radioactivity: physics form 5

Physics Form 5

Samura Physics Panel

CHAPTER 5: RADIOACTIVITY

5.1 Understanding the Nucleus of an Atom

Rutherford Atomic Model 1. An atom has a positively charged core or

nucleus, which contains the mass of the atom and which surrounded by orbiting electrons.

2. Geiger and Marsden planned and carried out an experiment proposed by Sir Ernest Rutherford and found evidence for Rutherford Model.

3. They fired a stream of alpha particles at a very thin gold foil and counted how many alpha particles were scattered at a number of different angles. The results agreed well with the theory.

4. The result and the conclusion of the experiment is simplified in the table below:

Today’s Model atom1. An atom consists of a nucleus which made of

protons and neutrons. It also has electrons orbiting the nucleus.

2. Protons and neutrons are called nucleons.

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Result ConclusionMost of the alpha particles passed straight through the foil in their original direction.

Most of the space taken up by an atom must be completely empty. A very small nucleus is placed at the centre of the atom.

A few alphas particles were deflected through very small angles

The nucleus are positively charged. The alpha particles also positively charged are repelled by the nucleus because repulsion force is produced between the like electric charges.

A very small number of alpha particles were bounced back by the gold foil.

When the alpha particles approach very close to the nucleus , they were exerted by a very large repulsion force because the repulsion obeys the inverse square law of the force between two charged objects( F α 1 )

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Proton Number , Z and Nucleon Number, A

1. The nuclide notation of an atom gives the symbol of the element:

A = nucleon number , Z = proton number (atomic number)

2. For a neutral atom, the number of protons equals the number of electrons.

3. The nucleon number, A is a total number of protons and neutrons

A = N + Z,

Nuclides and radioisotopes

1. Nuclide is a type of atom characterized by its proton number and its neutron number.

2. Isotopes are atoms of the same element with the same number of protons but different number of neutrons.

3. Isotopes have the same proton number but different nucleon numbers.

4. All isotopes have same chemical properties but different physical properties.

5. The unstable isotopes are called radioactive isotopes or radioisotopes. For example , the hydrogen element have three nuclides to form isotopes.

H1

1 H21 and H3

1

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Particle Charge Value of charge / C

Mass/kg

Proton(p)

+ve +1.6 x 10-19 1.67 x 10-27

Neutron(n)

Neutral 0 1.67 x 10-27

Electron(e)

-ve -1.6 x 10-19 9.11x 10-31

Element Number of fundamental particles

NucleonNumber, A

Proton numberZ

p n eHydrogen11 H

1 0 1 1 1

Carbon146 C 6 8 6 14 6

Beryllium94 Be 4 5 4 9 4

N = A - Z

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5.2 Analysing Radioactive Decay Radioactivity

1. Radioactivity is the spontaneous disintegration of an unstable nucleus into a more stable nucleus accompanied by the emission of energetic particles (radioactive rays) or photons.

2. The process is said to be spontaneous because it is not influenced by any physical factors such as time, pressure, temperature, etc.

3. The decay occurs randomly because each atom has the same probability of decaying at any moment of time.

4. Example of stable and unstable isotope:

5. There are three kinds of radiation emitted by radioactive materials :(1) Alpha particles, α(2) Beta particles, β(3) Gamma rays, γ

Radioactive Detectors

1. Radioactive detectors make use of the ionisation process to detect radioactive emission (except for the photographic plate).

2. The following are the common detectors for radioactive emissions.

Photographic Plate or Film

1. The photographic film or plate can be used as a special badge or tag to record the dosage of radiation a staff at radiation laboratories is exposed to.

2. The detector works on the principle that radioactive radiation can cause a chemical change on the plate and produce a dark trace.

3. The degree of darkening of the photographic film indicates the amount of radiation received.

4. The photographic film can detect all the three types of radioactive radiation.

Gold Leaf Electroscope

1. When the charged plate of the electroscope is exposed to the source of radioactive , the gold leaf will collapse slowly.This is due to the ions produced by radioactive source neutralize the charge in the electroscope.

2. This method is suitable for detecting alpha particles because alpha particles have high ionizing power.

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Element Stable isotopes

Unstable isotopes

Carbon 126 C 14

6 COxygen 16

8 O 158 O, 198 O

Lead 20682 Pb, 20882 Pb

21082 Pb, 21482

Pb

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Spark Counter

1. When the radioactive source is brought near the spark counter , the sparks are formed.

2. The radioactive rays will ionise the air molecules.

3. The sparks are formed due to collision between the ions and air molecules.

4. The spark counter can only trace alpha particle which have high ionising power.

Geiger-Muller tube (GM tube)

1. A GM tube is a very versatile , sensitive and useful detector of radiation.

2. When the radioactive radiations enter the GM tube through the mica window and ionises the argon gas. A pulse current is produced and counted by a scaler or ratemeter .

3. The actual reading of a GM tube is calculated as follow:

4. Background reading is produced by radioactive materials from Earth and the surroundings such as stones, sand, soils, etc and also from the cosmic rays in the sunlight.

5. The GM tube can detect alpha particles, beta particles and gamma rays.

Cloud Chamber

1. When the radioactive rays enter he upper part , the ionization of air will occur. The ions allow the saturated alcohol vapour to condense forming tiny alcohol droplet and will cause the formation of misty tracks.

2. The cloud chamber can detect all the three types of radioactive radiation.

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Radiation Track Characteristics and explanation.

Alpha particle - thick because have strongest ionizing power. A lot of alcohol droplets are formed along the ions produced along the track.

- straight because not easily deflected by air molecules with its greater mass.

- same length because each particle has equal amount of energy.

Beta particle - thin because ionizing power is weak.- curvy because the particles are light and deflected by air

molecules. - different length because each particle has different amount of

energy.

Gamma ray - thin, short and scattered because it has the lowest ionizing power.

Characteristics of alpha particles, beta particles and gamma rays.

Characteristic Alpha particles, α beta particles β

gamma rays γ

Nature Helium nucleus Fast moving electron Electromagnetic radiation.

Symbol 42 He 01− e -

Charge +2 (positive) -1(negative) No charge

Mass Large Very small No mass

Speed 10 % of the speed of light.

90 % of the speed of light Speed of light, c.

Ionising power Strongest intermediate Weakest

Penetrating power Weak Moderate Strong

Range in air A few cm A few m A few hundred meter

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Stopped by Human skin or a thin piece of paper.

A few mm of Aluminium A few cm of lead or concrete

Effect of electric field Deflected towards the negative plateα has a positive charge

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

No deflected because γ has no charge.

Effect of magnetic field Small deflection

because α has a large mass.

Greater deflection because β has a very small mass.

No deflection because γ has no charge.

Radioactive decay

1. Radioactive decay is the process of nucleus changing to a more stable nucleus while emitting radiation.

2. The nucleus before decay is called the parent nuclide and the product of decay is the daughter nuclide.

3. The radioactive decay results in changes in the number of protons and neutrons in the nuclei.

4. There are several types of decay:(a) Alpha decay(b) beta decay and (c) gamma decay

Alpha decay

1. The general equation of alpha decay is:

2. When a nuclide decays by emitting an alpha particle its proton number Z decreases by 2 and its nucleon number, A decreases by 4.

3. Example ;

U23892 → Th234

90 + He42

Beta decay


2. When a nuclide decays by emitting an beta particle its proton number Z increases by 1 and its nucleon number, A does not change.

3. Example ;

Sr9038 → Y90

39 + e01−

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Gamma emission

1. High frequency electromagnetic radiation coming from the nuclei of decaying atom is call gamma radiation.


3. Emitting a gamma does not change the atomic number of the atom; it also has very little effect on the mass.

4. Example ; Co60

27 → Co6027 + γ

Example 1

Balance the following equations:(a) Po214

84 → Pb82 + He4

(b) Bi83 → Po21484 + e0 + γ

Solution

Example 2

How many alpha particles and beta particles are

emitted when Th23290 decays into Pb208

82 ?

Solution

A decay series1. Radioactive substances often decay several

times in a series of steps , emitting radiations and producing a new substance at each step.

2. A parent substance produces daughter and grand-daughter substances in what is called a decay series.

3. For example the decay series of U238 can be represented as follows:

Example 3

The diagram shows part of a radioactive decay series.

Name the particles or radiations are emitted at part I, II and III.

Solution

Decay curve

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The number of atoms , mass or activity of a radioactive substance decreases with time.

Half-lifeThe half-life of a radioactive material is the time taken for half of the unstable atoms to decay.OrThe half-life of a radioactive material is the time taken for the activity of radioactive fall to half its original activity.

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Half-life T0 1T1/2 2T1/2 3T1/2

Number ofundecayed atoms. N (½)1 N = ½N (½)2N = ¼ N (½)3N= 18 N

% atoms Undecayed 100 % 50 % 25 % 12.5 %Mass 64 g 32 g 16 g 8 gActivity(s-1) 120 s-1 60 s-1 30 s-1 15 s-1

Number of atoms decayed N – N = 0 N–½N =½ N N–¼N = ¾ N

N–18 N =

78 N

Mass haveDecayed 0 g 32 g 48 g 56 g% atomsDecayed

0 % 50 % 75 % 87.5 %

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Example :

23491 Pa takes 20.8 hours to shrink from 80g to 5 g.

(a) How many half-life are there? (b) Determine the decrease in mass after 26

hours.

Solution :

Example 4The half-life of a radioactive material of mass 40 g is 2 hours. Determine the mass of the radioactive material that has decayed and has not decayed after 6 hours.

Solution

Example 5

The half-life of Sodium-24 is 16 hours. What is the time taken for Sodium-24 to shrink from 0.64 to 0.04 g?Solution

Example 7

The half-life of Ba-143 is 12 seconds. How long will it take for the activity of a Ba-143 sample to be reduced to 1/16 of its initial value? Solution

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5.3 RADIOISOTOPES

Radioisotopes

Radioisotopes are unstable isotopes which decay and give out radioactive emissions.Radioisotopes are naturally occurring or artificially produced.

Uses of radioisotopes

There are many uses for radioisotopes in a wide range of fields including medicine, agriculture, industry and archaeology

Medicine

In medicine field radioisotopes are used in the diagnosis of certain diseases , provides information of the specific organs of a patient or treat disease.

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Radioisotopes Half life Uses ProcessSodium-24 15 hours

Emit β and γdetect the positions of blood clots (thrombosis) in veins

injected into the blood stream and gamma rays and beta rays emitted is detected by a ray camera outside the body

Technetium -99 6 days

Emit γ

study the blood in heart

emit gamma and produces no harmful alphas or betas inside the body. The Technetium is combined into samples of the protein albumin, and this is injected into the patient.

Iodine-131 8 days.Emit β and γ

for detecting changes in the thyroid glands.

Patients are given an intravenous injection of iodine-131 . A detector is placed near the thyroid to read its activity or function.

Cobalt-60 5years

Emit β and γ

treatment of internal cancers

gamma radiation is carefully directed at cancer site from an external cobalt source. its operated by remote control from behind thick lead and concrete walls.Cobalt-60 also is used to sterilise medical equipments.

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Radioisotope in Industry

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Radioisotope Half life UsesStrontium-90 28 years

emits βthe thickness of paper in a paper industry

A radioactive source containing strontium-90 is placed at one side of the paper and a detector on the other side. The detector registers a higher count if the paper is too thin and a lower count if it is too thick.

Sodium-24 15 hoursEmit β and γ

test for leakage of underground pipes

A G-M tube is moved above the underground pipe , a leakage can be detected. The leakage can be detected when the tube registers a higher reading.

Cobalt-60 5 yearsEmit β and γ

check welds in steel structures and pipelines

cobalt-60 source placed on one side of a steel structure exposes a photographic plate at the other side. A flaw such as a bubble or crack inside a weld on a pipeline would be visible on the exposed film.

Americium-241

460 years emits α

used in a smoke alarm

When smoke enters the alarm , the smoke particles get in the way of the α radiation , reducing the ionisation and the current across the alarm. This causes the alarm to sound.

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Radioisotopes in Agriculture

Radioisotopes Halflife UsesPhosphorus-32

15 days.

a tracer in the study of the effectiveness of fertilizers

The plants are watered with a solution containing phosphorus-32. A leaf is plucked and tested for radioactivity everyday for a week. If the activity recorded increases then the plant has absorbed phosphorus.

Cobalt-60 15 years Emit β and γ

for food preservation,control insect pests which damage crops

Radiation treatment kill these pests and reduces the losses.Male insects are bred in laboratory and then irradiated. This does not kill them but damages their sex cells, making them sterile- unable to produce offspring. These males are then released in great numbers in affected areas. They breed as usual with normal females ,but no new generation of the insects is hatched

Radioisotopes in Archeology

Carbon-141. Carbon-14 has a half-life 5 700 years2. It is used to measure the age of a

archaeological specimen by carbon dating method.

3. Ordinary carbon contains a very small proportion of carbon-14 , produced when cosmic rays from space collide with nitrogen-14 in atmosphere.

4. Living plants take up the carbon-14 in the carbon dioxide they use for photosynthesis, as do animals when they eat the plants for food. While the plant or animal is alive, the proportion of cabon-14 to ordinary carbon-12 in their tissues stays constant, but once they die, the carobn-14 begins to decay – with a half-life of 5 700 years.

5. To date an archeological specimen , a small sample of carbon is extracted from it.

Uranium-238

1. Uranium-238 has a half-life of 5000 million years. It is used to measure the geological time.

2. During the formation of rocks, some radioisotopes such as uranium-238 are trapped.

3. As the decays continues, the proportion of uranium-238 decreases slowly resulting in the equally slow growth of its product lead-206 and the age of the rock can b estimated.

5.4 NUCLEAR ENERGY

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Atomic mass unit (a.m.u.)

The atomic mass unit (u) is the unit of mass for atoms and subatomic particles such as the proton, neutron and electron.

1 a.m.u or 1u is atom of the mass of the

carbon-12 atom.

1 u = 1.66 x 10-27 kg

Example 1The mass of an atom Cobalt-60 is 59.933820 u.What is the mass of the atom in kilogram?

Solution

Nuclear Fission

1. Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei, which subsequently emit either two or three neutrons and release of large amounts of energy. The example of a nuclear fission is shown as follow:

2. When a uranium-235 is bombarded by a neutron, it is split into two fission fragments (Kripton and Barium ) and three free neutrons.

Chain reaction1. One nucleus of isotope uranium-235 can

disintegrate with production of two or three neutrons, which cause similar fission of adjacent nuclei. These in turn produce more neutrons which go off and split other uranium atom - and so on.

2. A controlled chain reaction is used in nuclear power stations while an uncontrolled chain reaction is used in nuclear bombs.

Critical mass1. The minimum mass of fission material that will

sustain a nuclear chain reaction.2. For example , when a nucleus of uranium-235

disintegrates two or three neutrons are released in the process, each of which is capable of causing another nucleus to disintegrate , so creating a chain reaction. However, in a mass of U-235 less than the critical mass, too many neutrons escape from the surface of the material without hitting , preventing a chain reaction from happening.

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3. In the atom bomb, therefore, two or more sub-critical masses have to brought together to make a mass in excess of the critical mass before the bomb will explode.

Nuclear Fusion

1. Nuclear fusion is the combining of two lighter nuclei to form a heavier nucleus with the release of large amount of energy. The example of a nuclear fusion is shown as follow:

2. Nuclear fusion is believed to be process by which energy is released by the Sun. When two hydrogen-2 nuclei moving at high speed collide, they can join together to produce a heavier nucleus. A large amount of energy is released.

3. The temperature of a gas must be high giving a high average kinetic energy. Due to the requirement of high temperature, nuclear fusion is also known as a thermonuclear reaction.

4. Hydrogen bombs are made following the principle of nuclear fusion.

5. Another example of nuclear fusion is :

Differences between Nuclear Fusion and Nuclear Fission

Nuclear Energy

1. According to Albert Einstein, In a nuclear reaction (nuclear fission and fusion) neither mass nor energy are conserved separately but they can exchanged one for the other and only the “mass-energy” is conserved. A loss of mass means that the mass has changed to energy.

2. The relationship between the mass and the energy is given by the equation:

Where E=energy released, m=loss of mass or mass defect, c= speed of light =3 x 108 ms-1

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Nuclear Fusion Nuclear FissonDefinition Nuclear fusion is a

process whereby lighter nuclei fuse together to forma single heavier nucleus with the release of energy.

Nuclear fission is a process whereby a heavy unstable nucleus of an atom splits into lighter nuclei with the release of energy.

Where did the energy come from?

The reduction in mass, when two light nuclide fuse together, is converted into energy.

The reduction in the total mass of fragments compared into the mass of the original nuclide is converted into energy.

Process that takes place

Light nuclei at high speed and very high temperature overcome the repulsion force and fuse to form a single nucleus.

Moving particles, e.g. neutrons, hit and break up heavy nucleus and produce enough neutrons to break up other nuclei (chain reaction)

Can the rate of reaction be controlled?

Difficult to control. Can be controlled.

Examples Fusion is the process that powers the sun.

Fission is the process used in a nuclear reactor

E = mc2

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Example 2

Polonium-210 undergoes alpha decay to become plumbum-206 . The equation for the decay is:

210 206 4

Po → Pb + He + energy 82 84 2

Where,Mass Po = 209.982 u , Mass Pb = 205.969 u , Mass He= 4.004 u

1 u = 1.66 x 10-27kg, c = 3 x 108 ms-1

Using the equation and the information above , calculate (a) The mass defect(b) The energy released(c) The power generated in 2 ms

Solution

Generation of electricity from nuclear energy – Nuclear Power Station

1. The energy released from nuclear fission can be used to generate electricity. A nuclear power station consists two main components:(a) Nuclear reactor(b) Generator

2. The main components of nuclear reactor :

Component FunctionGraphite core Acts as moderator to slow

down the fast neutrons produced by the fission.In some nuclear power plant, the moderator is water.

Uranium rod (fuel)

To produce nuclear power when the fission reactions occur in the uranium rod

Boron control rod

To control the rate of fission reaction.The control rods are lowered into the reactor core to absorb some of the neutrons and thus reduce the rate of the fission reaction.Sometimes the rod is made of cadmium

Coolant To take away heat from the nuclear reactor.‘Heavy’ water and carbon dioxide are used as coolant because they have high specific heat capacity.

Concrete shield To prevent leakage of radiation from the reactor core

The main components of generator :

Component FunctionSteam generator To change water into

steam when the water in the generator is heated.The steam then drives the turbines

Turbine To turn the coils in the dynamo in the electrical generator to produce electricity

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The pros and cons of using nuclear fission to generate electricity

Nuclear power is controversial. Here are some arguments for and against using nuclear power station to generate electricity.

1. Nuclear power provides cheaper electricity than any other method because the nuclear power stations need less fuel than stations which use fossil fuels. The price of nuclear fuel is more stable than fossil fuels. Vast reserves of nuclear fuel in the world.

2. Safety procedures in the administration of nuclear reactors are very advanced and safe. Workers in nuclear power stations are at less risk than those in other energy industries. Many people have been killed in accidents in coal mining and oil rigs; very few comparable accidents have occurred in nuclear power stations.

3. Nuclear power is clean because produces less waste than fossil fuels. Burning fossil fuels in power stations does more damage to the environment than nuclear power stations. One of the major causes of acid rain is the sulphur dioxide and nitrogen dioxides released from burning coal in power stations. So nuclear power does not add to the greenhouse effect.

4. Produces useful radioisotopes as by-products

that can be used in industry, medicine, agriculture and research.

1. The initial cost to design and build a nuclear power station is very high. Used fuel rods are very hot and highly radioactive with very long halve-lives. Expensive procedures are required to cool down the rods and store them.

2. There is always a risk of accidents. If something goes wrong with a nuclear power station , it is very much more serious than an accident at a conventional power station. The effects cross national boundaries and can be felt many hundreds of kilometers away. The hot water discharged from the nuclear power stations can be caused thermal pollution. People who work in the nuclear power station and those living nearby may be exposed to excessive radiations.

5.5 MANAGEMENT OF RADIOACTIVE SUBSTANCES

The negative effects of radioactive substances.

1. People are exposed to a variety of radioactive radiations which are dangerous because the radiations have penetration power and ionization power.

2. As the radiations penetrate through living cells ,the ionizations process occur. Ionizations cause the ions react with other atoms in the cell to cause damage, changed permanently or die.

3. Factors affecting the severity of radiation are:(a) Types of radiation(b) Dosage and exposure time(c) Methods of insertion into the body(d) Exposure of different parts of the body.

4. The harmful effects of radiation on humans can be divided into two categories somatic effect and genetic effect.

Safety precautions in the handling of Radioactive Substances

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Harmful effect ExamplesSomatic effects- effects appear in the exposed to radiation (depends on the dose of radiation received)

Radiation burns (skin burn), Fatigue,Nausea, Hair loss, Leukemia, Cataracts, Vomiting, Infertility in male, Organ failure, Death

Genetic effect- damage of reproductive cells

Chromosome abnormalities, Birth defects, Congenital defects ( Down Syndrome, Klinefelter Syndrome and Turner Syndrome) , Premature death, Cancer in later life

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1. No eating, drinking , smoking or applying cosmetics are not allowed where any radioactive materials are handled.

2. Disposable gloves and protective clothing are worn.

3. Eye glasses made of lead are used at all times when handling radioactive substances.

4. Masks are worn in mines where radioactive dust particles are air-borne

5. Using shielding such as laboratory coats, long pants, close-toe footwear and especially to shield the sex organs using lead aprons.

6. Keeping a large distance between the person and the source

7. Keeping exposure times as short as possible8. Radioactive substances are kept in thick lead

containers9. Room, buildings, containers and radioactive

storage places must be labelled with the sign for radioactive substance.

10. Radioactive wastes must be disposed using suitable and safe methods

11. Nuclear reactors should be built on islands or areas far from residents

12. Use remote-controlled tools through a lead-glass screen.

13. Use tongs or forceps to move radioactive material

14. Sit behind a shielding wall made of lead and concrete

15. Wear a film badge which gives a permanent record of radiation dose received

16. Workers are checked for radiation contamination by using sensitive radiation monitors before they leave their place of work.

17. When radioactive material are used in medicine, the material with a short half-life is chosen.

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radioactivity: physics form 5

Education

number of electrons

neutron number

different number of

total number of protons

number of different

unstable nucleus

small nucleus

proton number atomic