Radioactive PDF

Chapter 5: Radioactivity Physics F5

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Chapter 5: Radioactivity Physics F5

5.1 NUCLEUS OF AN ATOM The Composition of the Nucleus

1. 2. 3. 4. 5.

1. 2. 3. 4.

Matter is made up of very small particles called atoms. An atom consists of a nucleus which is made up of proton and neutron with electron revolving around the nucleus. Proton and neutrons are called nucleons. A nuclide is an atom of a particle structure An atom of an element is represented by its symbol.

The proton number, Z, is defined as the number of protons in a nucleus. The nucleons number, A, is defined as the total number of protons and neutrons in a nucleus. For neutral atom, the number of protons equals the number of electrons. The number of neutrons, N in an atoms can be obtained by subtracting its proton number, Z from its nucleon number, A. Number of neutrons, N = A – Z Example 27 13 Al The proton number of aluminium is 13, The nucleon number of aluminium is 27 The number of neutron in this nucleus is 14

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Chapter 5: Radioactivity Physics F5

ISOTOPES 1. 2.

If two atoms have the same number of protons but different number of neutrons they are called isotopes of the same element. 1 2 3 1 Hydrogen has three isotopes, 1 H , 1 H , 1 H . However the three have their special names. 1 H is

3. 4.

called hydrogen, 1 H is called deuterium and 1 H is called tritium. Isotopes of an element have the same chemical properties and the same number of protons. However, they have different physical properties because their mass is different.

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10.2 RADIOACTIVE DECAY 1.

Radioactivity is the spontaneous disintegration of unstable nucleus into a more stable nucleus with the emission of energetic particle or photons.

2. 3.

Radioactivity is a random and spontaneous process. It is said to be a random process because there is no way to tell which nucleus will decay, nor is there any way to predict when it is going to decay. 4. A spontaneous process means the process is not triggered by an external factor. 5. There are three main types of nuclear radiation emitted. (a) Alpha particles, α (b) Beta particles, β (c) Gamma rays, γ 6. 7.

More than one type of radiation can be emitted at any one time during a radioactive decay. Table below give the nature and fundamental properties of the three types of nuclear radiation. Property

Beta particles, β

Alpha particles, α

Fast-moving electrons

Gamma rays, γ High frequency electromagnetic radiation

Nature

Helium nucleus

Symbol

4 2 He

Charge

+2e

-1e

No charge

Mass

Large

Very small

No mass

Speed

10% of the speed of light

99% of the speed of light

Moves with the speed of light in vacuum

0 1e

-

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Chapter 5: Radioactivity Physics F5

Ionising power

Strong

Moderate

Weak

When the energetic particles or photon passes through a medium, it can knock electrons out of the atoms and molecules of the medium to produce charged particles called ions. Penetrating power

Weak

Moderate

Strong

Thin piece of paper

A thin piece of aluminium

A few cm of lead or a thick concrete.

Effect of electric filed

Deflected towards the negative plate

Deflected towards the positive plate. The deflection is greater due to the small mass of electron.

Not deflected because no charge

Effect of magnetic field

Small deflection

Greater deflection

No deflection

Can be stopped by

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Chapter 5: Radioactivity Physics F5

DETECTOR OF RADIATION 1. 2.

The radioactive emission can be detected using detector of radiation. Table below summarises the detectors of radiation and the type of radioactive emission that can be detected. Name of detector

Type of radiation that can be detected

Geiger-Muller tube

α, β, γ

Cloud chamber

α, β, γ

Spark counter

α

Photographic badge

β, γ

Gold Leaf Electroscopes

α, β

Observation made The ratemeter shows a count rate higher than the background count. Tracks with specific characteristics are formed in the cloud chamber. Sparks are seen and heard between the wire gauze and the wire below it. Darkening of the photographic film in the badge. The electroscope discharge and the gold leaf falls.

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Chapter 5: Radioactivity Physics F5

Geiger-Muller (GM) Tube

1. 2. 3.

The radioactive emission enters the tube through the mica window and ionises the argon gas. The electrons and positive ions are attracted towards the anode and cathode respectively. When the electrons are collected by the anode, a pulse of current is produced.

4. 5. 6.

The pulses of current are counted by a ratemeter. The ratemeter gives the count rate in counts per second or counts per minute. Initially the GM tube is switched on without the presence of any radioactive substance. The reading displayed by the ratemeter is known as the background count rate. The background radiation is always present due to natural radioactivity in the ground, bricks of buildings and cosmic radiation. When the GM tube is used to detect radioactive emission, the background count rate is subtracted from the count rate obtained.

7. 8.

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Chapter 5: Radioactivity Physics F5

Cloud Chamber

1. 2. 3.

The cloud chamber contains supersaturated vapour which condenses into droplets when ionised by the passage of radioactive particle. The trails/tracks of these particles can be seen clearly (alpha particles are the best). Occasionally, a particle collides with an air molecules and changes direction.

Track of the three main types of radioactive emission.

RADIOACTIVE DECAY 1. 2.

Radioactive decay is a process where an unstable nucleus becomes a more stable nucleus by emitting radiations. The process is spontaneous (the rate of decay cannot be controlled, happens on its own, not affected by its chemical composition and not affected by physical factors such as temp. and pressure) and random ( it is impossible to predict which atom will decay at any moment of time)

Alpha Decay 1. 2. 3. 4.

An alpha particle is a nucleus of helium. It has 2 protons and 2 neutrons. When alpha decay occurs, the radioactive parent nucleus losses two protons and two neutrons which carry away energy. In other words, the proton number, Z, decreases by 2 to become (Z-2) and the nucleon number, A, decreases by 4 to become (A-4). The general equation for alpha decay: A 4 A 4 + Z 2 Z 2

X→

Y

He

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Chapter 5: Radioactivity Physics F5

5.

Examples of alpha decay: 238 92

4 U → 234 90 Th + 2 He + energy

Beta Decay 1. 2.

A beta particle is an electron with a charge of -1e. When beta decay occurs, a neutron disintegrates into a proton and an electron according to the 1 1 0 equation 0 + 1 1 The proton stays in the nucleus but the electron is shot out of the nucleus at high speed. In other words, the proton number, Z, increases by 1 to become (Z + 1). The nucleon number A, remains unchanged. The general equation for beta decay; A A 0 + Z 1 Z+1

n→ p

3. 4. 5.

e

X→

6.

Y

e

Example of beta decay;

234 90

0 Th → 234 91 Su + 1 e

Gamma Decay 1. 2. 3.

Gamma ray is a type of electromagnetic radiation produced during radioactive decays. Gamma radiation is often emitted during an alpha or beta decay. There is no change in the proton number and nucleon number for a nuclide that emits a gamma ray. It is just makes the nucleus more stable. The gamma decay of cobalt-60 is represented by the following equation; 60 60 +γ 27 27

Co →

4.

Co

Example of gamma decay;

210 84

4 Po → 206 Pb + 82 2 He + γ

198 79

0 Au → 198 80 Hg + 1 e + γ

RADIOACTIVE DECAY SERIES 1. 2. 3. 4.

When an unstable radioactive nucleus decays, the resulting daughter nucleus may also be unstable. The daughter nucleus will continue to undergo a series of successive decays until a stable configuration is achieved. A radioactive decay series can be displayed on a graph of nucleon number A, against proton number, Z, or a graph of number of neutrons, N, against proton number, Z. Below are two examples of radioactive decay series.

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Chapter 5: Radioactivity Physics F5

5.

The decay series above can also be represented as follows;

HALF-LIFE 1. 2. 3.

4.

Radioactive decay is a random and spontaneous process. This means that all the unstable nuclei do not decay at the same time. It is not possible to predict when a particular nucleus is going to decay. The nucleus will decay at different times, some decay earlier while others will decay at a much later time. Therefore the number of unstable nucleus that have not decayed decreases with time.

The half-life, T1/2 of a radioactive substance is the time for half of the radioactive nucleus to decay.

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Chapter 5: Radioactivity Physics F5 5.

Thallium-210 has a half-life of 1.3 minutes. This means that 16 g of thallium takes 1.3 minutes to decay to 8 g of thallium. The time taken for further decay to 4 g of thallium is also 1.3 minutes.

6.

Some radioactive nuclides have very short half-lives while others have very long half-lives.

Example 1 The nuclide thorium-232 (proton number 90) decays by emitting an alpha particle. Write down the equation for the decay process and name the daughter nuclide formed. Solution 232 90 232 = A + 4 A = 228

Th → AZ X + 42 He

90 = Z + 2 Z = 88 The equation can be written as 232 228 4 +2 90 88

Th →

Ra

Example 2 232 208 When 90 decays to 82 particles emitted.

Th

He

Pb , several α and β particle are emitted. Find the numbers of α and β

Solution 232 208 4 0 + x + y 90 82 2 1 The top and bottom numbers are balanced on both sides of the equation: 232= 208 + 4x + 0 …………………..[1] 90 = 82 + 2x + (-1)y …………………..[2]

Th →

Pb

He

e

From equation [1] 4x = 232 – 208 From euation ]2] 90 = 82 + 12 – y

⇒

6 α particle and 4 β particles.

Example 3 Sodium-24 has a haf-life of 15 hours. The original mass of the sample containing sodium-24 is 64 g. (a) What is the mass of the sodium-24 in the sample after 45 hours? Solution (a) T1/2 = 15 hours 45 hours = 45/15 = 3T Mass of sodium-24 after 45 hours

= ½ x ½ X ½ x 64 g = 8g 10

Chapter 5: Radioactivity Physics F5

Example 4 A radioactive substance has an initial activity of 960 counts per second. What is the half-life of the substance if its activity becomes 120 counts per second after 168 s? Solution T

T

T

480 240 120 960 This means the decay process has gone through 3 half-lives. 3T = 168 s T = 56 s.

→

→

→

Example 5 A sample of iodine-131 has a half-life 0f 8 days and an initial activity of 800 counts per second. What is the activity of iodine-131 after 16 days? Solutions T1/2 = 8 days 8 days = 16/8 = 2T Activity after 16 days = ½ x ½ x 800 = 200 counts per second

Example 6 Phosphorous-32 has a half-life of 15 days. How long does it take for 75% of the phosphorous atoms to decay? Amount of phosphorous atoms left behind after the decay = 25% T T 100%

→ 50% → 25%

Time for 75% of the atoms to decay

= 2T = 2 x 15 days = 30 days.

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Chapter 5: Radioactivity Physics F5

10.3 UNDERSTANDING THE USES OF RADIOISOTOPES RADIOISOTOPES 1. 2. 3. 4.

Isotopes of an element have the same chemical properties and the same number of protons. Radioactive isotopes are called radioisotopes Radioisotopes are unstable isotopes which decay and give out radioactive emissions. Table below gives a list of some radioisotopes.

Tritium

Nuclide notation 3 1H

Carbon-14

14 6C

β

5730 years

Sodium-24

β, γ

15 hours

Strontium-90

24 11 Na 90 38 Sr

β

28 years

Polonium-212

212 84 Po

α, γ

45 s

Radioisotopes

Radioactive emission

Half-life

β

12 years

APPLICATIONS OF RADIOISOTOPES Smoke Detectors Smoke alarms contain a weak source made of Americium-241. Alpha particles are emitted from here, which ionise the air, so that the air conducts electricity and a small current flows. If smoke enters the alarm, this absorbs the a particles, the current reduces, and the alarm sounds. Am-241 has a half-life of 460 years.

Thickness Control In paper mills, the thickness of the paper can be controlled by measuring how much beta radiation passes through the paper to a Geiger counter. The counter controls the pressure of the rollers to give the correct thickness. With paper, or plastic, or aluminum foil, b rays are used, because a will not go through the paper. We choose a source with a long half-life so that it does not need to be replaced often. Sterilising

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Chapter 5: Radioactivity Physics F5 Even after it has been packaged, gamma rays can be used to kill bacteria, mould and insects in food. This process prolongs the shelf-life of the food, but sometimes changes the taste. Gamma rays are also used to sterilise hospital equipment, especially plastic syringes that would be damaged if heated.

Radioactive Dating Animals and plants have a known proportion of Carbon-14 (a radioisotope of Carbon) in their tissues. When they die they stop taking Carbon in, then the amount of Carbon-14 goes down at a known rate (Carbon-14 has a half-life of 5700 years). The age of the ancient organic materials can be found by measuring the amount of Carbon-14 that is left.

Radioactive Tracers The most common tracer is called Technetium-99 and is very safe because it only emits gamma rays and doesn't cause much ionisation. Radioisotopes can be used for medical purposes, such as checking for a blocked kidney. To do this a small amount of Iodine-123 is injected into the patient, after 5 minutes 2 Geiger counters are placed over the kidneys. Also radioisotopes are used in industry, to detect leaking pipes. To do this, a small amount is injected into the pipe. It is then detected with a GM counter above ground.

Checking Welds If a gamma source is placed on one side of the welded metal, and a photographic film on the other side, weak points or air bubbles will show up on the film, like an X-ray. Cancer Treatment

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Chapter 5: Radioactivity Physics F5 Because Gamma rays can kill living cells, they are used to kill cancer cells without having to resort to difficult surgery. This is called "Radiotherapy", and works because cancer cells can't repair themselves when damaged by gamma rays, as healthy cells can. It's vital to get the dose correct - too much and you'll damage too many healthy cells, too little and you won't stop the cancer from spreading in time. Some cancers are easier to treat with radiotherapy than others - it's not too difficult to aim gamma rays at a breast tumour, but for lung cancer it's much harder to avoid damaging healthy cells. Also, lungs are more easily damaged by gamma rays, therefore other treatments may be used.

10.4 NUCLEAR ENERGY ATOMIC MASS UNIT 1. 2. 3. 4. 5. 6. 7.

Nuclear energy is the energy during the splitting and fusing of atomic nuclei. The atomic mass unit (a.m.u) is used to measure the masses of atomic particles. 1 a.m.u. is one twelfth of the mass of Carbon-12 atom. 1 a.m.u = 1.66 x 10-27 kg In most nuclear reactions, the sum of the masses of the particles before the reaction is more than the sum of the masses of the particles after the reaction. This difference is called the mass defect. The mass lost is converted into energy. According to Einstein’s Principle of Mass-Energy Conservation, the change of energy is linked to the change of mass by the equation:

E = mc2 Where m c E

= mass change (kg) = speed of light (ms-1) = energy changed (J)

Example: Below is an equation for the decay of radium-226. 226 222 4 +2 + Energy 88 86 The masses of each atom: 226 = 226.02536 a.m.u 88 222 = 222.01753 a.m.u 86 4 = 4.00260 a.m.u 2 (a) Find the mass defect in (i) a.m.u (ii) kg (b) Calculate the amount of energy released in

Ra →

Rn

He

Ra Rn He

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Chapter 5: Radioactivity Physics F5 (i) J (ii) eV [1 a.m.u. = 1.66 x 10-27 kg ; 1 eV = 1.66 x 10-19 J]

NUCLEAR FISSION 1. 2. 3.

Nuclear fission is a process in which a heavy nucleus splits into two or more lighter nuclei. Nuclear fission usually occurs when the nucleus of an atom is bombarded with a neutron. The energy of the neutron causes the target nucleus to split into two (or more) nuclei that are lighter than the parent nucleus, releasing a large amount of energy during the process.

4.

Figure 1 shows a slow neutron hitting a uranium-235 nucleus, causing it to split producing strontium-90, xenon-143 and three neutrons. The equation for the reaction can be written as;

235 92

90 1 U + 01 n → 143 54 Xe + 38Sr + 3 0 n + energy

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Chapter 5: Radioactivity Physics F5

Example:

235 Below is an equation involving the fission of 92 235 92

U by a fast moving neutron.

92 1 U + 01 n → 141 56 Ba + 36 Kr + 3 0 n + energy

235 Calculate the amount of energy released by 92 . 235 92 141 [ 92 = 235.04 a.m.u, 36 = 91.93 a.m.u, 56 a.m.u. = 1.66 x 10-27 kg]

U

U

Kr

Ba = 140.91 a.m.u, 01 n = 1.01 a.m.u, 1

Chain Reaction 1.

A nuclear fission reaction produced three free neutrons.

2.

In one of these neutrons bombards another uranium-235 nucleus then more fission will occur, releasing more neutrons. In this way, a chain reaction is produced. In nuclear reactions, a controlled chain reaction is used to generate electrical energy.

3.

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Chapter 5: Radioactivity Physics F5 If uncontrolled chain reaction takes place the huge amount of energy being released and resulting in an explosion such as an atomic bomb.

4.

Nuclear Fusion 1.

Another way for a less stable nucleus to become more stable is by fusion.

2.

In a fusion reaction, two light nuclei combine to form a heavier nucleus, releasing a vast amount of energy during the process. This process is accompanied by the release of a huge amount of energy. Two examples of fusion reactions: 2 1 3 (a) 1 +1 + γ + energy 2 2 3 4 1 (b) 1 +1 + 2 0 + energy Fusion is much more difficult to achieve than fission because the hydrogen nuclei with positively charge repel each other. This explains why fusion can only occur at very high temperature such as fusion in the sun. And also energy released in nuclear fusion is very much more than in nuclear fission.

3. 4.

H H

5. 6. 7.

H → He H → He

n

Example: The equation for the fusion reaction can be written as 2 3 4 1 +1 + 0 + energy 1 2 2 = 2.014102 a.m.u. where 1 3 = 3.016029 a.m.u. 1 4 = 4.002603 a.m.u. 2 1 0 = 1.008665 a.m.u. Calculate the mass defect and energy released in the reaction.

H

H → He H H He n

n

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Chapter 5: Radioactivity Physics F5

NUCLEAR REACTOR 1.

A nuclear reactor produces tremendous amount of energy through nuclear fission. This energy is used to generate of electricity.

2. 3.

The uranium fuel is formed into ceramic pellets. Fission generates heat and is used to produce steam. The steam turns huge turbine blades. As the turbines turn, they drive generators to produced electricity. Water is used to cool the structure of the power plant. Nuclear reactors is a place where a nuclear chain reaction will take place. The neutrons produced in a fission reaction are very fast neutrons. To slow down the neutrons graphite as a moderator is used. The rate of reaction is controlled by boron control rod which can absorb neutrons. In electric power plants, the reactors supply the heat to produced steam which drives the turbine– generators.

4. 5. 6. 7. 8.

REASON FOR THE USE OF NUCLEAR ENERGY - It is not expensive - It does not produce smoke or carbon dioxide and does not contribute to the greenhouse effect. - It produced huge amounts of energy from small amounts of fuel. - It produced small amounts of waste. - Nuclear power is reliable. - Nuclear reactors can also be used in research and in the production of medical and industrial isotopes. REASON AGAINST THE USE OF NUCLEAR ENERGY Radioactive waste is hazardous. Nuclear energy from uranium is no renewable The fuel and waste products have to be constantly monitored to prevent abuse. A nuclear accident can be a major disaster.

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