Radiography Presentation

Interaction of radiation with matter : When an x or gamma ray photon passes through a material, it looses energy as the photon may interact with the atomic electrons or nucleus of the object. The probability of interaction increases, if the photon is of lower energy and it has to pass thro ugh a large number of atoms of high atomic number. This energy loss reduces it’s ability to ionize and the intensity of radiation decreases. This process is known as attenuation. Attenuation depends on, the photon energy and the atomic number, material density and the thickness of the object in the path of radiation. Three major processes make up the total attenuation. In the Photo electric absorption process, the photon looses all of its energy to an atomic electron. The electron then leaves the orbit of the atom and moves through the material at high speed. This process occurs in steel with most of the low energy photons of 0.1 mev or less. As photon energy increases above 0.1 mev, probability of photoelectric absorption in steel decreases and rarely happens to photons with energies of 1 mev and higher. With high atomic number materials such as lead, photo electric absorption is dominant up to 0.5 mev. In the Compton scattering process the incident photon is deflected from its original line of travel. It loses some of its energy due to interaction with free electron in outer orbits and continues passing through the material in the new direction as a low energy photon The atomic electron involved in the interaction is ejected from its bound position. This loss of energy increases the probabil ity of further compton scattering and photo electric absorption. For photons of 0.1 to 10 mev, compton scattering is the major attenuation process. Compton scattering does not depend on atomic number, so a high intensity of scattered radiation can emanate from the object being radiographed and any other material that the radiation beam happens to strike. The third process Pair production, in which the photon is completely absorbed and an electron - positron particle pair is created, has a threshold energy of 1.02 mev, and becomes significant for high energy radiation above 4 mev. The created positron annihilates with simultaneous emission of two 0.51 mev photons, moving in different directions. The total attenuation is a combination of all three processes, plus some secondary processes such as generation of secondary x - rays within an object by slowing down process of ejected e lectrons. The absorption of x and gamma radiation through thickness of absorber follows an exponential pattern. hence transmitted intensity is expressed as, I = I0 e or

I = I0 e

- t - .693 t / HVT

where  = .693 / HVT HVT = half value thickness

 = linear absorption co-eficient , per cm of the material. I = radiation intensity after transmission through the material thickness t. [ excluding scatter build up ] I 0 = radiation intensity before transmission. HVT is the thickness of the material required to absorb half of the radiation. The reduction of radiation intensity is usually calculated from the number of half value thickness in the absorber, because the value of  is constant only for a single wavelength or energy. The intensity loss factor is, number of HVT LF = 2 material thickness / HVT for the material

= 2 Transmitted intensity = Initial intensity / LF.

Calculate intensity of transmitted radiation through 40 mm steel for an incident radiation intensity of 2000 mR / hr, produced by Ir 192 source ; Loss factor = 2 LF = 2

number of HVT

40 / 12.5

[ HVT steel = 12.5 ]

3.2

or, 2 = 9.2 Hence, intens ity is 2000 / 9.2 or 217.4 mR / hr. HVTmm

Steel

Lead

Uranium Concrete

----------------------------------------------------------100 kev 200 kev 300 kev Ir 192 Co 60

the radiation intensity to half of its initial value.

1.2 3.8 6.5 12.5 20

.25 .42 1.7 4.8 12.5

3 7

16 26 30 44, 2.7g / cc 66

Inverse Square law : Radiation intensity of X or gamma rays is measured by the rate of ionization [ knocked out electrons ] or charge produced by the radiation in a specified volume of air at a given location. X or gamma radiation diverge [ spread out ] after emission from their source [ usually a small area ] and covers a larger area as the distance from their origin increases. This property reduces the ionization per unit area with increase in distance from the source. The intensity of radiation varies inversely with the square of the distance from the source. I.e. if the radiation intensity is 100 mR / hr at 1 meter distance, then the intensity reduces to 25 mR / hr at 2 meter distance. The inverse square law is expressed as

I 1D 1

2

= I2 D2

2

where, I 1 = radiation intensity at distance D 1 I 2 = radiation intensity at distance D 2 Radiation intensity at different locations can be determined with reference to the intensity produced by the involved radioisotope at 1 meter distance. Radiation intensity, produced by a radioisotope at 1 meter is, Ci X RHM in Roentgen / hr. [ from energies between 70 kev to 3 mev ] RHM is the radiation intensity in air, produced by a 1 curie radioisotope in Roentgen per Hour at 1 Meter distance from the source. Roentgen is the quantity of radiation required to produce 1 electro static unit charge in 1 cc air at standard temperature and pressure [ 0.001293 gms of dry air ]. Total Intensity of radiation emitted by a radioisotope depends on the size of the source, because radiation is absorbed in the material of the source. Exposure = Radiation intensity X time.

Calculate the radiation intensity at 15 meter distance from a 10 Curie Ir 192 source ? Radiation intensity at 1 meter is, Ci X RHM or 10 X 500 = 5000 mR / hr So, I 1 = 5000 mR/hr D 1 = 1 meter I2 = ? D 2 = 15 meter or 5000 x 1 x 1 = I 2 x 15 x 15 I 2 = 5000 / 15 x 15 = 5000 / 225 = 22.22 mR / hr. Calculate the distance, where the intensity reduces to 2 mR / h r from a 10 curie Co 60 source ? Radiation intensity at 1 meter is, Ci X RHM or 10 X 1320 = 13200 mR / hr so, I 1 = 13200 mR / hr D 1 = 1 meter I 2 = 2 mR / hr D2 = ? or 13200 X 1 X 1 = 2 X D 2 X D 2 D 2 = Square Root of ( 13200 / 2 ) = 81.24 meters from the source.

If the radiation field at one meter distance is one unit area, then it spreads to four unit area at two meter distance, nine unit area at three meter distance…..

Radioisotope

RHM

Thulium 170 Ytterbium 169 Selenium 75 Cesium 137 Iridium 192 Cobalt 60

3 125 203 340 470 1320

mR / mR / mR / mR / mR / mR /

hr hr hr hr hr hr

Half life of Radioisotopes : All radioactive atoms finally disintegrate and becomes stable atoms. Stable atoms do not emit radiation. As a result there is a continuous loss of radioactive atoms in the source and the number of disintegrating atoms per second also decreases. The number of disintegrating atoms per second determines the activity of the source [ emission of radiation ] and with time the activity of the source decreases. Each radioisotope loses activity at its own unique rate which cannot be altered. A useful measure of this rate is the half life of the radioisotope. The half life, T is defined as the time required for the activity of any particular radioisotope to decrease to one-half of its initial activity. Thus a 20 Ci source will become 10 Ci after one half life, 5 Ci after two half lives, 2.5 Ci after three half lives and so on. After one half life the intensity of radiation emitted by the radioisotope will fall to one-half of its initial level. The rate of disintegration of the radioactive atoms follow an exponential pattern. hence it is expressed as, - t

N = N0 e - .693 t / T or N = N0 e

Half life of radioisotopes used in industrial radiography : Thulium 170 Ytterbium 169 Iridium 192 Cesium 137 Cobalt 60 Sodium 24 Selenium 75

128 32 74.3 30 5.27 14.9 119.8

days days days years years hours days

where  = .693 / T T = half life

 = disintegration constant of the isotope. N = number of remaining radioactive atoms after elapsed time t. N0 = initial number of radioactive atoms when the time t begins. Hence, Source activity can be calculated as

act present = act initial e

-.693 x days elapsed / T

The loss of activity of a radioisotope can also be calculated from the number of half lives elapsed. The activity loss factor LF is, no of half lives

LF = 2 days elapsed / half life period = 2 present activity = initial activity / LF.

Preparing a remaining activity table : Ir 192 loses 1 % activity per day, For example initial activity 52 Ci. Day 0 1 2 3 4 5 6

-------------52 X .99 49.4 X .99 46.9 X .99 44.5 X .99 42.3 X .99 40.2 X .99

52.0 Ci 49.4 46.9 44.5 42.3 40.2 38.2 Ci and so on .

Co 60 loses 2 % in 2 months. [ X .98 ] example initial activity 52 Ci. Day

0 60 120 180 240

------------52 X .98 50.9 X .98 49,9 X .98 48.9 X .98

52.0 Ci 50.96 49.94 48.94 47.96 Ci ------

Calculate the remaining activity of a 32 Curie Ir 192 radioisotope after 120 days. Half life of Ir 192 is 74 days. Loss factor = 2 =

2

days / half life 120 / 74 1.62

= 2 or 1 / 3.07 hence activity after 120 days 32 / 3.07 = 10.42 Ci. or

- .693 X 120 / 74

Ci = 32 e = 10.4 Ci.

Industrial Radioisotopes : The image contrast and flaw detection sensitivity depends on the energy used for the material and its thickness to be radiographed. Selection of suitable energy is one of the most important step in industrial radiography to achieve the desired sensitivity. Thulium 170 : Thulium is an expensive rare earth element. Thulium 170 is produced by neutron bombardment of thulium 169. The radioisotope decays to ytterbium 170 by emitting two beta particles [ 0.968, 0.884 mev ] and two gamma rays [ 0.084, 0.052 mev ]. It is a low energy radioisotope, useful for radiography of lower atomic number or low thickness medium atomic number materials. With a small physical source size and a thin specimen the radiographic image is produced by the gamma and x rays [ generated by the absorbtion of high energy beta particles of peak energy 0.9 Mev, in the test specimen ] and the radiograph will be good. For larger source size and thicker s pecimen the low energy gamma rays are absorbed and the radiograph is produced by the x rays only with a resultant loss of image quality. Result in steel above 10 mm thickness is poor. Containers of these sources are very small and light weight. Ytterbium 169 : Produced by bombarding enriched ytterbium. Disintegrates by electron capture to Thulium 169. Sources are constructed using a spherical ceramic pellet of ytterbium oxide in a welded titanium capsule. A 1curie source can be produced in a .3 - .5 mm spherical pellet, making it suitable for panoramic exposure in small - bore steel tubes. Yttebium 169 is a low energy radioisotope which can produce x ray quality radiographs on thin steel specimens. This element is not available in enough quantity and short half life make the source a very costly product. Selenium 75 : Selenium -75 is now generally acknowledged throughout the world to provide performance benefits relative to Iridium -192 in the working range of 5 –30 mm steel. Selenium -75 has a softer gamma ray spectrum than Iridium -192 and it has a significantly longer half life. The isotopes are produced in ceramic pellets and suitable lighter weight projectors are available. Iridium 192 : Produced by neutron bombardment of Iridium 191. The radioisotope disintegrates to platinum 192 and Osmium 192 and gives out beta particle and medium energy gamma rays. Its relatively lower energy radiation, high specific activity and reasonable activation time combine to make it an easily shielded powerful radiation source of small physical size. This is the most used radioisotope. It is available as encapsulated metallic pellets in medium weight, depleted uranium shielded, remotely operated portable projectors. Cobalt 60 : Produced by neutron bombardment of Nickel plated Cobalt 59 pellets. The radioisotope disintegrates to Nickel 60 and emits a beta particle and high energy gamma rays. Have a long half life, and takes a long time to activate [ 3 years ] Because of its higher energy, the isotope is suitable only for thick sections. However, It can be used for radiography of Steel, Copper and other medium weight metals of thickness ranging from 25 mm to 200 mm. This isotope is preferred for radiography of steel above 50 mm for shorter exposure time. The contrast of the radiographic image is poor when compared to iridium and more difficult to interpret. High energy of radiation requires heavy shielding. Cobalt 60 equipments are very heavy, costly and less mobile.

Thulium 170 properties : Half life : 128.6 days. Energy : 52, 84 kev gamma rays and .91 mev beta particle. Equivalent to 150 kev X-rays. RHM : 3 mR / hr / Ci at 1 meter distance. Suitable for radiography of 2 to 12 mm Steel or equivalent. Optimum range in Steel 2 to 10 mm Ytterbium 169 properties : Half life : 32 days. Energy : 63 to 308 kev. Equivalent to 200 kev X-rays. RHM : 125 mR / hr / Ci at 1 meter distance. Suitable for radiography of 1 to 15 mm Steel or equivalent. Optimum range in steel 2 to 12 mm. Selenium 75 properties : Half life : 120 days Energy : 66 to 401 kev RHM : 203 mR / hr / Ci at 1 meter. Suitable for radiography of 10 to 40 mm Steel or equivalent. Optimum range in steel 14 to 40 mm. Iridium 192 properties : Half life : 74 days. Energy : 206 to 612 kev. Avg energy : 400 kev. Equivalent to 600 kev X-rays. RHM : 470 mR / hr / Ci at 1 meter distance. Suitable for radiography of 6 to 75 mm Steel or equivalent. Optimum result in Steel 19 to 65 mm. Cobalt 60 properties : Half life : 5.27 Years. Energy : 1.17 and 1.33 mev. Avg energy : 1200 kev. Equivalent to 2 mev X-rays. RHM : 1.32 R / hr / Ci at 1 meter distance. Suitable for radiography of 35 to 200 mm Steel or equivalent. Optimum result in Steel 60 to 150 mm. Light alloys 150 to 450 mm.

Film preference with Ir 192 for steel : AA400, IX100, D7, R7 T200, IX80, D5, R5 MX125, IX50, D4, R4 M, IX25, D2, R2

.6” and above .4 to .6” .2 to .4” up to .2 “

Industrial X Ray sources : X - rays are electrically generated radiation and the energy of the emitted radiation can be controlled. X rays are produced when electrons traveling at high speed looses energy by collision with matter or change of direction. The usual type of x ray tube is a glass, ceramic or metal ceramic housing, where the spacing of electrodes and the degree of vacuum is such that no flow of electrical charge between the cathode and anode is possible until the filament is heated. The filament when heated with a suitable low voltage controlled supply, emits electrons and thus forms the cathode or negative electrode. The positive electrode is a solid block of copper with a tunnel at one end and a piece of tungsten embedded on the inside face of the tunnel end. The tungsten is the target or focal spot. A controlled high voltage is applied between these electrodes, drives the electrons rapidly towards the target. The sudden stopping of these rapidly moving electrons in the surface of the tungsten target results in generation of x rays. Much of the energy appears in the form of heat and intense heat is produced on the target. Many of the electrons knock out orbital electrons from the target atoms while others get deflected by the positive charge of the nucleus. These actions generate characteristics and continuous x-rays respectively. The continuous spectrum generates sufficient energy to penetrate materials and form the x-ray image. The machine is generally used for a shorter ‘on‘ time followed by sufficient ‘off ’ time for air cooling. The machine cannot be operated continuously without efficient cooling arrangement and is generally achieved by water circulation. The generated x rays have all the energies within the emitted spectrum. The lower energies contribute greatly towards image contrast and sensitivity a nd the radiograph is superior when compared to gamma rays. Sensitivity of 1% can be achieved in most cases. The intensity of radiation [ mR / min ] mainly depends on the number of electrons emitted by the heated filament and is controlled by the current passing through it. The energy of radiation [ keV ] depends on the voltage applied between the anode and the cathode and is selected considering the material, its thickness and radiographic sensitivity requirements. Machines most commonly used are 160 to 400 kV [ Tank type ] with 4 to 8 mA beam current within the portable range and 2 to 6 meV and more [ Betarons and Linear accelerators ] in the high energy range. Portable machines generate either directional or panoramic beam. Most of the machines are not suitable for working at heights and with all types of exposure setups. Linear Accelerators and Betatrons uses different tube arrangements and methods of electron acceleration. The heat generation is also less and the JME 6 mev betatron can operate without external cooling. The high energy machines are heavy, very costly and the energy of radiation cannot be adjusted continuously.

Suitable X ray energy range for Steel and equivalent Steel thickness : Inch Kev for 2% sensitivity .1 100 to 125 .2 100 to 160 .3 120 to 190 .4 140 to 225 .5 155 to 250 .6 165 to 275 .7 170 to 330 .8 180 to 420 .9 190 to 580 1 230 to 1000 1.25 230 to 2000

Source to film distance : In radiography, the most important requirement is to obtain the best possible image, an image with high contrast, low graininess and optimum sharpness. Unsharpness in a radiograph can be defined as the blurring of image edges, with a resultant loss of definition of fine cracks, LF, image detail and penetrameter detail. Unsharpness may produce gradual change in density with complete loss of image outline. The main causes of unsharpness are, geometric, film / screen, scatter and movement of source, object or film. As the film is located on the backside of the object, images of all internal conditions will be projected and magnified. Discontinuities located near the back of the object and closer to the fi lm have very little projection and magnification and images will be sharper than the others. Because of the physics of image formation with a non point source of radiation, the sharpness of all images depends on the amount of projection involved in the specific exposure arrangement. Images of small voids, cracks, penetrameter etc, located well above the film plane may be totally unsharp. Considering the available source size, a sharp image can be produced by selecting appropriate source to film distance, object to film distance and alignment of the radiation beam with respect to the plane of the object and the film. For a given radiography set up, Geometric unsharpness can be equated as, Ug = F X T / ( SFD -T ) or, SFD = { ( F X T ) / Ug } + T , the required source to film distance to limit a certain Ug. where, Ug unsharpness [ width of the shadow zone ]. F effective source size. T thickness [ height ] of the object. SFD source to film distance. From this equation minimum usable SFD can be found. Actual selected SFD must also satisfy the conditions to reduce angular image distortion. In practice, Ug can never be zero. Film unsharpness is also recorded. Ug can only be minimized by controlling exposure setup. Considering this fact, various codes permit a certain value of Ug depending on the thickness to be radiographed. Experience shows that sharper image can be obtained with SFD of 18" or more. SFD below 2" does not produce good image. Longer SFD increases exposure time. To produce high contrast and sharp image, the following conditions apply ; Source size should be very small. SFD should be as large as practicable. The film should be in close contact with the intensifying screens and the object and its recording plane perpendicular to the radiation beam. Source location should be such that radiation pass normal through the object thickness. Use of fine grain film and lower radiation energy to acheive high contrast. Control of forward and back scatter.

ASME V / Article 2 permissible Ug : Object thk Ug under 2 in .020'' 2 to 3 in .030'' >3 to 4 in .040'' above 4 in .070'' Calculate the minimum SFD required to radiograph a 40 mm thick weld, if the source size is 4 mm and permissible Ug is .5 mm ? F = 4 mm, T = 40 mm and Ug = .5 mm SFD = {( F x T ) / Ug } + T = {( 4 x 40 ) / .5 } + 40 = 160 / .5 + 40 = 320 + 40 = 360. Minimum SFD for this condition is 360 mm . [ Standard minimum SFD, 10 x Thickness ]

Unsharp

Sharp

Films : A thin, flexible, optically clear, polyester base, which remains flat is coated with Silver Halide - Gelatin emulsion using a binder. Another plain layer of gelatin protects the sensitive emulsion. Usually, the emulsion is coated on both the sides of the base, which records two composite images with a resultant increase in film density and speed of exposure. Some of the films have emulsion on one side only, which produces sharp image at the expense of longer exposure time and is ideal for optical image enlargements. When radiation or light strike the grains of the sensitive emulsion, a change takes place in the physical structure of the grains which can be made visible by treatment with a chemical solution known as developer. In the developer, a reaction takes place, causing formation of black metallic silver. The distribution of this silver particles creates an image on the film. Radiographic films are available in different sizes in variou s packages such as Box, Daylight, Roll, Vacuum pack etc. Boxes offer bare films in different sizes, pre cut or to be cut to size, and may have interleaving papers which separate each piece of film. The Interleaving paper is removed before the film is loaded into a reusable film holder. Interleaving paper is useful in separating the finished radiographs, protecting the films against scratches and dirt during handling and storage and provides a convenient place for writing notes and comments about the radiograph. Daylight and Roll pack films are a form in which each sheet of film is enclosed in a light-tight envelope and may have integrated thin lead screens . The film can be exposed from any side without removing it from the envelope. A rip strip makes it easy to remove the film in the darkroom for processing. These films eliminate the process of film loading in the darkroom and long length of roll film is a great advantage when setting up a panoramic exposure. Long films usually make use of an automatic film processor. Films must be handled very carefully to avoid pressure, creasing, buckling, friction etc. Films must also be protected from light, radiation, chemical vapors, high temperature and high humidity. During winters, films should not be drawn rapidly from its holders which may produce electrostatic discharge and exposure marks. Bulky parts must not be placed directly on the cassette during exposure. Unexposed films should be stored vertically at temperature 10 0 to 0 0 22 C. Exposure to temperature up to 35 C for 1 – 2 weeks will not have any significant effect on the characteristics of the film but long periods at high temperatures will accelerate fogging. Processing steps : Developing, Stop bath, Fixing and Washing. Developing : Exposed films are locked in stainless steel hangers, and immersed in the developer for 5 minutes at 20 0C. Immediately after immersion the hangers are tapped to dislodge air bubbles clinging to the surface of the films. The developer is then agitated by shaking the hangers for 10 seconds in every minute to maintain uniform developing action. Continuous agitation significantly reduces developing time. Developing time depends on solution temperature and concentration and must be corrected as required. By temperature control and replenishment of the developer, constant developing time can be maintained. Stop bath : developer solution is alkaline where as fixer is acidic. Hence developed films are washed in 2% acetic acid solution or water to remove traces of developer remaining on the films before they are immersed in the fixer solution. Fixing : The films are immersed in the fixer solution to make the developed image stable by removing unexposed silver halide grains by fixing action. Recommended fixing time is twice the clearing time and films can be left in the fixer for up to 15 minutes. Washing : following fixing, the films are washed in running water for 15 minutes to remove all traces of fixer from the surface. Washing time can be reduced by using suitable chemicals. Improperly washed films become brown with age. Removing water marks : Water droplets remaining on the surface of the films after washing can be removed by dipping the films in a wetting agent solution before drying. Drying : Films are finally dried uniformly by evaporation or by circulating hot air in a temperature controlled drying cabinet. Pre exposed film strips are available from the manufactu rers which can be used to monitor the performance of the processing system.

Process Marks : Film marks resulting from contact with contaminated fingers or chemical droplets can be eliminated by maintaining a clean dry workstation. Cleaning towels should be kept close at hand for frequent cleaning. Film handling and processing area should be opposite to each other. Film types : [ for information purpose only ] D2 / R2 : Low speed, high contrast, extremely fine grain for excellent image sharpness, can be used with or without lead screen. Suitable for thin-wall steel, lower density materials, plastics, composites etc. D4 / R4 : Medium speed, high contrast, extremely fine grain for good image quality, can be used with or without lead screen. Suitable for thin-wall steel, D5 / R5 : Standard speed, high contrast, very fine grain for good image quality, can be used with or without lead screen. For thin / medium thick steel. D7 / R7 : Higher speed, high contrast, fine grain, can be used with or without lead screen. Suitable for thicker parts with gamma and thinner with x - rays. This is the most used film for common applications. D8 / R8 : Very high speed, high contrast, medium grain, can be used with or without lead screen. Suitable for thick steel, cast parts with non uniform thickness etc. D10 / R10 : Very high speed, medium contrast, medium grain, can be used with or without lead screen. Suitable for thick steel, parts with non uniform thickness

Image quality indicators [ IQI ] : The image quality indicator or penetrameter is a device whose image in a radiograph is used to determine image quality recorded on the radiograph. It is used to determine radiographic sensitivity and image sharpness and is not intended for use in judging th e size or in establishing acceptance limits of the recorded discontinuities. Unless otherwise permitted, the penetrameter should always be placed on the ''source side'' of the test part. When it is not practical to do so, as in the case of double wall single image radiography, it can be placed on the film side with a lead letter " F " on or near the penetrameter. The sensitivity indicating hole must not be obstructed. The placement of the penetrameter should be in the most unfavorable location of the object with respect to the radiation beam. Penetrameters are available in different material compositions. The type to be used must have similar or lesser radiation absorption as the material being tested. Selecting a penetrameter to demonstrate 2 % sensitivity is a common practice. ASTM Hole type penetrameter : Hole type IQI consists of a small, uniform thickness rectangul ar strip of material with three drilled holes. The three hole diameters are equal to 1 T, 2 T and 4 T, T being the thickness of the strip. The identification lead numbers on the penetrameter indicates thickness of the s trip in mili-Inches [ 1 / 1000 inch ]. Sensitivity is described in terms of the thickness percent 1, 2 or 4, and the required hole size that must be recorded in the image on the film. For example, 2 - 2 T is the standard way to describe the use of a penetrameter whose thickness must be 2 % of the object thickness and whose 2 T hole must be recorded on the film. 2 - 2T is equivalent to 2 % sensitivity. Hole type penetrameter is generally placed parallel to the weld with the 2T hole towards the outside of the beam and in the worst radiographic position. The clearance between the weld edge and the penetrameter should be from 1 / 8” minimum to 1.5” maximum. It is generally required that the material under the hole penetrameter be equal to the thickness of the material being radiographed. The penetrameter should be raised to the height, equal to the maximum thickness being radiographed. If the reinforcement in a weld is not removed, a shim [ larger than IQI ] of similar material and suitable thickness can be placed under the penetrameter to equalize the thickness difference. The reinforcement should be included while selecting the penetrameter. Image density variation to be controlled within -15% [ weld area ] to +30% [ base metal ] with respect to the density through the body of the hole penetrameter [ reference ]. Alternatively two different penetrameters, one for the weld and the other for the base material should be used and the 2T holes to be recorded. If density difference cannot be controlled, then energy of radiation may be increased or filtered to reduce subject contrast. Hole penetrameters are usually selected from tables given in applicable procedures. The tabulated penetrameter may be permitted to be replaced with another penetrameter of equivalent sensitivity. A [ 5 – 4T ] requirement can be met with [ 10 – 2T ] recording.

Standard IQI thk / Nos : ASME 5 7 10 12 20 25 30 35 50 60 70 80 140 160 200 240 IQI size : up to 59 : 60 --179 : 200 -- 280 :

V / SA 1025 15 17 40 45 100 120 280 .5 X 1.5" 1 X 2.25" 4T round with 1T, 2T hole

available in material types : Magnesium for Magnesium alloys Aluminum for Aluminium alloys Titanium for Titanium alloys Steel for Cast Iron, CS, AS, SS Aluminium Bronze Nickel – Chromium Inconnel and similar Nickel – Copper Monel and similar Tin Bronze. [ material type identified by V- Notches on the body of the strip ] Quality level Equivalent sensitivity 1 - 1T .7 % 1 - 2T 1% 2 - 1T 1.4 % 2 - 2T 2 % [ standard ] Equivalent sensitivity equation : S =

2% IQI ; Job thickness in inches X 20 Job thickness in mm X 0.8 IQI selection table as per Weld Thk inch Upto .25 Over .25 thr .375 Over .375 thr .50 Over .5 thr .75 Over .75 thr 1.00 Over 1.00 thr 1.50 Over 1.50 thr 2.00 Over 2.00 thr 2.50 Over 2.50 thr 4.00 Over 4.00 thr 6.00 Over 6.00 thr 8.00

ASME V / Art 2. Source side Film side 12 10 15 12 17 15 20 17 25 20 30 25 35 30 40 35 50 40 60 50 80 60 Essential hole 2T

Equivalent penetrameter 2T 1T 4T 10 15 5 | 12 17 7 | 15 20 10 | 17 25 12 | 20 30 15 | 25 35 17 | 30 40 20 | 35 50 25 |

sensitivity : 2T 1T 40 60 50 70 60 80 80 120 100 140 120 160 160 240 200 280

4T 30 35 40 60 70 80 120 140

Wire Penetrameters : The wire type penetrameters consist of wires of various thickness [ diameter ] which are mounted side by side parallel with a distance between the axis of wires of not less than three times the wire diameter and not less than 5 mm. Minimum length of wire is 25 mm. The wires are to be placed across the weld [ 90 0 ] with the thinnest wires towards the outside of the radiation beam and in the worst radiographic position of the test part. Shim is not required for this penetrameter. The image of the thinnest wire visible on the area of interest indicates radiographic sensitivity. ASTM wire penetrameters : These penetrameters consist of 21 wires [ no 1 to no 21 ] of different diameters, packaged in 4 transparent plastic envelopes [ set A, B, C and D ] with necessary identification symbols. Each set has 6 wires, with the last wire of the previous set being repeated as the first wire of the next set. The diameter ratio between two successive wires is 1 : 1.25. These penetrameters are available in eight material compositions and 1 or 2 inch wire length. DIN wire penetrameters : The German, DIN wire type penetrameter comes in 16 sizes, from 3.2 mm [ No 1 ] to 0.10 mm [ No 16 ]. The wires are 50 mm long and distributed in three sets. Each set has 7 parallel wires, placed 5 mm or more apart and sealed firmly in a flexible plastic package with identification numbers. The diameter ratio between the adjacent wires is 1 : 1.25. These penetrameters are generally selected to indicate 2 % sensitivity. The sets are, 10 ISO 16 for 5 to 20 mm thickness. 6 ISO 12 12.5 to 50 mm. 1 ISO 7 40 to 160 mm. Available materials : aluminium, steel and copper.

ASTM wire diameter in Inches : Set A Set B Set C 1- .0032 6 - .010 11 - .032 2 - .004 7 - .013 12 - .040 3 - .005 8 - .016 13 - .050 4 - .0063 9 - .020 14 - .063 5 - .008 10 - .025 15 - .080 6 - .010 11 - .032 16 - .100 Identification : 03 Magnesium 02 Aluminum 01 Titanium 1 Steel

2 3 4 5

Set D 16 - .100 17 - .126 18 - .160 19 - .200 20 - .250 21 - .320

Aluminum Bronze Nickel Chromium Nickel Copper Tin Bronze

Wire length, 1 or 2 inches. [ See ASME V / SA 747 for details ]

Sensitivity [ S ] for all wire type penetrameter is calculated using the equation, smallest wire size visible X 100 S = ----------------------------------------------object thickness

Recomended Wire ASME for weld examination : Thickness in inch Upto .25 inclu Over .25 thr .375 Over .375 thr .50 Over .50 thr .75 Over .75 thr 1 Over 1 thr 1.5 Over 1.5 thr 2 Over 2 thr 4 Over 4 thr 6 Over 6 thr 8

V / Article 2 Source side .008 -5 .010 -6 .013 -7 .016 -8 .020 -9 .025 -10 .032 -11 .040 -12 .050 -13 .060 -14

Film side .006 -4 .008 -5 .010 -6 .013 -7 .016 -8 .020 -9 .025 -10 .032 -11 .040 -12 .050 -13

DIN penetrameter set:

Calculate 2 % wire in ASTM set for 1.25 inch and 42 mm thick welds respectively ; 1.25 “ / 100 = .0125”  1 % wire size. .0125” X 2 = .025”  2 % wire size. 5 th wire from left, in the Set B penetrameter. Thk 42 mm = 42 / 25 or 1.68 inch [ 1 inch = 25 mm ] 1.68” / 100 = .0168”  1 % wire .0168” X 2 = .033”  2 % wire. Set B 6 th or in Set C 1 st wire from left side. [ 2 % wire = Job thickness / 50 ]

1-7 3.2 2.64 2.0 1.6 1.32 1.0 0.8

6 - 12 1.0 0.8 0.66 0.5 0.4 0.32 0.25

10 - 16 0.4 0.32 0.25 0.2 0.165 0.15 0.1 mm

Gamma ray exposure time : To radiograph an object, the film is exposed to a pre determined dose of radiation [ film factor ] and then developed under standard conditions of temperature and time to achieve the target density [ blackness ]. An uniform object produces a single density because of nearly uniform radiation transmission. Each structural element, inclusion, void, and crack in the object uniquely alters the radiation passing through it. Different absorption takes place, the exposure to each area of the film varies accordingly, and film density differences in the image occur in the developed film. These areas, where blackness differs from the adjacent area, are interpreted for presence of discontinuities. The radiation exposure for a target density of 2 is known as the film factor for a particular brand of film. Hence, the intensity of radiation on the surface of the film, after passing through the object is to be determined. Dividing the film factor by this intensity determines the required exposure time for density 2. Exposure = Radiation Intensity X Time. Calculation of exposure time ; Step 1 Calculate radiation intensity at 1 meter from the source [ Ci X RHM ] Step 2 Calculate radiation intensity at the film distance 2 2 using I1 D1 = I2 D2 2 or Ci X RHM / SFD [ SFD in meter ] ------ ( x ) Step 3 ; Find out radiation absorption by the object. Thk / Hvt

2 --------------------------------- ( y ) Step 4 ; Divide radiation intensity by absorption value [ x/y ] --------------------------------------------------- ( z ) Step 5 Divide film factor by this radiation intensity ( z ). Exp time in hours = [ FF / z ] Example : Calculate exposure time for D7 film [ Film factor 1200 mR ] Source Ir 192 - 12 curie, RHM 500 mR / Ci. Material 40 mm steel, hvt 12.5 mm. Source to film distance 30 cm. Step 1 Step 2

12 x 500 = 6000 mR / hr 2 2 6000 x 100 = I2 x 30 or 66666.66 mR / hr 40 / 12.5 Step 3 2 or 9.18 Step 4 66666.66 / 9.18 or 7262 mr / hr or 7262 / 60, 121 mR / minute Step 5 1200 / 121 or 9.91 minutes exposure time. Exposure equation : Exp Time [ in minutes ] FF thk hvt SFD Ci RHM 100 60

= = = = = = = =

=

thk / hvt

x SFD 2 x 60 ---------------------------------2 Ci x RHM x 100 FF x 2

film factor of the film for a target density. thickness to be radiographed. half value thickness of the material. source to film distance in cm. source strength [ curie ] of the isotope. radiation out put at 1 meter / curie. 1 meter = 100 cm. hour  minute.

Approx Film factor in Roentgen with Ir 192 source, .004" lead screens F / B, Dev 5 min at 20 0C ] D7 D5 D4 D2

1.2 1.8 3.5 9.0

AA400 T 200 MX125

1.1

IX100 IX80 IX50 IX25

1.1

Using Exposure chart : Calculation of exposure time for an X Ray set up for 2 Cm weld, D7 film, FFD 50 Cm and Machine setting 180 kV 4 mA for a Andrex Model 160 / 200 keV Constant Potential. From the chart, for 2 Cm steel at 180 kV, the required exposure is around 9.3 mA – Min. [ Follow the Red line ] It means the product of mA and time in Min should be 9.3. Hence, for 4 mA machine setting the time is 9.3 / 4 = 2.32 Min. [ this is the exposure time for 70 Cm FFD as used in the chart ] For 50 Cm FFD, the corrected time is 2.32 X ( 50 2 / 70 2 ) or 1.2 Mins. Correction for SFD change : New SFD time = Old SFD time X ( New SFD 2 / Old SFD 2 ) Approximate change in exposure time to obtain other Density : For changing density, multiply with the given factor to the time for density 2. for 1 .5

1.5 .7

2 1

2.5 1.5

3 2

3.5 3

Note : Exposure charts are supplied by the X-Ray machine manufacturers for each model. In general, such charts can not be used for different models unless suitable correction factors are applied.

Calculation of exposure time for an Ir 192 set up for 2 Cm weld, D7 film, SFD 50 Cm, Source Strength 15 Curie. From the chart, for 2 Cm steel and D7 Film, the required exposure is around 3 Curie - Hrs. It means the product of Curie and time in Hrs should be 3 or in minutes be 180. [ 3 x 60 ] Hence, for 15 Ci strength, the time is 180 / 15 = 12 Mins. [ this is the exposure time for 1m or 100 Cm SFD as used in the chart ] For 50 Cm SFD, the corrected time is 12 X ( 50 2 / 100 2 ) or 3 Mins.

Single Wall Exposure Techniques [ welds ] : This is the most used and recommended technique for radiographic recording where two opposite sides of a solid test object are accessible. Exposure preparations : Radiation source / energy : is selected based on test material absorbtion, thickness to be examined and type of the film. Optimum contrast with minimum 2% recording sensitivity are the requirements. Visual examination : Visually detected surface imperfections which will produce images on the radiograph shall be rectified before shooting. Segment marking : The weld length is divided into suitable number of segments A –B / 0 –1 etc and marked such that the marks remain on the object till the weld is accepted. Identical segment marking is n ecessary on the source side and the film side of the object for accurately positioning the film and other accessories around the weld. Film Size : shall be at least 2” more than the length of the segment to be examined. Width shall be sufficient to record the weld, all markers and the complete penetrameter outline. SFD : Minimum SFD is to be calculated using the SFD equation. Thumb rule, 10 times the object thickness and 1.1 X length of the film, which is greater. Recommended minimum SFD is 18”. Location Markers : shall be placed on the marks near the weld, in sequence 1, 2, 3 / A, B, C etc on the source side of the object, unless a predetermined overlapping length between successive films is used. Identification Markers : as required, shall appear in each film, placed near the weld. Penetrameter : 2 % of the thickness being examined. Can also be selected from the table of the applicable specifications / procedures. Weld reinforcement to be included in penetrameter selection. Penetrameter must be attached to the source side of the object. Wire type : to be fixed near the location marker and across the weld, the thinnest wire in the set towards the location marker. Hole type : to be fixed near the location marker, 3.2 mm away and parallel to the weld, 2T hole towards the location marker. Shim : used to simulate the weld reinforcements, to be placed under the hole type penetrameter only, thickness of the shim should be nearly equal to the total weld reinforcement. Shim may be single or staked thin sheets and must be larger than the size of the penetrameter. Set up : Location markers are fixed on the source side marks and radiograph identification markers are fixed near the weld using adhesive tapes. The applicable penetrameter is fixed on the source side and near a location marker also with tapes. The film is then attached in close contact with the surface, opposite to the source side using magnets or adhesive tapes. Using a magnetic supporting stand, the exposure poi nt of the source guide tube is secured exactly at the central axis of the segment under examination and at a distance equal to the SFD. The object should be positioned such that the recording plane of the film is perpendicular to the imaging radiation beam. The film is then irradiated through the object for the required exposure time. Panoramic exposures : This is also a single wall technique used for hollow circular components where the inside of the bore is accessible for centering the source point. Circumferential weld joints in pipes and pressure vessels are frequently examined using this technique. The entire joint is recorded in a single exposure. A roll film or a number of films are used with 1” overlap between successive films. Location markers are fixed at regular intervals. Identification markers are fixed as required. Minimum three penetrameters must be attached at 120 0 to each other. Strict Control of exposure parameters are absolutely necessary.

Calculating SFD : source size X thickness SFD = --------------------------------- + thickness allowed unsharpness Selecting 2 % IQI : Wire dia = part thickness / 50 Strip thk = part thk in inches X 20 part thk in mm X 0.8 or Table Exposure time in minutes, SFD in CM : thkns / HVT

x SFD 2 x 60 t = -------------------------------------------2 Curie x RHM x 100 or Exposure chart from the manufacturer. Film Factor x 2

Double Wall Exposure Techniques : When it is not practical to use a single wall technique, double wall technique is selected. Single Wall viewing : For materials or welds, a technique may be used in which the radiation passes through two walls and only the weld [ material ] on the film side is viewed for acceptance on the radiograph. When complete coverage is required for circumferential weld [ material ], a minimum of three exposures taken 120 0 to each other shall be made. The IQI shall be placed on the film s ide with a lead letter “ F “ adjacent to or on the IQI without obscuring the 2T hole for a hole type IQI for each radiograph. Set Up [ for three exposures method ] : Visual examination : Any visually detected surface imperfections which will produce images on the radiograph shall be rectified before shooting. Segment marking : The circumference is divided into three equal segments and marked A, B, C / 0, 1, 2 etc, such that the marks remain till the joint is examined and accepted. Lead location markers are to be fixed at this spots. Film length : The length of each film is selected such that minimum 1” of overlap with next film is available at each end. The width shall be sufficient to completely record the weld, the markers and the penetrameters. Selection of IQI : The IQI is selected by calculating 2% of single wall thickness to be examined. The IQI may also be selected from tables as per applicable specifications / procedures. The single wall reinforcement in the weld must be included in IQI selection. Fixing Numbers and IQI : On the outside wall, at the ends of the segment under examination, lead location markers are attached with adhesive tape. Job identification numbers are generally attached at the center of the segment. The applicable IQI is fixed near one of the location markers such that the 2T hole or the thinnest wire in the IQI is closest to the marker. The lead letter „F‟ is then attached near or on the IQI without obscuring the essential hole to be recorded. All the numbers / IQI shall be arranged such that their im ages do not fall on the image of the weld. The film is wrapped over the set up ensuring good contact with the examination surface and with at least 1” projecting beyond the location markers at each end. The width of the film shall be sufficient to record the weld, all the markers and the IQI with identifications. Allignment of Source point : The exposure point of the source guide tube is then fixed to the surface on the location mark opposite to the selected segment to be exposed. Source point may touch the part surface or may be away from the surface to meet the SFD requirements based on applicable unsharpness value. The exposure point should be positioned such that the weld on the source side is not in line with the central beam of the radiation field being used for imaging. If the test parameter requires the source point to be away from the part surface, Minimum 4 exposures will be required to complete the examination. Safety precautions during radiography work : Use of personal monitoring device [ TLD ] and a dosimeter, while working with a radiation source is a must. A calibrated and properly functioning radiation survey meter must be used to monitor the radiation levels at the boundary of the radiation zone, when the source is driven out of the source projector and during the withdrawal of the source into the projector. High radiation level indicates that the source is outside the projector. Ropes / Tapes with radiation warning symbols must be used to separate the radiation zone and unauthorized entry into the radiation zone must be prevented. Red blinking light placed near the radiation source for safety awareness is recommended. Except when operating the projector, the radiation workers must be at a safe distance from the radiation source.

Double Wall Double Image Techniques : For materials and welds in components 89 mm or less in nominal diameter, a technique may be used in which the radiation passes through two walls and the weld [ material ] in both walls is viewed for acceptance on the same radiograph. For double wall viewing, only a source side IQI shall be used. Care should be exercised to ensure that the required geometrical unsharpness is not exceeded. If the geometrical unsharpness requirement can not be met, then single wall viewing shall be used. At least one location marker shall be placed adjacent to the weld or [ on the material in the area of interest ] for each radiograph. Ellipse : For welds, the radiation beam may be offset from the plane of the weld at an angle sufficient to separate the images of the source side and film side portions of the weld so that there is no overlap of the areas to be interpreted. When complete coverage is required, a minimum of two exposures taken 90 0 to each other shall be made for each joint. Superimposed : As an alternative, the weld may be radiographed with the radiation beam positioned so that the images of both walls are superimposed. In this case, images of the top and bottom section can not be separated. When complete coverage is required, a minimum of three exposures taken at either 60 0 or 120 0 to each other shall be made for each joint. Additional exposures shall be made if the required radiographic coverage cannot be obtained using the minimum number of exposures mentioned above.

Ellipse Exposure : a Incomplete Penetration b Excess Penetration c Inclusions