposures from an exposure chart is in every way similar to that used for x-rays except that the possibility to change the radiation energy has been removed. Before proceeding to carry out gamma radiography it will be necessary to establish the source activity at the time of exposure. This is done by reference to the decay chart supplied by the source manufacturer. Other convenient means of establishing gamma ray exposures are to use a specially designed slide rule or a programmable calculator. Slide rules can be obtained from some film manufacturers and from organisations such as ‘SCRATA.’ For the mathematically minded it
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The Welding Institute is a relatively simple task to program a calculator such that it can be used to predict gamma ray exposures. 10.0 SENSITIVITY 10.1 RADIOGRAPHIC SENSITIVITY Radiographic sensitivity can be defined as the ability of a radiographic system to reveal small changes in section thickness. It may also be defined as the ability of a radiographic technique to reveal the smallest possible flaw. True radiographic sensitivity is a difficult quantity to measure. 10.2 CONTROLLING RADIOGRAPHIC QUALITY Prior to interpretation of a radiograph it is necessary to establish adequacy of the radiographic technique used. National codes and standards describe devices known as ‘Image Quality Indicators’ (IQIs). Occasionally the word ‘Penetrameter’ is used when referring to the IQI. It is very important to realise IQI sensitivity is not a direct measure of radiographic sensitivity per se. Good IQI sensitivity does not necessarily indicate good radiographic sensitivity, but it does to some extent prove the quality of the radiographic technique in a general sense. These days the type of IQI most commonly in use is the wire type but other types exist, two examples being the plaque type and the step hole type. 10.3 BS EN 462-1 WIRE TYPE IQIs BS EN 462-1 wire type IQIs each consist of 7 wires taken from a list of 19 wires. Four standard wire groupings are available, designation ‘1’, wires 1 to 7, designation ‘6’, wires 6 to 12, designation ‘10’, wires 10 to 16 and designation ‘13’, wires 13 to 19. Each of these groupings is available in any of 4 types of material; steel, designated ‘FE’, copper, designated ‘CU’, aluminium, designated ‘AL’ and titanium, designation ‘TI’.
Figure 56. EN 462-1 wire type IQIs
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Designation W1 W2 W3 W4 W5 W6 W7 W8 W9 W10 W11 W12 W13 W14 W15 W16 W17 W18 W19
Diameter 3.2 2.5 2.0 1.6 1.25 1.0 0.8 0.63 0.5 0.4 0.32 0.25 0.2 0.16 0.125 0.1 0.08 0.063 0.05
Table 11. BS EN 462-1 wire diameters Note that it is fairly easy to remember the wire diameters: if you can remember the diameters of the first three, 3.2, 2.5 and 2.0 mm you can arrive at all other wire diameters by halving as shown below in figure 57.
Figure 57. Remembering the EN 462-1 wire diameters Looking along each row the wire diameters are successively halved, e.g. 3.2, 1.6, 0.8…..
The EN 462-1 material groupings are as follows: the FE designated IQIs are made from low alloy steel and cover all ferrous materials; the CU designated IQIs are made from copper and cover copper, tin, zinc and their alloys; the AL designated IQIs are made from aluminium and cover aluminium and its alloys; the TI designated IQIs are made from titanium and cover titanium and its alloys. Special IQIs can be used for materials lying outside these 4 groups, or the contracting parties could agree to use one of the four normal designations.
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The Welding Institute 10.4 OTHER WIRE TYPE IQIs Several other wire type IQIs are in common usage. Amongst these are the BS 3971, the DIN 54 109 and the ASTM E 747. The BS3971 series consisted of 21 wires where wire number 21 was the thickest at 3.2 mm and wire number 1 was the thinnest at 0.032 mm, the order of numbering was basically the reverse of that used by EN 462-1 with wire number 11 being the same diameter in both series (0.32 mm).
Figure 58. BS 3971 wire diameters The DIN 54 109 series consisted of 16 wires corresponding exactly to the first 16 wires in the EN 462-1 series. The ASTM E 747 series consists of 21 wires ranging from 0.08 mm to 8.1 mm in diameter; there are 4 overlapping groups of 6 wires, each designated by a letter (A, B, C or D), see table 10 below, and a large number of material groupings each designated by a number with ferrous being ‘1’. IQI Designation A B C D
WIRE DIAMETERS 0.08 0.25 0.81 2.5
0.1 0.33 1.0 3.2
0.13 0.4 1.27 4.0
0.16 0.5 1.6 5.1
0.2 0.63 2.0 6.3
0.25 0.81 2.5 8.1
Table 12. ASTM E 747 wire diameters 10.5 BS EN 462-2 STEP–HOLE TYPE IQIs BS EN 462-2 IQIs consist of stepped blocks of material with each step having a through drilled hole or pair of through drilled holes. Step thicknesses of 0.8 mm or less have two drilled holes, while the thicker steps have a single hole. In each case the step thickness and the hole diameter are equal.
Figure 59. BS EN 462-2 step hole IQIs (These IQIs are supplied encased in plastic complete with lead number identification similar to that used in EN 462-1 wire type IQIs)
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H1 X X X X X X
IQI DESIGNATION H5 H9
X X X X X X
X X X X X X
H13
X X X X X X
Hole / Step Number H1 H2 H3 H4 H5 H6 H7 H8 H9 H10 H11 H12 H13 H14 H15 H16 H17 H18
Hole dia. / step t’ness 0.125 0.16 0.2 0.25 0.32 0.4 0.5 0.63 0.8 1.0 1.25 1.6 2.0 2.5 3.2 4.0 5.0 6.3
Table 13. BS EN 462-2 Step-hole IQIs 10.6 ASTM E 1025 PLAQUE TYPE PENETRAMETERS ASTM E 1025 describes plaque type ‘penetrameters’ (penetrameter is just another word meaning IQI). When using this type of IQI the required sensitivity is typically specified as ‘2-2T’, ‘1-2T’ or perhaps ‘2-4T’. The number ‘2’ or ‘1’ indicates that the IQI thickness is 2% or 1% the thickness of the component under test. Where the component is a weld the reinforcement should be taken into consideration when choosing the IQI. ‘2T’ or ‘4T’ indicates the diameter of the drilled hole that must be clearly visible in the radiographic image if the radiograph is to be considered acceptable. The 2T hole has a diameter equal to 2 times the plaque thickness, the diameter of the 1T hole is equal to the plaque thickness while that of the 4T hole is 4 times the plaque thickness. Penetrameters up to 160 thousandths of an inch thick are rectangular and contain 1T, 2T and 4T holes. Thicker penetrameters are circular and contain 1T and 2T holes. Each rectangular penetrameter carries lead markers indicating its thickness in thousandths of an inch. Each circular penetrameter is identified by lead markers placed alongside which indicate its thickness in thousandths of an inch. A total of 8 material groups are identified by adding notches to the edges of the penetrameter.
Figure 60. ASTM E 1025 IQIs 10.7 IQI SENSITIVITY TWI Training & Examination Services – NDT 2/20 (Rev 3 05/08)
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IQI sensitivity is usually defined as the thickness of the thinnest wire, plaque or step which is visible on the radiograph expressed as a percentage of the specimen thickness. Exactly what is meant by specimen thickness varies from standard to standard and from technique to technique. BS EN 1435 contains tables of ‘essential wires’ for class A and class B techniques for IQI placed source or film side. ASME V article 2 also permits the use of wire type IQIs and takes a similar ‘essential wire’ approach. BS EN 1435 bases its requirements on nominal thickness while ASME V article 2 bases its requirements on the actual weld throat thickness. It used to be common good practice to place the IQI in the least favourable position within the diagnostic area of the radiograph. This would usually have meant placing the IQI upon the source side of the specimen and towards the extremities of the diagnostic area because this is where the contrast and definition would tend to be at their least favourable (highest value of geometric unsharpness and lowest film density). Nowadays, when performing radiography of a weld in accordance with BS EN 1435 the wire type IQI must be placed preferably source side, possibly film side, in an area of uniform film density. This usually means on the parent material and at the centre of the area of interest. The wires may or may not be visible in the image of the weld for double wall single image (DWSI) or single wall single image (SWSI) radiography but they shall be placed at 90° to the weld axis and at least a 10 mm length of wire shall appear on the parent material in an area of uniform film density. For double wall double image (DWDI) radiography the wires shall not be visible in the image of the weld. The IQI shall be placed with its wires parallel to the weld axis on the parent material adjacent to the weld. The requirement for visible wire length remains unchanged. In the past it was not uncommon for national codes or standards to specify an overall requirement for a radiographic sensitivity of 2% or better. This was easy to achieve on thicker sections but often impossible to achieve on thinner sections of material. Modern radiographic standards take account of the fact that the best achievable sensitivity for a given situation and technique is not a fixed quantity but a variable which depends upon such factors as the type of radiation source, the technique and the thickness of the specimen. Such standards specify a minimum sensitivity which should be achievable using a good quality radiographic technique. One such standard is BS EN 1435. Table 14 below gives some BS EN 1435 requirements for single wall single image (SWSI) radiography with source side IQI placement, double wall single image (DWSI) radiography with film side IQI placement and double wall double image (DWDI) radiography with source side IQI placement.
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CLASS ‘A’ RADIOGRAPHY 1. Single Wall Technique Source Side IQI Thickness Required Wire Average wire diameter Sensitivity
CLASS ‘B’ RADIOGRAPHY 1. Single Wall Technique Source Side IQI Thickness Required Wire Average wire diameter Sensitivity
≤ 1.2 > 1.2 ≤ 2 > 2 ≤ 3.5 > 3.5 ≤ 5 >5≤7 > 7 ≤ 12 > 12 ≤ 18 > 18 ≤ 30 > 30 ≤ 40 > 40 ≤ 50 > 50 ≤ 60 > 65 ≤ 85 > 85 ≤ 120 > 120 ≤ 220 > 220 ≤ 380 > 380
≤ 1.5 > 1.5 ≤ 2.5 > 2.5 ≤ 4 >4≤6 >6≤8 > 8 ≤ 12 > 12 ≤ 20 > 20 ≤ 30 > 30 ≤ 35 > 35 ≤ 45 > 45 ≤ 65 > 65 ≤ 120 > 120 ≤ 200 > 200 ≤ 350 > 350
18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3
0.063 0.08 0.1 0.125 0.16 0.2 0.25 0.32 0.4 0.5 0.63 0.8 1.0 1.25 1.6 2.0
2. DWSI Technique Film Side IQI Thickness Required Wire wire diameter ≤ 1.2 > 1.2 ≤ 2 > 2 ≤ 3.5 > 3.5 ≤ 5 > 5 ≤ 10 > 10 ≤ 15 > 15 ≤ 22 > 22 ≤ 38 > 38 ≤ 48 > 48 ≤ 60 > 60 ≤ 85 > 85 ≤ 125 > 125 ≤ 225 > 225 ≤ 375 > 375
18 17 16 15 14 13 12 11 10 9 8 7 6 5 4
0.063 0.08 0.1 0.125 0.16 0.2 0.25 0.32 0.4 0.5 0.63 0.8 1.0 1.25 1.6
3. DWDI Technique Source Side IQI Thickness Required Wire wire diameter ≤ 1.2 > 1.2 ≤ 2 > 2 ≤ 3.5 > 3.5 ≤ 5 >5≤7 > 7 ≤ 12 > 12 ≤ 18 > 18 ≤ 30 > 30 ≤ 40 > 40 ≤ 50 > 50 ≤ 60 > 65 ≤ 85 > 85 ≤ 120 > 120 ≤ 220 > 220 ≤ 380 > 380
18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3
0.063 0.08 0.1 0.125 0.16 0.2 0.25 0.32 0.4 0.5 0.63 0.8 1.0 1.25 1.6 2.0
> 5.25% 5% 3.64% 2.94% 2.67% 2.1% 1.67% 1.33% 1.14% 1.11% 1.14% 1.07% 0.98% 0.74% 0.53% < 0.53%
Average Sensitivity > 5.25% 5% 3.64% 2.94% 2.13% 1.6% 1.35% 1.07% 0.93% 0.93% 0.93% 0.76% 0.57% 0.42% < 0.43%
Average Sensitivity > 5.25% 5% 3.64% 2.94% 2.67% 2.1% 1.67% 1.33% 1.14% 1.11% 1.14% 1.07% 0.98% 0.74% 0.53% < 0.53%
19 18 17 16 15 14 13 12 11 10 9 8 7 6 5
0.05 0.063 0.08 0.1 0.125 0.16 0.2 0.25 0.32 0.4 0.5 0.63 0.8 1.0 1.25
2. DWSI Technique Film Side IQI Thickness Required Wire wire diameter ≤ 1.5 > 1.5 ≤ 2.5 > 2.5 ≤ 4 >4≤6 > 6 ≤ 12 > 12 ≤ 18 > 18 ≤ 30 > 30 ≤ 45 > 45 ≤ 55 > 55 ≤ 70 > 70 ≤ 100 > 100 ≤ 180 > 180 ≤ 300 > 300
19 18 17 16 15 14 13 12 11 10 9 8 7 6
0.05 0.063 0.08 0.1 0.125 0.16 0.2 0.25 0.32 0.4 0.5 0.63 0.8 1.0
3. DWDI Technique Source Side IQI Thickness Required Wire wire diameter ≤ 1.5 > 1.5 ≤ 2.5 > 2.5 ≤ 4 >4≤6 >6≤8 > 8 ≤ 15 > 15 ≤ 25 > 25 ≤ 38 > 38 ≤ 45 > 45 ≤ 55 > 55 ≤ 70 > 70 ≤ 100 > 100 ≤ 170 > 170 ≤ 250 > 250
19 18 17 16 15 14 13 12 11 10 9 8 7 6 5
0.05 0.063 0.08 0.1 0.125 0.16 0.2 0.25 0.32 0.4 0.5 0.63 0.8 1.0 1.25
> 3.33% 3.15% 2.46% 2.0% 1.79% 1.6% 1.25% 1.0% 0.98% 1.0% 0.91% 0.68% 0.5% 0.36% < 0.36%
Average Sensitivity > 3.33% 3.15% 2.46% 2.0% 1.56% 1.07% 0.83% 0.67% 0.64% 0.64% 0.59% 0.45% 0.33% < 0.33%
Average Sensitivity > 3.33% 3.15% 2.46% 2.0% 1.79% 1.39% 1.0% 0.79% 0.77% 0.8% 0.8% 0.74% 0.59% 0.48% < 0.5%
Table 14. BS EN 1435 Sensitivity requirements for wire type IQIs.
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The Welding Institute 11.0 RADIOGRAPHIC TECHNIQUES (FOR WELDS IN PLATE AND PIPE) Three basic techniques are used for the radiography of butt-welds in pipe, these being the ‘single wall single image (SWSI)’, ‘double wall single image (DWSI)’ and ‘double wall double image DWDI’ techniques. All radiographs of butt-welds in plate will in general be single wall single image. Each of these techniques is discussed below, paying particular respect to the extent of the diagnostic area, the minimum source to film distance, the placement of location markers and the placement of IQIs. 11.1 IQI TYPE AND PLACEMENT It is important that IQIs are placed source or film side and at a position within the diagnostic film length (DFL) in accordance with the requirements of the contract specification. As a general rule, wherever possible, the IQI should be placed source side. IQIs placed source side are affected both by radiographic contrast and by geometric unsharpness. Film side IQIs indicate radiographic contrast only, thus source side IQIs give a more accurate measure of the overall radiographic quality. It used to be standard good practice to place wire type IQIs towards the end of the diagnostic area, with the thinner wires toward the outside of the DFL; the wires were invariably placed across the weld and sensitivity was assessed on the weld allowing for any weld reinforcement present. This way of working would still meet ASME V article 2 requirements, although this document does not specify where within the DFL the IQI should be placed. In Europe matters are different. When working in accordance with BS EN 1435 sensitivity should generally be assessed at the centre of the DFL on the parent material. Plaque type and step hole type IQIs should (preferably) always be placed at the centre of the diagnostic area on the parent material. Should the image of these IQI types encroach on the weld area the radiograph should be re-taken. If working with a wire type IQI in accordance with ASME V article 2 sensitivity would probably be measured on the weld. ASME V article 2 then has a requirement that the film density through the diagnostic length shall not vary by more than + 30% or – 15% from that measured at the IQI. The same allowable density variation applies to plaque type IQIs, but these, of course, must be placed alongside, not on the weld. Plaque type IQIs may be shimmed to compensate for any weld reinforcement. If a technique produces a wide range of film density the placement of several IQIs may be necessary in order to meet the allowable density variation requirement. BS EN limits the diagnostic film length (DFL) by specifying that the penetrated thickness at the ends of the DFL shall not exceed 110% (class ‘B’ techniques) or 120% (class ‘A’ techniques) of the thickness penetrated at the centre of the DFL.
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The Welding Institute 11.2 LOCATION MARKERS All national codes and standards require the use of location markers, usually in the form of lead letters or numbers that appear in the radiograph as a radiographic image. It is very important that the markers are placed in such a way as to prove coverage of the weld where a multiple exposure technique is used. Three general rules apply: (1) When performing radiography of welds in flat plate location markers must be placed source side. Film side markers will not prove coverage because of ‘parallax.’ (2) When performing radiography of welds in curved surfaces location markers should be placed on the convex surface for all techniques where the source or focus to film distance is equal to or exceeds the radius of curvature. (3) When performing radiography of welds in curved surfaces location markers should be placed source side for all techniques where the source or focus to film distance is less than the radius of curvature.
Figure 61. Location marker placement – parallax effect on flat plate
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The Welding Institute 11.3 IDENTIFICATION OF RADIOGRAPHS All national codes and standards require unique and permanent identification of radiographs. In general this can be applied by any suitable means although in some cases identification using lead numbers that appear as radiographic images is required. Where not prohibited by the contract specification ‘flashing’ the radiographic identification is a good method. The required identification is written on a scrap of white paper, the radiograph is suitably masked and the scrap of paper is placed on the unmasked area. The radiograph is then flashed with a suitable light source and the identification becomes visible during subsequent film processing. Exactly what constitutes an acceptable unique identification varies widely from specification to specification, but the minimum is a unique number. ASME V article 2 requires a unique weld number, the date and the manufacturer’s name or symbol. Most codes require radiographs of repair welds to be marked with R1, R2, R3 etc depending on the number of repair attempts. RW is commonly used to identify a complete re-weld. Items such as heat treatment condition, welder number and welding procedure reference may also be required. 11.4 RADIATION ENERGY BS EN 1435 specifies the maximum x-ray tube voltage which may be used based on the component thickness (see figure 17 above). BS EN 1435 also specifies the minimum and maximum thickness on which each type of gamma ray isotope may be used (see table 15 below). ASME V article 2 specifies a recommended minimum steel thickness for iridium 192 of 19 mm and a minimum of 38 mm for cobalt 60, a minimum of 63 mm of aluminium is specified for iridium 192. RADIATION SOURCE Thulium 170 Ytterbium 169 (1) Selenium 75 (2) Iridium 192 Cobalt 60 X-ray equipment, 1 MeV to 4 MeV X-ray equipment, 4 MeV to 12 MeV X-ray equipment, 12 MeV and above
PENETRATED THICKNESS (w) Test Class A Test Class B w≤ 5 w≤5 1 ≤ w ≤ 15 2 ≤ w ≤ 12 10 ≤ w ≤ 40 14 ≤ w ≤ 40 20 ≤ w ≤ 100 20 ≤ w ≤ 90 40 ≤ w ≤ 200 60 ≤ w ≤ 150 30 ≤ w ≤ 200 50 ≤ w ≤ 180 w ≥ 50 w ≥ 80 w ≥ 80 w ≥ 100
(1) For aluminium & titanium, the penetrated thickness is 10 ≤ w ≤ 70 for class A and 25 ≤ w ≤ 55 for class B. (2) For aluminium & titanium, the penetrated thickness is 35 ≤ w ≤ 120 for class A.
Table 15. BS EN 1435 applicable thickness ranges for gamma ray sources and high energy x-rays 11.5 SOURCE TO FILM DISTANCE The minimum source to film distance for BS EN 1435 is calculated using the formula: f/d ≤ kb2/3 where f is the source to object distance, d is the effective source or focus size, b is the object to film distance and k is a constant equal to 7.5 for class A techniques and
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The Welding Institute 15 for class B. BS EN 1435 also includes a nomogram for the less mathematically minded. Figure 53 above shows the BS EN 1435 requirements graphically. ASME V article 2 limits the minimum source or focus to film distance by specifying maximum geometric unsharpness, 0.51 mm for component thickness up to 50.8 mm, 0.76 mm for component thickness greater than 50.8 & up to 76.2 mm, 1.0 mm for component thickness greater than 76.2 and up to 101.6 mm and 1.78 mm for component thickness exceeding 101.6mm 11.6 SWSI TECHNIQUES 11.6.1 SINGLE WALL SINGLE IMAGE TECHNIQUE FOR PLATE
Figure 62. BS EN 1435 SWSI technique for flat plate Figure 62 above shows a typical set-up for exposure of a butt weld in flat plate. The captions refer to BS EN 1435 requirements. The source should be positioned on the centre line of the weld, directly above the centre of the diagnostic area.
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The Welding Institute 11.6.2 SINGLE WALL SINGLE IMAGE TECHNIQUE: SOURCE INTERNAL, PLACED CENTRALLY (PANORAMIC TECHNIQUE)
Figure 63. Single wall single image ‘panoramic’ technique (BS EN 1435) Required number of exposures = 1 (see figure 68 below for BS EN 1435 requirements)
This technique is commonly used for pipeline welds where specially designed, remotely operated, devices known as crawlers are often used. These machines can travel distances of up to several kilometres along the inside of the pipeline in order to reach the desired position to radiograph a particular weld. The typical battery life for an x-ray crawler will usually allow about 100 exposures to be made between successive battery charges. Gamma ray crawlers are also used. This technique may also be used for examining girth welds in cylindrical pressure vessels. Using Thulium 170 isotopes boiler tube welds which may have an outside diameter of only 40 mm are occasionally examined by this technique. The major advantage of this technique is that it can radiograph an entire girth weld in a single exposure. With this technique location marker placement is not critical, but it usually more convenient to place the markers film side. In most cases it will be impractical to place the IQI source side for this technique, although source side IQIs would be preferred if access is not a problem. Film side IQIs are therefore generally used. Comparitor radiographs having IQIs placed source and film side can be used to establish sensitivity requirements for film side IQIs. In most cases three IQIs are placed at 120° intervals around the circumference, although some specifications require more or less than this. The radiograph may consist of a number of overlapping films or it may be a single length of ‘rollpack’ film. Identification of the film may be included as a radiographic image but it may also be added later. Where several overlapping films are used each film must be uniquely and permanently identified.
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The Welding Institute 11.6.3 SINGLE WALL SINGLE IMAGE TECHNIQUE: SOURCE INTERNAL, OFFSET.
Figure 64. Single wall single image source internal & offset technique (BS EN 1435) Required number of exposures: see figure 68 below for BS EN 1435 requirements
In some cases it may not be possible to satisfy the requirements of the applicable specification for geometric unsharpness if the panoramic technique is used. Where this is the case it may be possible to achieve a satisfactory geometric unsharpness by offsetting the source towards the inner wall of the pipe. Location markers should be placed film side if the SFD or FFD is longer than the radius of curvature of the test item. If the converse of this is true (as may be the case for a large diameter pressure vessel) then the location markers should be placed source side. 11.6.4 SINGLE WALL SINGLE IMAGE TECHNIQUE: FILM INSIDE, SOURCE OUTSIDE
Figure 65. Single wall single image film inside, source outside (BS EN 1435) Required number of exposures: see figure 66
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Figure 66. BS EN 1435: Exposures required for FISO techniques FISO = film inside source outside
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The Welding Institute 11.6.4 (continued) SINGLE WALL SINGLE IMAGE TECHNIQUE: FILM INSIDE, SOURCE OUTSIDE This is a rather unpopular technique because a large number of exposures (usually 8 or more) are required in order to cover the entire circumference of the weld. In general it will only be used when an acceptable radiograph cannot be achieved using either of the two single wall techniques described in 10.6.2 and 10.6.3 above and can also not be achieved using the double wall techniques described below. Location markers MUST be placed source side. The IQI should always be placed source side, there is no excuse for using a film side IQI when using this technique. Identification of the films may be included as radiographic images (although it will probably impractical to use long identifications due to the limited amount of area available on the film) but may also be added later. 11.7 DOUBLE WALL SINGLE IMAGE Where there is no access to the inside of a pipe double wall techniques have to be employed. In the double wall single image technique the source of radiation is usually placed at the minimum possible distance from the film. The reason for this is that as the source to film distance increases so does the number of exposures needed to cover the entire circumference of the weld. In addition, any improvement in image quality due to the reduced geometric unsharpness associated with an increase in SFD or FFD has to be offset against a reduction in image quality due to increased scatter. Geometric unsharpness limitations permitting gamma sources can be placed almost in contact with the outside surface of the pipe. In many cases this reduces the required number of exposures to just three (see figure 68 below for BS EN 1435 requirements). X-ray tubes are bulky and the minimum achievable FFD will usually be about 125 mm plus the outside diameter of the pipe. A minimum of 4 exposures per weld is therefore required when using an x-ray source for this technique. Being able to place the source of radiation in close contact with the pipe gives gamma ray techniques another significant advantage over x-ray techniques particularly on smaller pipe diameters. Less offset is needed with gamma ray sources in order to ensure that the image of the source side portion of the weld is not superimposed upon the film side part of the weld. This can increase the chance of finding vertical defects such as lack of root fusion in the weld being radiographed. As the wall thickness to diameter ratio increases the double wall single image technique becomes increasingly difficult to apply, the number of exposures required increases and the quality of the radiographs produced diminishes. For these reasons doublewall-double-image (superimposed) techniques tend to be preferred for heavy wall small diameter pipes. Because there will in general be no access to the inside of the pipe when this technique is employed the location markers and IQI are always placed film side.
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The Welding Institute 11.7 (continued) DOUBLE WALL SINGLE IMAGE
Figure 67. Double wall single image technique Required number of exposures: see figure 68
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Figure 68. BS EN 1435: Exposures required for DWSI & SWSI SIFO techniques SIFO = source inside film outside
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The Welding Institute 11.8.1 DOUBLE WALL DOUBLE IMAGE (ELLIPTICAL)
Figure 69. Double wall double image technique (elliptical)
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The Welding Institute 11.8.1 (continued) DOUBLE WALL DOUBLE IMAGE (ELLIPTICAL) In accordance with BS EN 1435 this technique is limited to girth welds in pipe having an outside diameter of less than 100 mm. In accordance with ASME V article 2 welds in pipe of up to 3½ inch nominal diameter ( OD about 101.9 mm, this is slightly outside the BS EN 1435 requirement) may be radiographed using DWDI. For pipes with a wall thickness to outside diameter ratio in excess of about 0.15 the double-wall-double-image (superimposed) technique is to be preferred. The minimum number of exposures required by both BS EN and American standards is two at 90° to each other. Long source to film distances are needed because the minimum value of object to film distance is equal to the outside diameter of the pipe. Exposure times for this technique therefore tend to be rather long especially in the case of gamma ray techniques. A single location marker on each exposure is generally sufficient, although some specifications require pitch markers (A to B, B to C, C to D and D to A etc). Location markers may be placed source side or film side. IQIs should always be placed source side. BS EN 1435 requires wire type IQI s to be placed on the parent material with their wires parallel to the weld axis (see figure 69). Special BS EN 462-1 having wires just 10 mm long are available for this purpose. Working in accordance with ASME V article 2 standard wire type IQIs should be placed with their wires across the weld at 90° to the weld axis. In the double wall single image technique the film is wrapped around the pipe so as to remain as close as possible to the weld. Conversely, in the double wall double image technique the film should be kept as flat as possible (see figure 69). 11.8.2 DOUBLE WALL DOUBLE IMAGE (SUPERIMPOSED) This technique has the same range of application as the elliptical technique, but is preferred when the thickness to outside diameter ratio exceeds 0.15. Welds having difficult geometry that may prevent them from being radiographed using the elliptical technique can generally be radiographed successfully using this technique. As the image of the source side part of the weld is superimposed on the image the film side part of the weld it is often not possible to accurately locate a weld defect when using this technique. This is not usually much of a handicap because small diameter welds tend to be cut-out and re-welded rather than being repaired locally. A single location marker per exposure is usually sufficient when using this technique and it may be placed either source or film side. IQIs should always be placed source side. BS EN 1435 and ASME V article 2 both require a minimum of 3 exposures at 120° to spacing (or 3 at 60° spacing for difficult access situations) for this technique.
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The Welding Institute The double wall double image superimposed technique may be more likely (than the elliptical technique) to successfully detect lack of root fusion due to the more favourable angle of incidence of the primary beam.
Figure 70. Double wall double image technique (superimposed)
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The Welding Institute 12.0 INTERPRETATION OF RADIOGRAPHS 11.1 INTRODUCTION Competent interpretation of radiographs is a skill which can only be gained through long experience. This chapter is intended to give the reader a guide to radiographic interpretation and should be regarded as a base upon which to build. The interpretation of a radiograph should not be confused with the acceptance or rejection of a component. The radiograph must first be interpreted and any defects observed assessed against applicable standard. A weld or casting must be accepted on its merits or rejected for its faults and should neither be accepted nor rejected due to difficulties encountered in the interpretation of radiographs. Any radiograph not meeting code requirements with regard to radiographic quality must be rejected. In circumstances where there is doubt as to the nature of a radiographic image it is often necessary to visually inspect the component or to cross check the radiographic results using another NDT method. 12.2 VIEWING CONDITIONS The success or failure of radiographic interpretation is highly dependent upon the film viewing conditions. The eye is very sensitive to small variations in film density once it has developed ‘night vision’. Anyone carrying out radiographic interpretation should therefore not begin to view radiographs until ‘night vision’ has developed. Since this cannot be achieved in a brightly lit room it is important that the films are viewed in low ambient light. ‘Night vision’ takes several minutes to develop and so the films should not be viewed immediately upon entering the viewing room. Five minutes is the recommended period that should elapse before critical interpretations are made. It is also important that film is properly masked on the viewer so that the light falling on the eye comes from the radiograph only. If the film is not adequately masked the eye will be blinded by the bright light coming from around the film. Radiographs are easily damaged, therefore the viewing room must be clean and dry and the radiographs must be handled with care. The viewer should be mounted on a table or bench large enough to allow the films to be spread out without the danger of them falling to the floor. A well shielded reading lamp will allow reports to be read or notes to be made, without unduly increasing the overall ambient lighting. The radiographs should be viewed at a normal reading distance ( normally less than 400 mm). A low power magnifier (2 or 3X) may occasionally be helpful, but it should not be necessary for routine examination. In accordance with PCN and CSWIP requirements the visual acuity of the radiographic interpreter must be J1 in at least one eye (corrected or uncorrected). The viewing of radiographs is often undertaken in the dark room where the film was processed. This is satisfactory provided that the viewing bench or table is clean and well away from the processing tanks. Under normal circumstances films should NEVER be viewed whilst wet. There are two reasons for this:
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The Welding Institute (a) The film emulsion is swollen with water and the images are not as clear as when the film is dry. (b) The emulsion is very delicate and any attempt to mask the film will result in scratches or marks on the film, effectively ruining it. National standards generally require that the illuminance of a radiographic film viewer be sufficient to produce a transmitted light intensity of at least 30 and preferably 300 candela per square metre (cd/m2). This means that a viewer suitable for viewing radiographic film with a density of 3.0 must have an illuminance of at least 30,000 cd/m2 with as much as 300,000 cd/m2 being desirable. BS EN 25580 requirements for radiographic film viewers are given by table 16 below. Note that these are minimum requirements. Film Density 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Minimum Screen luminance (cd/m2) 300 1,000 3,000 10,000 10,000 30,000 100,000 300,000
Transmitted light Illuminance (cd/m2) 30.0 31.6 30.0 31.6 10.0 9.5 10.0 9.5
Table 16. BS EN 25580 requirements for radiographic film viewers 12.3 REPORTING The initial interpretation of a radiograph should always be undertaken by the manufacturer or his designated representative. Other interested parties should be presented with a report which includes an interpretation of each film. It is their job to check this and to agree or disagree with it. The radiographic report should contain the following information as a minimum: (i) (ii) (iii) (iv)
Identification of the item radiographed. The date of manufacture. The date of radiography. Exposure details including the type of equipment used and the tube voltage for xray techniques and the type of isotope for gamma ray techniques. (v) The type of film used. (vi) The type and thickness of the intensifying screens used. (vii) Geometric details, particularly the FFD or SFD and the effective focus or source dimension. (viii) Details of the component being radiographed, including the type of material and method of manufacture, the thickness, the heat treatment condition and the repair status. (ix) The method of film processing. (x) The film density achieved. (xi) The radiographic sensitivity achieved.
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The Welding Institute (xii) The technician’s name, signature and date. (xiii) The interpreter’s name, signature and date. (xiv) An interpretation of each film and a statement of the component’s acceptability or not. (xv) The code or standard applicable to the radiographic technique. (xvi) The acceptance code or standard. (xvii) Reference to a written procedure or technique sheet. 12.4 FILM QUALITY The success of radiographic interpretation is dependent upon the quality of the film presented. If the film does not meet the minimum applicable standards for quality then it should be rejected and reshot. The manufacturer’s interpreter may, for economic reasons, not be inclined to reject radiographs which do not meet the minimum quality standards. Therefore any third party viewing the radiographs should be extremely careful to correctly assess the quality of the radiographs prior to endorsing the relevant report. Otherwise the third party will be open to criticism should the film become the subject of any subsequent legal inquiry. When assessing a film for quality a number of items must be considered. These are discussed below. 12.4.1 COMPONENT IDENTIFICATION All radiographs must be permanently and uniquely marked with sufficient information so as to permit their identification with the component radiographed at a later stage. It is often useful to include such items as the date of test and heat treatment or repair status of the component in the identification. Radiographic identification could appear on the radiograph as a radiographic image but there is usually no reason why it should not be added by any other suitable means. A written procedure should be in force describing the standard method to be used for identifying radiographs. 12.4.2 LOCATION MARKERS Location markers on a radiograph serve two functions: they permit the radiograph to be identified with the area of the component radiographed and they serve to prove that the component has been fully covered by the technique used. Refer to the sections above on radiographic techniques for details. Wherever possible location markers should permanently identify the radiograph with the area radiographed. Items such as pressure vessels are usually hard stamped with a permanent radiographic datum. A written procedure should be in force which describes the standard method used for the placement of location markers. 12.4.3 FILM DENSITY It is important that the film density is within the specified range since a film having low film density will also have inferior film contrast. BS EN 1435 requires a minimum film density of 2.0 for class A radiography and a minimum of 2.3 for class B. ASME V Article 2 requires a minimum of 1.8 for x-ray techniques and minimum of 2.0 for gamma ray techniques. In most cases (including BS EN 1435 and ASME V article 2) the minimum figures for film density apply to the area of interest (the diagnostic area) on the radiograph. In weld radiography, for example, film density should generally be measured on the weld
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The Welding Institute area between the location markers (which identify the ends of the diagnostic film length). Density can be assessed either by comparison with a calibrated density strip (which preferably should have been made using film identical to that of the radiograph) or by using a measuring device known as a densitometer. Anyone accepting radiographs which do not meet the applicable density requirements is open to criticism at a later stage should litigation follow a component failure. ASME V article 2 requires that the film density within the area of interest must not vary by more than minus 15% or plus 30% from the value measured through the body of the IQI. If necessary additional IQIs can be used in order to satisfy this requirement for exceptional areas. Occasionally an upper limit is specified for film density. ASME V article 2, for example, specifies an upper limit of 4.0. 12.4.4 RADIOGRAPHIC SENSITIVITY Radiographic sensitivity is not directly related to the minimum detectable defect size. However, a radiograph that meets the applicable code requirement for radiographic sensitivity, is much more likely to provide good defect sensitivity than a radiograph which fails to meet the code requirements. The sensitivity of a radiograph depends upon the parameters chosen to produce that radiograph (see the section above on the production of a radiograph). If any of the relevant parameters are altered the sensitivity will be affected. It is therefore essential to use Image Quality Indicators (IQIs) in order to prove that adequate radiographic quality has been attained. Except in the case of the ‘panoramic technique’, which has been described above, at least one IQI should generally appear each radiograph. Anyone viewing radiographs should be careful to check that the radiographic sensitivity meets the requirements of the applicable code. Anyone who fails to do is open to criticism should litigation follow a component failure. 12.4.5 ARTEFACTS AND OTHER UNWANTED IMAGES In film radiography an artefact can be defined as ‘any image resulting from a cause that is not directly associated with the object that has been radiographed. Artefacts can be produced by mechanical or chemical damage to the film and by damaged or dirty intensifying screens. Sometimes radiographic images may be formed by things such as debris on the internal of a pipe. These images, while they are strictly speaking not artefacts, can also interfere with the proper interpretation of the radiograph. When radiographs are being produced on a commercial basis it is not possible for every film to be free from all artefacts. An artefact only becomes significant when it cannot be identified as being an artefact or when it hinders the interpretation of the film. These two factors are rather subjective but if any doubt exists then the interpreter should call for a repeat radiograph. A list of possible artefacts is given in the next section.
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12.5 INTERPRETATION OF RADIOGRAPHIC IMAGES There are three types of image which may appear on a radiograph: (i) (ii) (iii)
Images due to artefacts. Images due to surface irregularities in the component. Images due to internal discontinuities in the component.
Every image within the diagnostic area of a radiograph must be identified as one of these three. It is not permissible to reject a component simply because an image appearing within the diagnostic area cannot be interpreted. Similarly it is not permissible to reject a radiograph for artefacts which are not within the diagnostic area. The following sections attempt to give a description the various types of image which may be seen on a radiograph. The ability to successfully identify all radiographic images is a skill which can only be perfected with time and experience. 12.6 ARTEFACTS 12.6.1 PRESSURE MARKS (CRIMP MARKS) These are produced by careless film handling. If the film is crimped or buckled either before or after exposure crescent shaped images in the processed radiograph will result. Light marks indicate crimping before exposure. Dark marks indicate crimping after exposure but before film processing. It is usually possible to identify crimp marks by viewing the film in reflected light. They should appear as indentations in the surface of the film. Lead screens which have been crimped should be discarded. 12.6.2 SCRATCHES: ON THE FILM Radiographic film emulsion is delicate, it is easily damaged if handled carelessly at any stage during the production of a radiograph. Areas used for film handling must be free from dust and films must be handled carefully at all times. Depending upon how severe and when or how formed film scratches may produce either light or dark images. Film scratches can usually be identified using reflected light. 12.6.3 SCRATCHES: ON LEAD INTENSIFYING SCREENS These may appear as either light or dark images which cannot be seen in reflected light. If the intensifying screens used to make the radiograph can be positively identified then it may be possible to trace the shape and position of such an image to a scratch on the screens. Even if this can be done it will probably be necessary to reshoot the radiograph. Scratched lead screens should be discarded.
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The Welding Institute 12.6.4 DIRT: ON THE FILM OR SCREENS Dirt which finds it’s way between the film and the screens will in general produce a light image on the resultant radiograph which is not visible in reflected light. Greasy fingers will produce dark marks on a finished radiograph which can easily be seen in reflected light. Greasy fingers before development produce light marks. 12.6.5 STREAKINESS OR MOTTLING: POOR DEVELOPMENT This is usually caused by insufficient agitation in the early stages of development and is due a process known as ‘bromide streaming’. Reaction products from the chemical interaction of the developer with the silver halides in the film emulsion tend to build up around high film density zones. These reaction products slow down the action of the developer. Since they are relatively heavy they will tend to flow down the surface of the film leading to a light coloured streak in the finished radiograph. Under or over development usually leads to a mottled effect on the finished radiograph. A similar effect will be produced by developer which has passed the end of its service life. In less severe cases such artefacts may not be a cause for rejection of the radiograph but darkroom procedures should be reviewed in order to prevent a recurrence or a further deterioration in radiographic quality. 12.6.6 DEVELOPER SPLASHES These will appear as dark spots on the film and indicate poor dark room practice. Such marks are usually visible in reflected light. 12.6.7 FIXER SPLASHES These will appear as light spots on the film and again indicate poor dark room practice. Such marks are usually visible in reflected light. 12.6.8 WATER SPLASHES These may appear as either light or dark images on a radiograph. Water splashes before exposure tend to cause light marks. Water splashes after exposure tend to cause dark marks. Such marks are usually visible in reflected light. 12.6.9 WATER MARKS These are easily seen on the radiograph in both transmitted and reflected light and are due to uneven drying. They commonly occur where a dry or partially dry film is wetted locally either by splashing or by excess water running down from a film clip. The appearance of water marks can be reduced or eliminated by the use of a squeegee to remove excess water or by the use of a final wash that contains a small amount of detergent (i.e. a wetting agent). 12.6.10 AIR BELLS
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The Welding Institute These are light marks caused by air bubbles adhering to the film in the early stages of development. They will not occur if the film is properly agitated. 12.6.11 DIFFRACTION MOTTLING This can be a problem when x-rays are used to radiograph large grained material, one example being cast austenitic steels. Diffraction is an apparent bending of a beam of radiation that is due to ‘interference.’ Diffraction occurs when radiation passes through a grating that has a spacing approximately equal to one wavelength. The spacing of atoms in a metallic crystal is about 0.1 nanometres. This corresponds to x-ray radiation with a photon energy in the region of 10 keV. If low energy components are removed from the x-ray beam by filtration the problem with diffraction mottling will disappear. Diffraction mottling does not occur in gamma radiography because of the absence of low energy beam components. Diffraction can be used to advantage. It is the basis for the study of metal crystals by x-ray crystallography. 12.6.12 STATIC MARKS Penetrating radiation is by definition ‘ionising’. It always causes the build up of an electric charge on the film during exposure but under normal circumstances this is not a problem because the charge quickly flows to earth. In dry climates, however, a static charge may remain on the film up to the point where it is unloaded in the darkroom, whereupon it flows to earth suddenly in a manner which could be painful for the radiographer. Such a sudden dissipation of electrical energy leads to the emission of a sudden burst of light. This light produces dark tree-like marks on the finished radiograph. Static marks can be avoided by careful film handling. 12.6.13 DICHROIC FOGGING Radiographs affected by dichroic fog will appear reddish when viewed using transmitted light and greenish in reflected light. Dichroic means two-coloured. This artefact is caused when the development process continues during the fixing process. It happens when the fixer solution has become insufficiently acidic to stop the development process. The use of an acidic stop bath between the development and fixing processes will in general prevent the occurrence of this seldom seen artefact. 12.6.14 RETICULATION This artefact appears on the radiograph as an orange peel like mottling effect. It is caused when the film emulsion is subjected to a temperature shock at any stage during the film processing. A sudden change in temperature causes the film emulsion to wrinkle. It will not generally occur as long as the sudden change in temperature is less than 10°C. 12.6.15 FILM FOGGING BY X OR GAMMA RAYS If radiographic film is not stored well away from sources of ionising radiation then it is likely to become ‘fogged.’ Films which have been fogged in this way will produce reduced radiographic contrast (fogging has much the same effect as scattered radiation which
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The Welding Institute is explained in a section above). If it is suspected that the film is fogged then the fog level can be checked by processing a piece of unexposed film. Film which has a density due to fogging of 0.3 or more is not suitable for use in high quality industrial radiography. 12.6.16 LIGHT FOGGING Exposure to light other than that from darkroom safelights (actually prolonged exposure to safelights will cause also fogging) at any stage prior to fixing the film will cause the film to become fogged. Such fogging may be localised or general. Localised fogging is not a problem unless it encroaches onto the diagnostic film area. General fogging by light has the same effect as fogging due to exposure to ionising radiation. 12.6.17 FILM FOGGING DUE TO INADEQUATE STORAGE CONDITIONS Film stored at too high a temperature or which has been exposed to chemical fumes may become fogged. The fog level of all film increases with age, even under ideal storage conditions, therefore all film boxes are marked with an expiry date. High speed films deteriorate more quickly than do slower films. 12.6.18 SOLARISATION Image reversal due to extreme over exposure to x or gamma rays or caused by exposure to light during film development. 12.6.19 A FINAL WORD ON ARTEFACTS It should be stressed again that artefacts are cause for rejecting the film only if they interfere with interpretation. A large number of artefacts present on the radiographs indicates poor practice and the interpreter should take time to inspect the radiographic facilities and review darkroom procedures. 12.7 INTERPRETATION OF WELD RADIOGRAPHS 12.7.1 RADIOGRAPHIC INDICATIONS DUE TO SURFACE GEOMETRY It is usually possible to successfully interpret radiographs of welds in the as welded condition. Experience will help the interpreter to identify the sort of surface marks which are normal for a particular welding process and technique. Where there is doubt it a visual examination of the weld will often help. Where it is felt that an indication resulting from surface geometry could mask a significant defect indication, or where visual examination proves inconclusive, it may be necessary to dress the weld to a smooth contour and reshoot the radiograph. The severity of weld defects such as excessive penetration or undercutting is difficult to judge using radiographic evidence alone. Wherever possible defects of this type should be judged for acceptability by visual means. Listed below are some of the common surface conditions that can produce radiographic images.
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The Welding Institute 12.7.1.1 EXCESSIVE ROOT PENETRATION This is excess weld material protruding through the root of a single sided fusion weld. It appears in the radiograph as a continuous or intermittent light irregular band within the image of the weld. Common causes of excessive penetration are, no root face, root gap too wide, excessive amperage, travel speed too slow and incorrect polarity.
Figure 71. Excessive root penetration
12.7.1.2 ROOT CONCAVITY This takes the form of shallow groove which may occur in the root of a single sided weld. It appears in the radiograph as a series of dark areas along the centre of the weld varying in density according to the depth of imperfection. It is often seen in welds made with the use of a backing gas. The pressure of the backing gas can cause the weld root to collapse during welding of the first subsequent weld run (hot-pass). Other possible causes are no root face, travel speed too slow on the hot pass, amperage too high on the hot pass, incorrect polarity on the hot pass, excessive pre-heat and root gap too narrow.
Figure 72. Root concavity
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The Welding Institute 12.7.1.3 INCOMPLETELY FILLED GROOVE (LACK OF FILL) This is a continuous or intermittent channel along the edge of the weld, due to insufficient weld material. Incompletely filled groove is a fusion defect and should not be confused with lack of reinforcement or undercutting. It produces an image in the radiograph of a straight edged (on one side at least) dark band. Incompletely filled groove is caused by poor welding practice.
Figure 73. Incompletely filled groove or lack of fill 12.7.1.4 LACK OF REINFORCEMENT This is a concave area of the weld cap where the weld is locally thinner, sometimes thinner than the parent material. In the radiograph it appears as a dark area towards the centre of the weld which has diffuse edges. Lack of reinforcement is caused by poor welding practice.
Figure 74. Lack of reinforcement
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The Welding Institute 12.7.1.5 UNDERCUT This is an irregular groove at the toe the weld in the parent material due to burning away during welding. It appears in the radiograph as a dark / irregular /intermittent band in a position adjacent to either the cap or root weld toe or between adjacent capping runs. It may therefore appear inside or outside the weld image on the radiograph. The major causes of undercutting are excessive amperage and poor welding technique. Welds in the vertical or horizontal – vertical position tend to be prone to undercutting.
Figure 75. Undercut 12.7.1.6 SPATTER Spatter consists of globules of molten filler metal expelled during arc welding on to the surface of the parent material or weld. Spatter appears in the radiograph as small light spots. The major causes of spatter are incorrect polarity and welding current too high. Spatter particularly affects MIG, MAG, MMA and FCAW, spatter is highly unlikely to be seen in association with welds made by TIG or SAW. In pipe welding spatter is possible on both the external and internal surfaces.
Figure 76. Weld spatter
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The Welding Institute 12.7.1.7 EXCESSIVE DRESSING / GRINDING MARKS This is a reduction in material thickness caused by the removal of the surface of a weld and adjacent areas to below the surface of the parent material. Excessive dressing appears as a dark area with diffuse edges. A grinding mark appears as a dark area that will usually have clearly defined edges. Caused by poor practice or poor access for welding.
Figure 77. Excessive dressing 12.7.1.8 HAMMER MARKS (TOOL MARKS) These are indentations in the surface of the parent material or of the weld resulting from the application of a tool, for example a chipping hammer. They usually appear in a radiograph as dark half moon shaped areas usually having clearly defined edges. Caused by poor fabrication practice. They often result from attempts to correct welding distortion. 12.7.1.9 TORN SURFACE This is a surface irregularity due to breaking off of temporary attachments. The radiographic indication produced has a shape corresponding to that of the affected area which may be may be either light or dark depending on whether part of the attachment has remained or whether parent material has been torn away. Caused by poor fabrication practice, often seen in association with storage tank or ship hull welds. 12.7.1.10 SURFACE PITTING This is a surface imperfection, usually of the parent material but also the weld metal where a component has been in service. It usually takes the form of small depressions resulting from localised corrosion. Pitting appears in a radiograph as small dark rounded images. It is possible to mistake this for a welding defect, its appearance in the radiograph can be identical to that of porosity.
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The Welding Institute 12.7.2 INTERNAL DEFECTS 12.7.2.1 CRACKS In weld radiography four basic types of crack are sometimes detected by radiography. These are: (i) (ii) (iii) (iv)
Centreline cracks (shrinkage cracks). Transverse cracks (including chevron cracks). Heat affected zone cracks or toe cracks. Crater cracks.
A crack is a linear discontinuity produced by a fracture. In association with welding, cracks can occur at a time after the completion of welding, during the deposition of subsequent welding runs or at the point of solidification. Cracking can affect both the weld deposit and the parent material. Cracks are often invisible on radiographs but when they are detected they appear in the radiograph as dark, fine often branching lines which are usually diffuse or discontinuous. The ability of the radiographic technique to detect a crack is dependent on the crack’s orientation relative to the direction of the radiation. Figure 71 below shows how even a slight deviation from the optimum orientation will greatly reduce the change in section thickness which the radiation experiences due to a planar defect such as a crack. In the case shown a variation from optimum incidence of just ± 1° will reduce the change in penetrated thickness from 10 mm to 1 mm for a planar defect measuring 10 mm by 17 μm.
Figure 78. Detectability of planar defects
Centreline cracks (also called shrinkage or solidification cracks) Centreline cracks are caused by excessive restraint or the deposition of too large an amount of weld metal in a single pass. Too large an amount of weld metal can result from excessive amperage or travel speed too slow. Centreline cracks are possible for all arc welding methods. Centreline cracks occur at the point of solidification when the weld metal has a very low tensile strength. They are the welding equivalent of a hot tear. Of all the types of crack that can affect welds centreline cracks are probably the easiest to detect by
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The Welding Institute radiography. They tend to be much wider than other types of crack and because of this their detectability is less strongly affected by changes in the direction of the primary beam. Transverse cracks & chevron cracks Any crack that lies across the weld axis is called a transverse crack. Basically there are two distinct types of transverse crack. Both types usually occur when the compressive strength of the parent material is significantly greater than the tensile strength of the weld metal. The first type is a shrinkage or solidification crack. Cracks of this type usually occur at 90° to the weld axis, often affecting the root pass of single sided welds. In nature they are very similar to centreline cracks, but the source of restraint is different. They are relatively easy to detect by radiography. The second type is a chevron crack. This type of crack occurs at an angle of about 45° to the weld axis, usually at some time after the completion of welding. Chevron cracks are a special type of hydrogen induced crack; the stress that causes the crack being due to an excessive amount of dissolved hydrogen in the weld metal. They are sometimes detected by radiography, but in situations where there is a known problem, other NDT methods with a higher probability of detection should be used. Heat affected zone cracks & toe cracks Various mechanisms can lead to cracking in the heat affected zone of a weld. Heat affected zone cracks will often start at or run to the toe of the weld since there is always a high stress concentration at this point. In ferrous welds the hardest, most martensitic, most brittle microstructure is usually to be found in the heat affected zone. It is this ‘susceptible grain structure’ that makes the heat affected zone a prime site for cracking. Heat affected zone cracks are usually caused by one of two mechanisms. The first of these involves dissolved hydrogen. Molten iron has a very high solubility for hydrogen while solid iron has a very low solubility. Thus as the metal freezes hydrogen will attempt to leave solution and escape from the weld pool but this process is slow compared to the process of freezing, therefore most of the hydrogen becomes trapped in the solidified metal. The trapped hydrogen then diffuses through the metal crystals and begins to build up an internal pressure at points of weakness, usually the grain boundaries. In some cases the internal pressure exceeds the strength of the material and hydrogen cracking occurs. Hydrogen induced cracking may occur at any time up to 48 hours after welding. Where ferrous materials operate in a hydrogen rich environment, for example in sour gas service, hydrogen cracking can occur as an in-service defect. High strength, high carbon equivalent steels are the most prone to hydrogen cracking. The presence of trace elements, especially sulphur and phosphorous can make hydrogen cracking much more likely to occur. Hydrogen induced cracks are not likely to be detected by radiography and other methods such as ultrasonic testing should be used in any situation where there is a high probability of occurrence.
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The Welding Institute A second type of cracking that can occur in the heat affected zone of a weld is sometimes called weld decay. This type of cracking can affect stainless steels and is caused by the precipitation of brittle material at the grain boundaries. The brittle material is chromium carbide. All stainless steels contain a small proportion of carbon which is generally held in solution within the austenitic grains. The heat from welding can cause this carbon to combine with the chromium which is present forming chromium carbide which is an extremely brittle material. Weld decay can be avoided by reducing the carbon content of the parent material and filler wire. Cracking caused by weld decay is unlikely to be detected by radiography. Crater cracks This type of crack occurs when the heat source is removed too suddenly at the end of a weld run. The cracking mechanism is the same as that for centreline cracking. The major dimension of a crater crack is usually less than 5 mm. They often have a star shaped appearance in a radiograph and they are relatively easy to detect. Many welding standards will permit this type of cracking provided that it does not exceed a specified maximum dimension.
Figure 79. Typical radiographic appearance of a crack 12.7.2.2 LACK OF FUSION Lack of fusion in welding can occur either between the weld deposit and the parent material or between successive layers of weld material. Lack of fusion may also occur due to lack of fill (see 11.6.1.3 above) or due to lack of penetration (see 11.7.2.3 below). Lack of fusion is an area where the solid material immediately adjacent to the molten weld pool failed to become molten during the welding process leading to a lack of union between the molten weld material and the adjacent solid material. The ability of radiographic techniques to successfully detect lack of fusion is strongly dependent on the orientation of the defect with respect to the incident beam of radiation (see figure 78). Given favourable orientation lack of fusion with the parent material will appear in the radiograph as a fine dark straight line which may be continuous or intermittent. Unfavourably orientated lack of fusion with the parent material may sometimes still be detected due to the presence of associated slag inclusions or porosity. A slag inclusion with a straight edge normally indicates lack of fusion and gas escaping from an area lack of fusion during the deposition of a subsequent welding run may lead to a line of ‘linear porosity’.
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The Welding Institute Lack of fusion between subsequent layers of weld material will generally not be detected by radiography unless it is associated with some other type of defect such as slag.
Figure 80. Types of lack of fusion
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Figure 81. Lack of fusion in the radiograph
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The Welding Institute 12.7.2.3 INCOMPLETE ROOT PENETRATION This can be described as the failure of the weld material to extend into the root of a joint. Incomplete penetration is a fusion defect, not to be confused with root concavity (see 11.7.1.2 above). Incomplete root penetration appears in a radiograph as a dark continuous or intermittent linear shadow, the edges of which will usually be straight. Where welds are deposited without a root gap lack of penetration may appear as a single continuous or intermittent dark line. It should be noted that root gaps frequently close during welding so even in cases where there should have been a root gap lack of penetration may appear in the radiograph as a single dark line.
Figure 82. Lack of root penetration 12.7.2.4 NON-METALLIC INCLUSIONS These are usually formed by slag, but occasionally by other foreign matter such as wind blown sand may become entrapped within the molten weld material. Slag inclusions are irregularly shaped, they may be either rounded / isolated or linear / elongated. Linear slag inclusions with a straight edge often indicate lack of fusion. Sometimes linear slag will appear on the radiograph as two parallel lines. this type of slag inclusion is often referred to as ‘tram lines’ or ‘wagon tracks’. Most welding slag and other possible sources of non-metallic inclusions are radiographically much less absorbing than the surrounding metallic material, therefore they appear in the radiograph as dark images. Although very rarely used, some types of covered welding electrode have a high barium content in the flux coating. These electrodes produce a slag which is radiographically denser than steel. In this case, therefore, a slag inclusion may appear as a light image. 12.7.2.5 METALLIC INCLUSIONS Dependent upon the nature of the welding process it is possible for foreign metallic material to become entrapped within the molten weld material. Associated with the gas tungsten arc welding process, tungsten inclusions are probably the most commonly encountered form of metallic inclusion. They are caused by the break-up of the nonconsumable tungsten electrode during welding. Since tungsten has a melting point well in
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The Welding Institute excess of 3000°C particles of tungsten falling into the weld pool do not become molten. Tungsten is radiographically extremely dense, therefore tungsten inclusions always appear as bright - light images which tend to be angular. They are usually quite small - typically around 0.5 mm. Copper inclusions can occur particularly with the submerged arc or other welding process where the consumable electrode is fed through a copper contact. If the copper contact gets too near to or if it touches the weld pool molten copper (melting point about 900°C) will become included in the weld pool. Copper is radiographically more absorbing than most other materials including steel so copper inclusions may produce light rounded images with extremely diffuse edges. Copper inclusions in ferritic steel welds usually cause severe transverse cracking. Metallic inclusions are quite common in aluminium welds, where such welds are not properly segregated from their steel counterparts. Aluminium melts at around 660°C, so particles of steel or iron oxide falling into the weld pool will not become molten (the melting point of steel is about 1400°C). Contamination can easily occur if tools such as grinding disks which have been used for steel are used on aluminium. Steel inclusions in aluminium appear as very bright angular shapes with sharp edges.
Figure 83. Slag inclusions
Figure 84. Tungsten inclusion 12.7.2.6 GAS PORES: POROSITY The solubility for gas of the molten weld material is many times that of the solid weld material, thus as the material freezes there is a tendency for any dissolved gases to precipitate from solution causing gas pores or porosity in the finished weld. Gas pores are extremely easy to detect by radiography since they are not sensitive to the direction of radiation and the gas which fills them is many times less radiographically dense than the surrounding material. Gas pores appear on a radiograph as sharply defined dark circular spots. They may be isolated, grouped or evenly distributed. Aligned porosity is usually an
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The Welding Institute indication of lack of fusion (see 11.7.2.2 above). Evenly distributed porosity generally indicates that the electrode was in some way faulty, group porosity usually occurs at restarts and is due to poor welding technique. 12.7.2.7 ELONGATED CAVITIES (HOLLOW BEAD) These will generally only occur in the root run of welds deposited by manual metallic arc. Welds deposited using cellulosic coated electrodes (AWS E6010, 7010 etc.) are more likely to suffer from this defect than welds deposited with other types of electrode. Hollow bead can be caused by holding the arc at too shallow an angle with respect to the work piece or by a strong draught of air along the inside of the pipe during welding. On the radiograph it has the appearance very similar to that of slag. The radiographic indication usually has rounded ends and it is always situated along the centre of the root bead. 12.7.2.8 WORM HOLES These are gas pores which have become frozen in the weld pool while attempting to migrate to the surface of the weld pool. In addition to occurring due to an excess of dissolved gas in the weld pool wormholes sometimes occur due to laminations in the parent material which extend to the weld face. Lack of fusion contains a small amount of entrapped air and this can cause wormholes in a similar way. Wormholes appear on the radiograph as a dark shadow the shape of which depends on the orientation of the defect. If the wormhole is end on to the radiation a very dark rounded shadow is formed. If the wormhole is side on then the appearance is somewhat like a tadpole. Where a lamination in the parent material or a lack of fusion is the source of wormholes they are often apparent in the radiograph in a herringbone like array.
Figure 85. Wormholes due to a lamination in the parent material 12.7.2.9 CRATER PIPES & CRATER CRACKS These occur due to shrinkage at the end of a weld run where the source of heat was removed too suddenly causing the weld pool to freeze too rapidly. This defect is quite common when the welding process is gas tungsten arc but it may also occur with shielded metallic arc and other welding processes.
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The Welding Institute A crater pipe will appear in the radiograph with an image very similar to that of a wormhole. It can only be distinguished from a wormhole by its position in the weld. Crater cracks are shrinkage cracks and as such have a relatively greater volume than do most other cracks. They often have a star like appearance in a radiograph. Their radiographic image rarely measures more than 3 or 4 mm. 12.8 INTERPRETATION OF CASTING RADIOGRAPHS Five groups of defect images may be seen in radiographs of metal castings, these being: (i) (ii) (iii) (iv) (v)
Voids. Cracks. Cold Shuts. Segregation. Inclusions.
12.8.1 VOIDS Voids in castings are formed by gases dissolved in the molten material precipitating from solution during the solidification process or by shrinkage caused by inadequate feeding. 12.8.1.1 MACROSHRINKAGE This is a large cavity formed during the solidification process which occurs to lack of sufficient feed material. With good mould design macroshrinkage (also called piping) should be confined to the feeder heads. Macroshrinkage appears on the radiograph as a dark continuous or semi-continuous area of varying film density with diffuse edges. 12.8.1.2 FILAMENTARY SHRINKAGE (ALSO CALLED SPONGINESS) This is a coarse form of shrinkage which has smaller physical dimensions than a macroshrinkage cavity. These cavities may be extensive and branching in nature. Filamentary shrinkage occurs at the point in a casting which freezes last. Theoretically this should always be at the centre of a section but this is not always the case. On some occasions the defect may actually extend to the surface of the casting. Filamentary shrinkage has diffuse branched appearance on the radiograph of variable film density. 12.8.1.3 MICROPOROSITY / MICROSHRINKAGE This is a very fine form of filamentary shrinkage due to lack of sufficient feed metal or gas or both, in which numbers of cavities occur either round the grain boundaries or between the dendrite arms (a dendrite is a material crystal which in the initial stages of growth is tree-like). These cavities tend to link up in a three dimensional network throughout the material.
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In the radiograph the images of these cavities are superimposed and generally produce a mottled or cloudy effect. In non-ferrous alloys, particularly magnesium based alloys, microshrinkage may occur in layers and produce dark streaks in the radiograph. 12.8.1.4 PINHOLE POROSITY Pinhole porosity consists of small cavities less than 1.5 mm diameter which are formed due to the evolution of gas from the molten material The defect may be evenly distributed throughout the casting or localised to a particular area. When it occurs local to the surface of the casting, due to gas evolved at the mould face, it is known as subcutaneous pinhole porosity. The defect appears in the radiograph as an assemblage of small, rounded, widely distributed dark images. This condition is distinguished from microporosity by the size and also by the rounded nature of the images which do not show the same tendency to interconnect. This defect can arise from the accidental injection of air during pressure die casting. 12.8.1.5 GASHOLES A gashole is a discrete cavity greater than about 1.5 mm diameter caused by gas evolved from the material as it freezes. It may also arise from gas evolved from the core or mould (in which case the defects are called blowholes). The radiographic image appears as a dark area of smooth outline which may be circular or elongated and can be associated with pinhole porosity. Gasholes occasionally become elongated as they try to rise to the surface of the molten material during cooling, in this form they are known as wormholes. The radiographic image of a wormhole may vary from a circular to an extremely elongated image depending upon the angle of view. 12.8.1.6 AIRLOCKS (ENTRAPPED AIR) These are cavities formed by air which has been trapped in the mould by the material during pouring. The defect appears in the radiograph as dark area with an outline which is generally smooth but which may have irregularities. An airlock cannot always be distinguished radiographically from a gas hole but a helpful guide to identification is the shape, size and position in the casting. In pressure die casting air may be injected with the material. In this instance the defect is usually more severe in the runners and may assume an angular form. In pressure and gravity die castings this defect may occur in clusters or as strings of small voids. In investment casting it may appear as small rounded voids.
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The Welding Institute 12.8.2 CRACKS Cracks are discontinuities caused by fracture of the material at the point of solidification or at some time thereafter. Cracks appear on the radiograph as one or more dark lines. The width and form of the indication depends on the type of crack and on the radiographic technique used. 12.8.2.1 HOT TEARS These are discontinuities of a decidedly ragged form resulting from stress developed near the solidification temperature when a material has low mechanical strength. These stresses usually arise when the natural contraction of the casting is restrained by the mould or core. The defect occurs mainly at or near to a change of section. The defects are not necessarily continuous, they may exist in groups and will often terminate at the surface. Hot tears may sometimes be referred to as a pulls. Radiographically hot tears are revealed as wavy, ragged dark lines which are often discontinuous, with areas appearing as approximately parallel dark lines which may possibly be overlapping. Generally the ends of the indication taper to become fine. 12.8.2.2 STRESS CRACKS These are well defined and approximately straight cracks formed after the material has become completely solid, quite large stresses being required to cause fracture. Distinctions are sometimes drawn between types depending upon the time at which fracture occurred. In the radiograph stress cracks are often revealed as clearly defined smooth dark lines - thus differing from the ragged appearance of a hot tears. 12.8.3 COLD SHUTS These are discontinuities caused by the failure of a stream of molten material to unite with either a confluent stream, or solid material, such as a chaplet or internal chill or pouring splash. In the radiograph these defects usually appear as a dark lines. They may be difficult to distinguish from a hot tears except by the typical involute appearance of the end of the defects. The shape of an unfused chaplet or unfused chill in a radiograph is dependent upon orientation of the beam. A cold shut resulting from a splash may appear as a dark crescent or circle.
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The Welding Institute 12.8.4 INCLUSIONS These consist of foreign matter entrapped in the casting. This may be of many types including sand, slag, flux, or dross. As an inclusion may be of greater or less opacity then the surrounding material it may appear radiographically as a light or dark area (e.g. a sand inclusion will appear dark in steel and light in aluminium). Slag usually gives a rounded image whereas material included in the casting as a solid (e.g. dross and sand), will give an irregular shape. If dross is trapped as an oxide film it will often produce a characteristic folded appearance in the radiograph. Inclusions may in many respects resemble voids in radiographic appearance but they will generally exhibit a greater variation in density. 12.8.5 SEGREGATIONS These result from local concentrations of any of the constituents of an alloy. They may be classified as general, localised or banded. Detection of such defects by radiography depends upon the segregating constituents producing a local variation in the absorption of the radiation. 13.0 LOCALISATION A radiograph is a two dimensional image of a three dimensional object. When a flaw is detected using a standard technique there is no certain way of telling how far below the surface of the object the flaw is. In some cases it might be desirable to have three dimensional information about the position of a flaw. A technique called ‘localisation’ can be used to estimate the through wall position of a volumetric flaw such as a slag inclusion. It is important to note that localisation of planar flaws such as cracks or lack of fusion is generally not possible by radiographic methods. 13.1 90° METHOD This is the simplest method of localisation, but it is rather limited in its field of application. A typical test object, where this method might be useful would be a small to medium sized casting that has a fairly simple cross section. Figure 79 below shows how this method would work on a small cylindrical casting. Two radiographs are taken with primary beam mutually at 90° to each other. In an ideal situation the component would be placed on some kind of turntable so that it could be moved accurately keeping the two exposures in the same plane relative to the axis of the component. The apparent defect position in each radiograph can be measured relative to convenient datum point and the results plotted on a sketch. The defect position is then deduced by triangulation.
13. 2 TUBE (SOURCE) SHIFT METHOD Figure 87 below shows how this method could be used to locate a slag inclusion in a butt weld. In order for this method to work well a high degree of dimensional accuracy is needed. The source to object distance, the object to film distance and the distance that the
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The Welding Institute source is moved between the successive exposures must all be accurately measured and controlled. Two ‘half ‘exposures are made from different source positions using the same radiographic film to produce two flaw images. The distance between the two images, ‘m’ is then measured and the flaw depth is calculated as shown. The source shift, which is usually about one sixth of the source to film distance, has been exaggerated in the figure.
Figure 86. The 90° method for a small cylindrical casting
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Figure 87. Tube shift method NOTE: Source shift distance exaggerated for clarity
Using similar triangles:
But:
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so:
multiply both sides of the equation by Bm:
add dB & subtract Am both sides:
which can be written as:
divide both sides by (B + m):
12.3 TUBE (SOURCE) SHIFT METHOD WITH LEAD MARKERS Placing lead markers on the component source and film side as sown in figure 88 below removes the need for accurate measurement of the source to object distance, the object to film distance and the distance that the source is moved between the successive exposures. Refer to figure 88 below. The three triangles in the enlarged view will be very close to similar as long as the source or focus to film distance is long in comparison with the thickness. The calculation below assumes that the triangles are similar. if the triangles are similar then:
so it follows that:
and:
we also know that:
so:
and:
and:
therefore we can write:
and:
(argument continued below figure 88)
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Figure 88. Tube shift method with lead markers
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we have already established that:
and
from this we can see that:
and:
so:
and:
now divide equation 1 by equation 2 to get:
from this it follows that:
t, the thickness of the plate is known and x, y and z can be measured on the radiograph. Therefore d can be calculated.
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The Welding Institute 14.0 UNITS USED IN RADIOGRAPHY 14.1 IONISATION (EXPOSURE) The quantity of ionising radiation can be measured in terms of its ionising effect or exposure on air at standard temperature and pressure (STP). The SI unit of ionising effect is the ‘coulomb per kilogram’, the quantity of ionising radiation that produces a total electric charge of 1 coulomb per kilogram (Ckg-1) of air at STP. The centimetre-gram-second (cgs) unit of ionising effect is the ‘roentgen’ (R), the quantity of ionising radiation that produces an electric charge of 1 electrostatic unit (esu), which is equivalent to 2.08 x 109 ion pairs, per cubic centimetre of air at STP. One cubic centimetre of air at STP weighs 0.001293 grams. One esu is equal to 3.336 x 10-10 coulomb so: 1R = 2.58 x 10-4 Ckg-1 or 1 Ckg-1 = 3876 R 14.2 ABSORBED DOSE The SI unit of absorbed dose is the ‘gray’ (Gy). The gray is defined as the quantity of ionising radiation which releases 1 joule of energy per kilogram of absorber. The cgs unit of absorbed dose is the ‘roentgen absorbed dose’ (rad). The rad is defined as the quantity of ionising radiation which releases 100 ergs of energy per gram of absorber. 1 Gy = 100 rad The units of radiation absorbed dose can be approximately related to the units of ionising effect as follows: 1R = 0.88 rad 1 Ckg-1 = 3411 rad = 34.11 Gy The conversions above are approximate since the relationship between the roentgen and the rad or the coulomb per kilogram and the gray varies to some extent with radiation energy.
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The Welding Institute 14.3 MAN MAMMAL EQUIVALENT or RADIOBIOLOGICAL EQUIVALENT The effect which ionising radiation has on our bodies varies with the type of radiation and also, to some extent, with radiation energy. In order to compensate for this a quality factor (QF) is introduced. Quality factors for several types of ionising radiation are listed in table 17 below. Type of Radiation x-rays gamma rays beta particles alpha particles thermal neutrons** fast neutrons*** protons heavy ions
Quality Factor (QF) 1.0 1.0 1.0* 20 2 10 10 20
* may in some cases exceed 1.0 ** energy < 10 keV *** energy > 10 keV
Table 17. Quality factors In the cgs system multiplying the dose in rad by the appropriate quality factor gives the dose in ‘roentgen equivalent man’ (Rem) where 1 Rem is the amount of ionising radiation which has the same biological effect as 1 rad of x-rays. In the SI system multiplying the dose in gray by the appropriate quality factor gives the dose in ‘sievert’ (Sv) where 1 Sv is the amount of ionising radiation which has the same biological effect as 1 Gy of x-rays. Thus: 1 Sv = 100 Rem or 1 Rem = 0.01 Sv
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The Welding Institute 14.4 DOSE RATE Dose rate in the SI system is generally measured in microsieverts per hour (μSv/h), but may also be measured in millisieverts (mSv) or sieverts (Sv) per hour. Alternatively dose rate can be expressed in micrograys (mGy), milligrays (mGy )or grays per hour In the cgs system dose rate is generally measured in millirem per hour (mRem/h) but may be measured in Rem per hour (Rem/h). 1 mRem/h = 10 μSv/h or 1 mSv/h = 0.1 mRem 14.5 SOURCE STRENGTH OR ACTIVITY For radioactive sources the source strength or activity is the number of disintegrations occurring each second. This is proportional to the number of active atoms present in the source. The cgs unit of source strength or activity is the curie (Ci). One curie is equal to 3.7 x 1010 disintegrations per second. The SI unit of source strength or activity is the becquerel (Bq) or the gigabecquerel (GBq). One becquerel is equal to one disintegration per second; one gigabecquerel is equal to 109 disintegrations per second. 1 Ci = 37 GBq or 1 GBq = 0.027 Ci 14.6 SPECIFIC ACTIVITY The specific activity of a radioactive source is equal to the source activity divided by the weight of the source. In the cgs system it is expressed in ‘curies per gram’ (Ci/g) while in the SI system it is expressed in ‘becquerels per gram’ (Bq/g) or ‘gigabecquerels per gram’ (GBq/g). 14.7 OUTPUT The output of a source of ionising radiation is the dose rate per hour at some fixed distance, usually 1 metre, from the source. For radioactive isotopes it is useful to state output in grays, sieverts, rads or Rems per hour per curie at one metre. Table 18 below gives some examples.
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ISOTOPE NAME
OUTPUT (mSv per hour per Ci)
Thulium 170 Ytterbium 169 Selenium 75 Iridium 192 Cobalt 60
0.026 1.25 1.8 4.8 13.0
Table 18. Output of various radioactive isotopes The output of radiation from a typical 200 kV industrial constant potential x-ray machine is as much as 1,000 mSv per milliampere of tube current at a distance of 1 metre from the focal spot.
15.0 RADIATION MONITORING DEVICES Ionising radiation cannot be detected by human senses; it is extremely harmful to health, therefore it is imperative that we have available to us reliable equipment that can measure radiation dose. Two basic types of radiation monitoring device exist: (1) devices which give a read out of the current dose rate and (2) devices which measure accumulated dose over a given period of time. 15.1 SURVEY METERS Survey metres give a real time measurement of dose rate. They are of 5 basic types, ionisation chambers, proportional counters, Geiger counters, scintillation counters and solid state devices. Each of these is discussed and described in the sections below. 15.1.1 IONISATION CHAMBERS An ionisation chamber is part of the family of radiation detectors known as ‘gaseous detectors’. The ionisation chamber can take many forms, but basically it consists of 2 electrodes separated by a layer of gas. As ionising radiation interacts with the gas, causing ionisation, it becomes electrically conductive and pulses of current flow as each photon of ionising radiation is received. Compared with the other types of gaseous detector the ionisation chamber operates at low electrical voltage, see figure 89 below. The actual voltage needed depends on the geometry and size of the ionisation chamber. Ionisation chambers can detect alpha, beta and gamma or x-ray radiation but they give no information as to the photon energy of the radiation detected. Ionisation chambers are occasionally used in conjunction with an electronic circuit that counts the current pulses but it is more usual that the output is a reading of the average current flowing across the chamber. The measurement range of ionisation chamber instruments is comparatively narrow and they tend to be bulky and fragile when compared to the Geiger counter described in paragraph 14.1.2 below. They are therefore seldom seen in industrial applications.
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Figure 89. Gaseous detectors, pulse size versus applied voltage
Figure 90. Simplified lay-out of an ionisation chamber instrument 15.1.2 PROPORTIONAL COUNTERS Neither the Geiger counter below, nor the ionisation chamber above can give any information as to the photon energy of the ionising radiation received. The best that can be achieved with these instruments is to shield the chamber such that alpha and beta radiation is excluded from the measurement. The gas chamber used in a proportional counter often contains multiple electrodes. Proportional counters operate in a voltage range intermediate between the ionisation chamber and the Geiger counter. In addition to gauging radiation dose rate or intensity they are able to give information as to the type and photon energy of the radiation received. They are also able to determine the direction from which the radiation is
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The Welding Institute coming. They are often used as fixed monitoring instruments within and around nuclear installations, but are rarely seen in other workaday industrial applications. 15.1.3 GEIGER COUNTERS Geiger counters operate at higher voltages than the proportional counter; typical operating voltages vary from 400 to 1,000 volts or more dependent on the size and geometry of the gas chamber. At such voltages the pulse size is very large and no amplification is needed. A The original 1928 version of the Geiger tube contained a special self quenching gas mixture consisting of an inert gas doped with a small amount of hydrocarbon (e.g. butane). This design was greatly improved upon in 1947 when Liebson designed a tube containing inert gas with a small proportion of halogen (e.g. bromine). All modern instruments follow the Liebson design. Geiger tubes can be made very small, a cylinder of less than 6 mm diameter and length 25 mm is not untypical. Geiger tubes are extremely durable and reliable. A Geiger tube constructed of a light metal such as aluminium will detect only x or gamma rays. Tubes provided with a window made from thin glass will also detect beta radiation while those having a similar window made from mica can detect alpha in addition to beta and gamma. The measurement range of the instrument can be extended by shielding the tube. Geiger tube instruments are otherwise insensitive to changes in photon energy. In general Geiger counter instruments give little information as to the direction from which the detected radiation is coming. Geiger counters may give a reading in counts per second, but usually the average current flowing across the tube is measured with the ammeter scale being calibrated to read microsieverts or millisieverts per hour. As radiation intensity increases to high levels a Geiger counter will become increasingly inaccurate. This is because the instrument suffers from a short dead time after a pulsing event has occurred – if another photon of radiation arrives during the dead time it will not be detected. Some instruments will cease to function at all if exposed to a very high dose rate. 15.1.4 SOLID STATE RADIATION DETECTORS Solid state radiation detectors have been available since the 1950s. Various types of semiconductor are available which begin to conduct electricity under the influence of ionising radiation. Instruments based on this type of semiconductor are able to differentiate between different photon energies. Thus in addition to measuring dose rate they can provide information as to the spectrum of radiation that is present. 15.1.5 SCINTILLATION COUNTERS Various materials known as phosphors will emit flashes of light when placed in a beam of ionising radiation. Phosphors can be manufactured to respond to one or more types of ionising radiation. Table 19 below lists some common phosphorescent materials. Many other phosphors exist, including a number of organic liquids and solids. Phosphors have been used as radiation detectors since the very early days of the discovery of ionising radiation, both Roentgen and Becquerel used them. The amount of light produced can be quite small so phosphors are always used in conjunction with a light amplification system such as the photomultiplier tube. Modern instruments use ‘charge coupled devices’ or CCDs in conjunction with a radiation sensitive phosphor. A CCD is at the heart of any modern digital camera. The CCDs used for radiation detection measure the
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The Welding Institute intensity of light emitted from the phosphorescent layer under the influence of ionising radiation. Whichever system is used scintillation counters relate the intensity of light produced by the phosphor to the intensity of the ionising radiation received. In general they give a reading in counts per second but occasionally they will be calibrated to read directly in microsieverts or millisieverts per hour. Scintillation counters are extremely sensitive, they can detect very low levels of ionising radiation. They are direction sensitive instruments and are very useful when searching for radioactive contamination. They are used in industrial radiography to check for leakage of fissile material from a sealed source. PHOSPHOR (ACTIVATOR) Sodium Iodide (Thallium) Lithium Iodide (Europium) Zinc Sulphide (Silver) Bismuth Germanate (N/A)
SENSITIVE TO: Gamma Gamma & Neutrons Alpha Gamma
Table 19. Common phosphorescent materials 15.2 PERSONAL MONITORS Survey meters, with a few exceptions, give a real time reading of dose rate but do not integrate this to give a total dose received over a given period of time. Several types of device exist which are capable of integrating the dose received over a period of time. One convenient use of such a device is for monitoring the total dose that a person receives during the course of his or her working day. When used in this way such devices are referred to as ‘personal monitors’. Four types of personal monitor are commonly used in industrial radiography. 15.2.1 FILM BADGES
Figure 91. Film Badge
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The Welding Institute The principle of a film badge is that when exposed to ionising radiation followed by developing under tightly controlled conditions the film density produced can be related to the radiation dose received. Film badges (as shown in figure 91) can be used to detect x, gamma and beta radiation. Coupled with the right type of intensification screen radiographic film can be used to detect and measure other types of ionising radiation. The film badge of the type shown in figure 91 contains a section of carefully manufactured radiographic film having two emulsions, one fast and one slow. The use of two emulsions extends the measurement range of the badge. The badge holder is equipped with various filters which extend the range of measurement and additionally enable the badge to give some information as to the type and photon energy of the ionising radiation received. The film badge has in large part, been replaced by the thermoluminescent dosimeter (TLD) (see below). Table 20 below gives a comparison of typical film badge and TLD specifications. FILM BADGES Radiation Type Measuring Range (photon energy) Measuring Range (dose) Typical period of use Radiation Type Measuring Range (photon energy) Measuring Range (dose) Typical period of use
Gamma
X-ray
Beta
10 keV to 7 MeV
10 keV to 7 MeV
700 keV to 3.5 MeV
0.1 mSv to 10 Sv
0.1 mSv to 400 mSv
0.1 mSv to 10 Sv
2 to 4 weeks THERMOLUMINESCENT DOSIMETERS Gamma X-ray
Beta
10 keV to 10 MeV
10 keV to 10 MeV
700 keV to 3.5 MeV
0.05 mSv to 10 Sv
0.05 mSv to 10 Sv
0.05 mSv to 10 Sv
4 weeks
Table 20. Film badge & thermoluminescent dosimeter specifications 15.2.2 THERMOLUMINESCENT DOSIMETERS (TLD) The thermoluminescent dosimeter or TLD offers several significant advantages over the film badge: (a) A TLD is much less easily damaged than a film badge. (b) The TLD has slightly wider measurement range than the film badge. (c) The TLD is much less subject to possible errors or failures in processing – the measurements obtained have a better degree of accuracy. (d) The TLD can be reused many times. (e) The absorption characteristics of the TLD more closely resemble those of the human body, thus dose calculations are simplified. Most TLD badges contain two or more discs of a thermoluminescent material. This material is usually lithium fluoride but occasionally other materials are used. During exposure to ionising radiation lithium fluoride stores energy. When subsequently heated to a temperature
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Figure 92. Quartz Fibre Electrometer The quartz fibre electrometer (QFE) is a gaseous detector like the ionisation chamber, proportional counter and Geiger counter described above. When raised to the light a scale like the one on the left of figure 92 can be seen through the lens of the instrument. The vertical line is the quartz fibre. When a static electrical charge is applied to the instrument the quartz fibre moves to the zero point of the scale. As the gas inside the QFE becomes ionised the static charge is gradually dissipated and the fibre begins to move to the right. The corresponding total dose received can be read on the upper scale. The QFE has quite a narrow measuring range, typically 0 to 50 mSv or less. The example shown above has a measurement range of 0 to 200 mRem which is equivalent to 0 to 2 mSv. The QFE is sensitive to x and gamma radiation in the photon energy range 45 keV to 3.5 MeV. The QFE is a very convenient means for checking how radiation doses are accumulating during a working day but it suffers from fragility and is very easily damaged. 15.2.4 SOLID STATE INTEGRATING DOSIMETERS The QFE has largely been replaced by solid state integrating dosimeters. These devices are extremely shock proof and have a wider measuring range than the QFE. They are typically combined with an audible warning device which bleeps if the wearer unwittingly enters a high radiation area.
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The Welding Institute 16.0 RADIATION SAFETY All personnel working with ionising radiation should be aware that such radiation is injurious to the human body (or any other biological tissue). Anyone working at a place where radiography is carried out should make himself fully aware of the safety procedures and regulations which are in force and take care observe all warning barriers. Ionising radiation cannot be detected by the five human senses. Ionising radiation has cumulative effects upon the human metabolism. Ionising radiation causes genetic damage to the human body the full effects of which may not be apparent until as much as 15-35 years after the initial exposure. Regardless of any nominal safe limits it is always prudent to avoid exposure to radiation whenever possible. N.B. Where industrial radiography is concerned there is little or no danger from contamination because all gamma sources in use are of the sealed variety. X or gamma rays are not capable of producing any residual radioactivity in the items subjected to exposure. 16.1 PRECAUTIONS 16.1.1 EXPOSURE BOOTHS At locations where a large volume of industrial radiography is carried out exposure booths of various shapes and sizes will generally be available. These usually consist of enclosures having lead lined walls. Some exposure booths have walls filled with spent casting sand or other radiation absorbing material. Such exposure booths should be regularly monitored to ensure that the radiation dose rate is within safe limits in the areas outside the booth where personnel can move freely. Safety switches are usually fitted to doors of exposure booths in order to prevent the operation of x-ray sets or gamma ray equipment whilst the door is open. In cases where overhead cranes might have to pass over an open topped exposure booth similar safety switches are normally installed so as to trip out the x-ray set, or wind back the gamma ray source, should the crane encroach upon the irradiated area during exposure. In many countries (including Britain) it is a legal requirement that an audible warning is given before any exposure takes place. Exposure booths should be equipped with switches inside the x-ray compound which can be operated in order to prevent the operation of the x-ray or gamma ray equipment should any personnel be accidentally trapped inside. Radiation detectors should be installed inside the exposure booth to indicate when gamma ray sources are being used.
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The Welding Institute 16.1.2 SITE WORK A demarcation barrier is required showing the zone where radiation is in excess of the legally permitted limit (in Britain this is 7.5 μSv/h). The barrier is usually consists of brightly coloured rope or tape suspended at about 1 metre above the ground, with warning signs at 5 metre intervals. Areas which will be irradiated at greater than the legal limit must be cleared of all non classified personnel prior to any exposure. Audible and visible warnings must be given before any exposure takes place. The barrier should be monitored with an efficient radiation detector and should be guarded by classified personnel during exposure. 16.1.3 SCATTER Personnel should be aware that radiation can be scattered by structures. Apparently safe locations may be subject to stray scattered radiation. 16.2 EXPOSURE LIMITS FOR RADIATION WORKERS In Britain classified workers are allowed to receive an accumulated dose of 20 millisieverts (20 mSv) per year from the age of 18 yrs to 65 yrs. A formal investigation is required if a classified worker receives a dose of 15 mSv or more within any single calendar year. The investigation has to establish the source of the dose received. The investigation may or may not include a thorough medical check for the person receiving the dose. These requirements are typical for all countries within the European Union, but requirements in other countries may differ widely. 16.2.1 DOSIMETERS For work in radioactive environments (i.e. nuclear reactors) personnel must be equipped with direct reading dosimeters which will display immediately the accumulated dose received. Personnel working in these locations must take particular care to avoid ingesting radioactive particles. Tightly fitting breathing masks are required and protective clothing should be worn. 16.3 PERMITTED LEVELS The figures given in the paragraphs below relate to Statutory Instrument 1999 Number 3232, ‘Ionising Radiation Regulations 1999’. These regulations exclude radiation doses received due to medical reasons. 16.3.1 CLASSIFIED WORKERS The maximum permitted dose rate for personnel equipped with film badges (or TLDs) is 20 mSv per year . This is approximately equivalent to a constant dose rate of 10 μSv/hr for a 40 hour working week if a 48 working week year is assumed.
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The Welding Institute 16.3.2 UNCLASSIFIED PERSONNEL, CONTROLLED & SUPERVISED AREAS Controlled area Unclassified personnel must be excluded from any area where radiation dose is deemed likely to exceed three tenths of the annual allowable dose for a classified worker (6 mSv). The maximum permissible dose rate at the boundary of a controlled area is 7.5 Sv/hr. Supervised area A supervised area is defined as an area where the annual dose is expected to equal or exceed 1 mSv. Such areas should be clearly signed. Unclassified persons are permitted to pass through such areas but they must not remain in them for extended periods. Where possible verbal warnings should be given by the radiographer. 16.4 ‘SAFE’ WORKING DISTANCES The dose rate from a source of ionising radiation reduces in proportion to the reciprocal of the square of the distance from the source. For any source of ionising radiation:
If the source of ionising radiation is x-ray then it will not be possible to calculate the dose rate at one metre although the dose rate will be proportional to the tube current. Halving the tube current at a given tube voltage will halve the radiation dose rate. If the source is gamma ray then the dose rate at one metre can be calculated if the source strength (curies or gigabecquerels and ‘output’ of the source are known. Output for any given isotope is the dose rate per curie or gigabecquerel at one metre from the source. Thus:
Output for the various radioactive isotopes used in industrial radiography is tabulated below:
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The Welding Institute If we take 7.5 μSv/hr to be the ‘safe’ dose rate then we can calculate the ‘safe’ distance using the formula below:
The above formula can be simplified to:
Where ‘C’ is a constant for each isotope. For Thulium 170 C = 1.86, for Ytterbium 169 C = 12.91, for Selenium 75 C = 15.49, for Iridium 192 C = 25.30 and for Cobalt 60 C = 41.63. 16.4.1 SHIELDING If shielding is introduced then the reduction in the minimum safe working distance can be calculated if the magnitude of the ‘half value layer’ or ‘tenth value layer’ of the shielding material is known. The half value layer for any material is the thickness of material that will reduce the radiation dose rate, for a given radiation energy, by a factor of two. The tenth value layer is similarly the thickness of material that will reduce the dose rate by a factor of ten. For example the half value layer of lead for cobalt 60 is about 12.5 mm while for iridium 192 it is about 4.8 mm. The tenth value layer of lead for cobalt 60 is about 41.5 mm while for iridium 192 it is about 16 mm. If the shielding thickness is an exact multiple of the half or tenth value layer then the dose rate after shielding can be found simply by dividing the unshielded rate by two for each half value layer or by ten for each tenth value layer. Where this is not the case the formulae given below can be used.
Where: Ru = the unshielded dose rate Rs = shielded dose rate t = the thickness of shielding material hvl = the half value layer tvl = the tenth value layer
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The Welding Institute 17. A GLOSSARY OF TERMS (USED IN RADIOGRAPHIC TESTING) The following is a compilation of the more common terms used in connection with radiographic testing. Other lists can be found in BS EN 1330-3 and ASME V, Appendix A.
Absorbed Dose: (of ionising radiation) The energy per unit mass imparted to the irradiated material. Absorbed dose is measured in Grays [Gy] (1 Gy = J kg-1) Absorption: The reduction in intensity of a beam of radiation during its passage through matter. Absorption Coefficient: usually abbreviated as ‘μ’. Where I = I0 x e-μt and I is the shielded radiation intensity, I0 is the unshielded radiation intensity while t is the thickness of the absorber. Alpha Radiation: A type of ionising radiation consisting of high velocity charged particles emitted from the nucleus of heavy radioactive isotopes. The alpha particle consists of 2 protons and 2 electrons – a helium nucleus, and has a positive charge of 2. Alpha radiation has very low penetrating power, but it is very strongly ionising. Anode: The positive electrode of a discharge tube. In an x-ray tube the anode carries the target. (see ‘target’) Atom: The smallest indivisible part of a chemical element, it consists of a nucleus formed from positively charged protons and neutrons surrounded by orbiting negatively charged electrons. Atomic Mass Number: The total number of protons plus neutrons in the nucleus of an atom, standard abbreviation A. Atomic Mass Unit (amu): A measure of atomic weight, roughly speaking the proton has a weight of 1 amu while that of the neutron is marginally greater than 1 amu. The weight of the electron is roughly 0.00054 amu.
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The Welding Institute Atomic Number: The number of protons in an atomic nucleus, standard abbreviation Z. Atomic weight: The weight of an atom expressed in atomic mass units, it is approximately equal to the atomic mass number. Back-Scatter: Scattered radiation caused by the presence of objects behind the film or radiation detector. (see ‘scatter’) Background Count The ionising radiation dose rate due to natural causes (see ‘background radiation’). Background Radiation: Ionising radiation which is present at any given site which is due to natural causes: sunlight contains a proportion of ionising radiation; some natural rocks such as granite are weakly radioactive. Base Fog Level: The film density of a radiographic film prior to exposure to ionising radiation (measured by processing a sample of unexposed film). Base fog level increases with the age of the film. Poor storage conditions – temperature too high, humidity too high are a common cause of excessive base fog level. Other possible causes of high base fog level are exposure to chemical or solvent fumes, exposure to ionising radiation and exposure to light. Most international standards specify a maximum base fog level of 0.3. Becquerel: The SI unit of radioactivity which is defined as 1 disintegration per second (1 curie = 37 GBq. (Gigabecquerels)) (see ‘curie’). Beta Radiation: Ionising radiation consisting of very high velocity electrons emitted from the nucleus of a radioactive isotope. In beta emission a neutron converts to a proton while emitting a very high speed electron. Beta radiation has low penetrating power but must not be ignored when assessing radiation safety. Beta radiation causes severe skin burns and can lead to fatality. Betatron: An apparatus in which electrons are accelerated along a spiral path by means of the electric force associated with a varying magnetic field.
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Bunsen – Roscoe Law: The reciprocity law, which basically states that the film density produced by a dose of ionising radiation is independent of the radiation dose rate (i.e. a low dose rate for a long exposure time will produce the same film density as a high dose rate for a short exposure time so long as [dose rate] x [time] remains constant). Build-up: As ionising radiation is scattered the majority of the scattered radiation continues in approximately the same direction as the primary beam. When working with radiation shielding thicknesses amounting to several half value layers this causes the transmitted radiation intensity to be higher than would be expected based on the number of half value layers. The problem of build-up increases as radiation energy increases. Calcium Tungstate: A complex salt of calcium which fluoresces in the blue part of the visible light spectrum during exposure to ionising radiation. Calcium tungstate is the common base material for salt or fluorometallic intensifying screens. Cassette: A light proof container for holding an x-ray film during exposure. Cassettes may be rigid or flexible. They must be designed so as to maintain good contact between the x-ray film and intensifying screens. Casting: The formation, by pouring molten base material into a mould, of a useful product shape, or any component produced by such a process. The great majority of metallic components begin life as a casting. Cathode: The negative electrode of a discharge tube which usually consists of a heated tungsten filament. Characteristic Curve: The curve, for a given photographic film (or x-ray film) which relates the logarithm to the base of 10 of the relative amount of radiation exposure (i.e. radiation intensity x exposure time) to the achieved photographic density under specified processing conditions. Also known as Hunter & Driffield curve, H&D curve or sensiometric curve. The process by which a characteristic curve is produced is called sensiometry.
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The Welding Institute Compton Scattering: An important scattering mechanism in industrial radiography which is predominant for x-ray photon energies between 0.6 and 6 MeV. Constant Potential: A uni-directional voltage of constant (or nearly constant) magnitude Contrast: The difference in brightness to the human eye of two adjacent areas in a radiographic image. Curie: The unit of radioactivity for any radioactive isotope. A radioactive isotope with a source strength of 1 Curie is decaying at the rate of 3.7 x 1010 disintegrations per second. (abbreviation: Ci). The SI unit of radioactivity is the Becquerel. (see ‘Becquerel’) Curie-Hours or Curie-Minutes gamma ray exposures are usually expressed in curie hours or curie minutes because the intensity of radiation is proportional to the source strength and the total amount of radiation received is proportional to the source strength multiplied by the exposure time. Definition The degree of sharpness of delineation of image detail in a radiograph. (see ‘geometric unsharpness’, ‘inherent unsharpness’ and ‘penumbra’) Densitometer The instrument used for measuring radiographic density. (see ‘density’) Density The degree of darkness of a radiograph. This is expressed as the logarithm to the base 10 of the ratio of the intensity of incident light to intensity of the light transmitted through the film. Density Strip A strip of film exposed to form gradations of film density. Once calibrated using a suitable densitometer density strips form a convenient means of comparing film densities. Alternatively density strips of known density can be used to check the calibration of densitometers.
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The Welding Institute Developer The chemical solution used in the development of a radiographic film. All developers are reducing agents which reduce the sensitised silver halide grains in the film emulsion to metallic silver thereby producing an image on the film. Development The chemical process by which a latent image is converted to form a visible image. (see ‘developer’). Die Casting: A casting process for producing small to medium sized components, mainly applicable to low melting point alloys, involving the use of a reusable mould generally constructed from steel. In pressure die casting the molten charge is forced into the mould under pressure. Die castings have a better surface finish and better mechanical properties than equivalent sand castings. Dose Rate The total quantity of radiation energy per unit time. It usually is expressed in Sieverts or Rems per unit time. (see ‘sievert’ and ‘Rem’) Electromagnetic Radiation Light waves, radio waves and ionising radiation (x or gamma rays) are all forms of ‘electromagnetic radiation’. All electromagnetic radiation travels at the same velocity (299,274,000 metres per second in a vacuum), the different types of electromagnetic radiation differ only in their wavelengths. Electron: Very tiny, negatively charged fundamental particle. The mass of the electron is roughly 0.00054 times that of a proton. Electron Volt A unit of energy which is equal to the amount kinetic energy acquired by an electron when it is accelerated through a potential difference of one volt. (abbreviated as eV 1 keV = 1,000 eV, 1 MeV = 1,000,000 eV). Electron volts are a convenient unit for expressing ionising radiation energies. Emulsion Photographic emulsion; a suspension of photo-sensitive material (silver halide grains) in a matrix such as gelatine.
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The Welding Institute Exposure Chart A chart which relates the required radiographic exposure for a given radiographic film and film density to the penetrated thickness of a specified material. Filament (in an x-ray tube) The heated cathode which usually consists of a thin tungsten wire through which a heating current is passed in order to stimulate the thermionic emission of electrons. Film Badge A piece of photographic film used as a radiation monitor. Film badges are usually partially shielded so as to increase the effective measuring range. Film Contrast This is the degree to which a particular radiographic film when viewed by the human eye can differentiate between two adjacent areas of different radiation exposure. Film contrast is related to the change in film density per unit increase in radiation exposure. This increases with the achieved film density. Filter Material (usually thin copper sheet) interposed in the path of radiation in order to reduce selectively the intensity of radiation of a certain range of wavelengths or energies (usually the lower range of energies). Filters are useful for reducing the effect of scattered radiation in xradiography. Fixer The chemical solution containing principally sodium or ammonium thiosulphate which takes into solution the excess silver halides in a film emulsion which remain after the development process has been completed. Fixing The chemical removal of unused silver halides from an emulsion after development. (see ‘fixer’) Flaw Sensitivity The ability of a radiographic technique to detect flaws. This is not easy to quantify but is expressed as the minimum detectable thickness of a specific flaw measured in the direction of the radiation beam, expressed as a percentage of the total thickness of a specimen of specified homogeneous material.
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The Welding Institute Fluorescence The ability of certain chemical compounds to convert invisible incident radiation to a visible radiation emission. Calcium tungstate is useful in radiography because it fluoresces in the blue part of the light spectrum under the action of ionising radiation. Fluorescent Screen A suitably mounted layer of material (e.g. calcium tungstate, barium platino-cyanide or zinc sulphide) which fluoresces in the visible region of the spectrum under the action of ionising radiation: Fluorescent screens can be used to produce a radiographic image directly or to produce an intensifying effect when used in conjunction with radiographic film. Fluorometallic Screen An intensifying screen used in radiography which combines the scatter reducing properties of a lead screen with the image intensification properties of a salt screen. Focal Spot The area of the target on which the electron stream impinges and from which X-rays are emitted. The effective focal spot size in an x-ray tube is usually less than the actual focal spot size. Focus-to-film Distance: The distance from the focus of an x-ray tube to a film set up for radiographic exposure. (abbreviation: FFD). Gamma Radiation: Electromagnetic radiation emitted during the decay of some radioactive isotopes. Gamma photons are by-product of some alpha or beta decay events. All gamma emitters used in industrial radiography emit gamma as a by-product of beta decay. Gamma Radiography: Radiography by means of gamma rays. Gray The SI unit of absorbed dose.
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The Welding Institute Half-Life A fixed property of any radioactive isotope. Half-life is the time taken for the number of radioactive atoms in a given sample to reduce by half. It can be anything from a few seconds to millions of years dependent on the isotope. The decay process itself is random but the half-life is fixed because each radioactive atom has the same probability of decay – averaged over a very large number of radioactive atoms this gives rise to a fixed half-life. Half-value Thickness (half-value layer) The thickness of a specified substance which when introduced into the path of a given beam of radiation, reduces the radiation intensity by half. It may be used as an indication of the quality of the beam or the opacity of the substance. (abbreviation: HVL) Identification Marker A marker, usually of heavy material (lead), used to provide a reference point or identification mark in a radiograph. Image Intensifier A device used in fluoroscopy (a form of real time radiography), incorporating a vacuum tube in which electrons released by X-rays from a special type of screen, are accelerated and focused onto a fluorescent screen thus producing a brighter image than that produced directly using a fluorescent screen. Image Quality Indicator: Any device which gives an indication of radiographic quality. The commonest varieties are wire type, step hole type and plaque type. The quality (or sensitivity) of the image is defined as the smallest discernible wire diameter or step thickness expressed as a percentage of the total thickness. Inherent Fog Unwanted blackening of an emulsion caused by the development of grains which are inherently developable without exposure. This type of fog varies with the age of the emulsion and with conditions of storage. (see base fog level) Intensifying Factor The ratio of the exposure time without intensifying screens to that when screens are used, all other conditions being the same.
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The Welding Institute Intensifying Screen: A layer of suitable material, e.g. lead foil, which when placed in close contact with photographic emulsion, adds to the photographic effect of the incident radiation. Injection Moulding: Die casting process whereby molten raw material is forced into a mould under pressure. The process of injection moulding is common for plastics and low melting point metal alloys, especially those of zinc, magnesium and aluminium. Investment Casting: Also called the lost wax process. In investment casting a wax model of the required item is made and mould is formed around this using some type of refractory material. After firing to harden the mould and burn out the wax molten metal is poured to produce the desired component. Investment casting is an expensive method but it produces the best surface finish and the best material properties of all of the casting processes. Isotopes: Nuclides having the same atomic number but different mass number (i.e. the nucleus contains the same number of protons but a different number of neutrons). Some isotopes are stable while others undergo nuclear fission thus producing emissions of radiation. Latitude: The ability of a radiographic technique to display a wide range of material thickness at an acceptable film density. In general latitude will be reduced if a high contrast ultrafine grain film is used. Macroradiography: Radiography of thin sections of material in such a way that the resulting image may be enlarged to reveal microstructure. MAG: Metal active gas welding, often referred to as CO2 welding, mainly applicable to carbon steel. MAG is an automatic or semi-automatic arc welding process involving a reel fed consumable electrode; the arc is shielded by an active gas, usually carbon dioxide. Porosity and lack of sidewall fusion are common defects in the MAG process. MIG: Metal inert gas welding. MIG is an automatic or semi-automatic arc welding process involving a reel fed consumable electrode; the arc is shielded by an inert gas, usually argon. MIG welding is applicable to most metals and alloys. Porosity and lack of sidewall fusion are common defects in the MIG process.
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Milliampere hours, minutes or seconds: A measure of X-ray exposure expressed as the product of the milliammeter reading (i.e. tube current) and the of exposure time in hours, minutes or seconds. MMA: Manual metallic arc welding. An arc is struck between a flux coated consumable electrode and the work piece. The flux coating decomposes to form a shielding gas, usually carbon dioxide and a molten slag which protects the hot metal. Neutron: The fundamental particle having a mass slightly greater than 1 amu and zero electrical charge. Neutron Radiography: Neutrons emitted by nuclear reactors and some radioactive isotopes are a form of penetrating radiation and can be used to perform radiography. Neutrons are heavily absorbed by substances such as water or plastic which contain significant amounts of hydrogen. Neutrons pass easily through metals such as steel or aluminium. Neutron radiography is useful for the detection of water ingress into aeroplane wing structures and other similar applications. Pair Production: Pair production is the scattering mechanism which predominates at x-ray photon energies exceeding 6 MeV. Pair production does not occur until the threshold photon energy of 1.02 MeV is exceeded. Penumbra: The partial shadow extending beyond the edges of the main shadow (umbra) of an object due to the finite size of the radiation source: the width of this partial shadow. Photoelectric effect: An important scattering mechanism in industrial radiography which is predominant a radiation photon energies of below 0.6 MeV. Positron: Basically an electron but with opposite electrical charge. Positrons are emitted during pair production which is an important scattering mechanism in high energy x-radiography.
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The Welding Institute Pressure Mark A variation in photographic density caused by the application of local pressure to the emulsion; the mark may be light or dark according to circumstances. Primary Radiation Radiation which is incident on the absorber and which continues unaltered in photon energy and in direction after passing through the absorber. Processing A series of operations, such as developing, fixing and washing, associated with the conversion of a latent image into a stable, visible image. Quality Factor: In order to take account of the fact that for instance 1 Gy or 100 R of alpha radiation is biologically much more damaging than 1 Gy or 100 R of x or gamma radiation a quality factor is used. The dose in Sieverts or Rem is then equal to the dose in Grays or Rads multiplied by the quality factor, 1 Sievert or 100 Rem of any type of ionising radiation has the same biological effect. Rad: The old unit of radiation absorbed dose. The SI equivalent is the Gray. Radiograph A photographic image produced by a beam of penetrating ionising radiation which has passed through an object. Radiographic Contrast Contrast in radiograph; usually expressed in terms of density difference. The ability of the combination of a radiograph and the human eye to differentiate between two areas of different subject thickness. Radiographic contrast is the combined effect of subject contrast and film contrast. Radiographic Exposure The subjection of an emulsion to radiation for the purpose of producing a latent image; commonly expressed in milliampere-minutes or curie-hours.
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The Welding Institute Radiographic Range (latitude) The maximum range of thickness of a specified homogeneous material which can be recorded satisfactorily in a single radiograph with a specified technique. Reciprocity Law A law which states that, all other conditions remaining constant, the time of exposure required to produce a given density is inversely proportional to the intensity of the radiation. (see Bunsen-Roscoe Law) Rem Roentgen Equivalent Mammal: The old unit of man mammal equivalent absorbed radiation dose. 1 Roentgen of alpha radiation has a much greater biological effect than does 1 Roentgen of x or gamma rays whereas 1 Rem has the same biological effect whatever the type of ionising radiation. Despite the name the Rem is arrived at by multiplying the dose in rad (radiation absorbed dose) by a quality factor. The SI equivalent of this unit is the Sievert. Resolution The smallest distance between recognisable images on a film or screen. It may be expressed as the number of lines per millimetre which can be seen as discrete images. Reticulation An effect due to rupture of an emulsion coating, usually caused by a rapid change of temperature. It gives an appearance similar to the grain of leather. Rod-Anode Tube A type of unipolar (grounded anode) X-ray tube in which the target is situated near the end of a long tubular anode. Roentgen: The old unit of exposure or ionising effect. The SI equivalent is the coulomb per kilogram. Salt Screen An intensifying screen consisting of a material such as calcium tungstate, which fluoresces in the visible or ultra-violet region of the spectrum under the action of ionising radiation. Salt screens are seldom used in industrial radiography. Sand Casting: Casting process where a molten charge is poured into a mould formed from compressed sand. Sand casting is the most versatile of all the casting processes, but suffers from coarse grain structure and poor surface finish.
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SAW: Submerged arc welding. An automatic or semi-automatic arc welding process that offers deep penetration and a high deposition rate. A consumable wire is reel fed and the arc is struck under a layer of powdered flux. Scattering The redirection of radiation, with or without a change in photon energy (but usually with a reduction in photon energy), during its passage through matter. Screen-Type Film X-ray film designed for use with salt screens. It is sensitive to the fluorescent light emitted by such screens under the action of x-rays. Secondary Radiation Radiation, other than primary radiation, emerging from the absorber. Sievert The SI unit of Man/mammal equivalent dose. (abbreviation: Sv) Source-to-Film Distance The distance from a source of radiation to a film set up for a radiographic exposure. (abbreviation: SFD) Specific Activity The amount of radioactive material per unit mass of a sample. Usually expressed in Curies/gram. Speed The relative rate at which a photographic emulsion reacts to exposure to radiation. TIG: Tungsten inert gas welding. A manual or fully automatic arc welding process. TIG is extremely versatile but requires a high degree of operator skill. The heat source is an arc struck under a shield of argon or helium gas between a non-consumable tungsten electrode and the work piece. Filler wire may be fed into the arc, or in some circumstances a weld may be produced without the need for filler wire.
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The Welding Institute APPENDIX 1 IONISING RADIATION REGULATIONS
IONISING RADIATION REGULATIONS 1999 (SI 1999 N. 3232) – SUMMARY Item Details Requirement(s) 1) ALL AREAS WHERE 1) CONTROLLED AREA A controlled area is one to THE ANNUAL which access is restricted due to RADIATION DOSE IS the possibility of high radiation EXPECTED TO EXCEED dose rates. Controlled areas 6 mSv SHALL BE must have well signed physical DESIGNATED AS boundaries. Flashing lights CONTROLLED AREAS. shall be used to warn of an exposed source or energised x2) THE RADIATION DOSE ray tube. Flashing lights and a RATE AT THE clear audible warning shall be BOUNDARY OF A used to warn of imminent CONTROLLED AREA exposure of a source or SHALL NOT EXCEED energising of an x-ray tube. 7.5 Sv/h. Access to a controlled area is generally limited to classified personnel, but other persons may enter under strict supervision. 1) ALL AREAS WHERE 2) SUPERVISED AREA A supervised area is one in THE ANNUAL which the ionising radiation RADIATION DOSE IS dose is likely to significantly LIKELY TO EXCEED exceed natural background 1 mSv SHALL BE radiation levels. There are no DESIGNATED AS specific requirements for SUPERVISED AREAS. signposting supervised areas. Physical barriers are not required. Persons should not be 2) THE RADIATION DOSE RATE WITHIN A allowed to linger in, but they are SUPERVISED AREA not restricted from passing SHALL NOT EXCEED through a supervised area 7.5 Sv/h
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The Welding Institute APPENDIX 1 IONISING RADIATION REGULATIONS Item 3) CLASSIFICATION OF PERSONNEL
4) TRAINEES
Details Personnel who are likely to receive a significantly higher than normal ionising radiation dose in the course of their work duties should be classified. Classified personnel have their exposure to radiation monitored by an approved dosimetry service (e.g. the NRPB); in addition some employers operate their own internal dose monitoring schemes. Employers are required to investigate where the recorded dose of any classified worker exceeds 15 mSv in one year; employers are encouraged to set their own investigation level some way below 15 mSv per year. All classified personnel shall annually be declared fit by the appointed doctor. The appointed doctor may carry out a full medical or he may consider it sufficient to base this declaration on dose records alone. There is no specific requirement for blood tests; such things are at the discretion of the appointed doctor.
The annual dose of a trainee under the age of 18 must not exceed the stated limit of 6 mSv, but it is likely to be higher than that of a general member of the public. Typically a trainee would be spending a higher than average amount of time within a supervised area.
Requirement(s) 1) ALL PERSONNEL AGE 18 OR OVER WHO ARE LIKELY TO EXCEED AN IONISING RADIATION DOSE OF 6 mSv PER YEAR SHALL BE CLASSIFIED. 2) ALL RADIOGRAPHERS SHALL BE CLASSIFIED. 3) A PERSON UNDER THE AGE OF 18 CANNOT BE CLASSIFIED AND CANNOT WORK AS A RADIOGRAPHER. 4) CLASSIFIED PERSONNEL SHALL BE DECLARED FIT BY THE APPOINTED DOCTOR ON AN ANNUAL BASIS. 5) THE RECORDED WHOLE BODY DOSE OF A CLASSIFIED WORKER SHALL NOT EXCEED 20 mSv. 6) A FORMAL INVESTIGATION IS REQUIRED IF THE RECORDED WHOLE BODY DOSE FOR A CLASSIFIED PERSON EXCEEDS 15 mSv IN ANY PERIOD OF ONE YEAR. 1) THE ANNUAL IONISING RADIATION WHOLE BODY DOSE FOR A TRAINEE UNDER THE AGE OF 18 SHALL NOT EXCEED 6 mSv. 2) A PERSON UNDER THE AGE OF 18 CANNOT BE CLASSIFIED
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The Welding Institute APPENDIX 1 IONISING RADIATION REGULATIONS Item 5) GENERAL PUBLIC
Details Requirement(s) 1) THE ANNUAL IONISING In setting the dose limit for RADIATION WHOLE members of the public (and BODY DOSE FOR A other mammals) it is taken into MEMBER OF THE consideration that such GENERAL PUBLIC members of the public (or other SHALL NOT EXCEED mammals) could be pregnant. A 1 mSv. developing foetus is particularly sensitive to ionising radiation. 6) ANNUAL DOSE LIMITATIONS – SUMMARY (SI 1999 N0. 3232 SCHEDULE 4) DESCRIPTION CLASSIFIED TRAINEE GENERAL PUBLIC WORKER WHOLE BODY 20 mSv (1.) (2.) (3.) 6 mSv 1 mSv(4.) LENS OF THE EYE 150 mSv 50 mSv 15 mSv SKIN 500 mSv 150 mSv 50 mSv HANDS, FOREARMS, 500 mSv 150 mSv 50 mSv FEET, ANKLES NOTES 1) Where special circumstances apply – an employer is able to show that the annual limit of 20 mSv is impractical, up to 50 mSv can be received in a single calendar year but not more than 100 mSv over any five year period. Any employee exceeding these limits is likely to be suspended from work pending an investigation by the HSE. 2) Where person in question – note 1 above, is a pregnant female the dose shall not exceed 13 mSv in any period of 3 months. 3) Where the recorded dose exceeds 20 mSv in one year the employer is required to make a formal investigation to determine whether the dose limitations of note 1 are likely to be complied with. The employer must report the matter to the HSE and must put in place a program to ensure that the dose limitations of note 1 are not exceeded. 4) For persons who act as carers to others who receiving exposure to ionising radiation for medical purposes the dose limit is 5 mSv in any five year period.
7) ANNUAL DOSE LIMITATIONS – GENERAL. Item 8) RADIATION MONITORS
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The dose limitations, items 1 to 6 above are additional to exposure to radiation for medical purposes. Details Radiation monitors must be checked before use to ensure correct functioning; typically this would involve a battery check and a check to see that a reading is produced when the instrument is exposed to a source of ionising radiation. Radiation monitors must have a scale appropriate to the magnitude of the doses being measured. General good practice is to have portable monitors calibrated on an annual basis. Appropriate calibration periods can vary dependent on the type and usage of the radiation monitor in question.
Requirement(s) 1) RADIATION MONITORING EQUIPMENT SHALL BE PROPERLY MAINTAINED. 2) RADIATION MONITORING EQUIPMENT SHALL BE FIT FOR THE DESIGNATED PURPOSE. 3) RADIATION MONITORING EQUIPMENT SHALL BE CALIBRATED AT APPROPRIATE INTERVALS.
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The Welding Institute APPENDIX 1 IONISING RADIATION REGULATIONS Item 9) RADIATION PROTECTION ADVISER
10) RADIATION PROTECTION SUPERVISOR
11) APPOINTED DOCTOR
Details Usually a person, but may be an organisation, meeting the HSE’s criteria of competence. In general the RPA must be fully aware of the company’s activities involving ionising radiation. The local rules should be approved by, if not written by the RPA. The RPA is often called upon to undertake training of the radiation protection supervisors. A person appointed by the employer and named in the local rules to act as such. The RPS has in depth knowledge of the local rules. The RPS would generally take control where an emergency situation occurs and carry out initial investigation of any recorded or suspected overdose. General good practice requires the presence of at least one RPS where radiography is performed at an on-site location. A registered medical practitioner appointed in writing by the HSE.
12) APPROVED DOSIMETRY SERVICE
An organisation, approved in writing by the HSE, which monitors and records the ionising radiation doses of classified personnel.
13) MINIMUM NOTIFICATION PERIOD
Under most circumstances all work involving the use of ionising radiation must be notified to the HSE
Requirement(s) 1) ALL ORGANISATIONS WORKING WITH IONISING RADIATION SHALL APPOINT A RADIATION PROTECTION ADVISER.
1) ALL COMPANIES WORKING WITH IONISING RADIATION MUST APPOINT AT LEAST ONE RPS. 2) THE NAMES AND CONTACT DETAILS OF ALL RPS SHALL BE LISTED IN THE LOCAL RULES
1) ONCE A YEAR ALL CLASSIFIED PERSONNEL SHALL BE CERTIFIED FIT BY AN APPOINTED DOCTOR. 1) ALL COMPANIES EMPLOYING CLASSIFIED PERSONNEL MUST CONTRACT OUT THEIR DOSE MONITORING TO AN APPROVED DOSIMETRY SERVICE COMPANY. 1) THE MINIMUM NOTIFICATION PERIOD IS 28 DAYS PRIOR TO THE PLANNED COMMENCEMENT OF WORK.
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The Welding Institute APPENDIX 1 IONISING RADIATION REGULATIONS Item 14) LOCAL RULES
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Details The local rules are employer specific and describe in detail how the employer will control ionising radiation work such that the ionising radiation regulations are fully complied with.
Requirement(s) 1) A COPY OF THE LOCAL RULES MUST BE PRESENT AT ALL WORK SITES. 2) THE LOCAL RULES MUST BE REGULARLY UPDATED TO REFLECT REGULATORY CHANGES, CHANGES IN WORKING CONDITIONS AND CHANGES IN PERSONNEL.
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