RADIOACTIVITY

Table of Contents
Atomic Structure
Types of Radiations
Radioactivity
Penetrating Power
Ionising Effect of the Radiations Connections
Radiation Detectors
Building on…..
Background Radiation
Decay Law  Electromagnetic
Applications of Radioactivity Spectrum-Gamma
Hazards of Radiation rays
 Atomic Structure –
Nuclear Fission
form 2 Chemistry.
Nuclear Fusion
 Introduction to
Revision Exercise Physics –
relationships
between Physics and
Chemistry,
Specific Objectives History(carbon
By the end of this topic, the learner should dating)
be able to:
a) define radioactive decay and half-life Arriving at……
b) describe the three types of radiations emitted in
natural radioactivity  Explaining
c) explain the detection of radioactive emissions Radioactivity and
d) define nuclear fission and fusion radioactive decay
e) write balanced nuclear equations  Identifying – and
f) explain the dangers of radioactive emissions explaining – the
g) state the applications of radioactivity types of Radiations
h) solve numerical problems involving half-life.  Explaining the
applications – and
(15 Lessons) dangers of
Radioactivity
Content
1. Radioactive decay Looking forward to….
2. Half-life
3. Types of radiations, properties of radiations  Nuclear and atomic
4. Detectors of radiation, physics!
5. Nuclear fission, nuclear fusion  Nuclear energy –
6. Nuclear equations and the dangers of
7. Hazards of radioactivity, precautions
8. Applications nuclear weapons.
9. Problems on half-life (integration not required)
 2 Modern Physics

RADIOACTIVITY

Radioactivity is the spontaneous disintegration of the nucleus of an atom.

It can also be defined as the spontaneous random emission of particles from the
nucleus of an unstable nuclide.
The process of radioactivity is not affected by such external factors as temperature,
pressure or chemical composition.

Atomic Structure and nuclear stability

The number of protons in the nucleus is called the atomic or proton number, while
the sum of the number of protons and neutrons is called the mass or nucleon number.
Some atoms have same atomic number but different mass numbers. Such atoms are
said to be isotopes.
Stable nuclides have a proton to neutron ratio of about 1:1.
Unstable nuclides undergo radioactive decay. Nuclides with too many neutrons decay
in such a way that the proton number increases. Those with too many protons decay in
such a way that their proton number decreases.

Types of Radiations
To identify types of Radiations, a radium source is placed in a thick lead box with a

small opening. When a strong magnetic field is introduced perpendicular to the path of
radiations, some are deflected. Using Fleming’s left-hand rule, it can be shown that
radiation P is positively charged, R negatively charged and Q carries no charge.
The positively charged radiation is called alpha (α) particles, the negatively charged
beta (β) and the uncharged gamma (γ) radiation.
Alpha particles are deflected less compared to beta particles. This suggests that alpha
particles are heavier than beta particles. An α-particle is basically a helium nucleus,
denoted by 42He .
0
The β-particles are electrons, denoted by −1 e
If an electric field is used, the alpha particles are deflected towards the negative plate
showing that they are positively charged while the beta particles are deflected towards

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 RADIOACTIVITY 3

the positive plate since they are negatively charged. The gamma rays go straight
through the electric field since they are uncharged.

Radioactive decay
Radioactive decay is the process by which an element changes into another element
by emitting a particle or radiation from the nucleus.

Radioactive decay involves emission of alpha or beta particles and this may be
accompanied by a release of energy in form of gamma radiation.
Alpha Decay
If the nuclide decays by release of an alpha particle, the mass number decreases by 4
and the atomic number by 2. This is expressed as;
A −4 4
A
Z X Z−2 Y + 2He
→

(Parent ¿nuclide) (daughter ¿nuclide) (alpha¿ particle)

Uranium, for example, decays by emitting an alpha to become thorium.
234 4
238
92 U 90 Th + 2He
→

206 4
210
84 Po 82 Pb + 2 He
→

Beta Decay
If the nuclide decays by release of a β-particle, the mass number remains the same but
the atomic number increases by 1. This is expressed as;
A 0
A
Z X Z +1 Y +−1e
→

(Parent ¿nuclide) (daughter ¿nuclide) (beta¿ particle)

Radioactive sodium, for example undergoes beta decay to become magnesium.
24 0
24
11 Na 12 Mg +−1e
→
Gamma Radiation
Some nuclides might be in an excited state and to achieve stability, they may emit
energy in form of gamma radiation, without producing new isotopes. For example:
Thorium-230;
230
230
90 Th 90 Th +γ
→
 4 Modern Physics

Example 1
Uranium – 238 (23892U ) undergoes decay to
become lead-206( 82 Pb ) . Find the number x=8
206

of α and β-particles emitted in the Also;

process. 92 = 82 + 2x – y
Solution 92 = 82 + 16 – y
Let the number of α and β-particles 92 = 98 – y
emitted be x and y respectively.
y=6
Pb + x ( 2 He ) + y ( −1e )
238 206 4 0
92 U 82
Eight α-particles and six β-particles
→

238 = 206 + 4x are emitted.

4x = 32
Example 2
234
Uranium 92 U decays to polonium
218
84 Po
by emitting alpha particles.
Write down the nuclear equation
representing the decay.
Solution
Let the number of alpha particles
(helium) be x.

234
92 U
218
84 Po+ x ( 42 He )
→

234 = 218 + 4x
16 = 4x
x=4
The decay equation is, therefore;

Po+ 4 ( 2 He )
234 218 4
92 U 84
→

Properties of Radiations
Penetrating Power

 Alpha particles are able to penetrate a thin NB: 1 cm thickness of lead

metal foil, but are stopped by a paper few is referred to as half-
millimetres thick. Their range in air is about 5 thickness for lead, since it
centimetres. lowers the intensity of the
 Beta particles are able to penetrate thin metal foil radiation to half the
and paper but are stopped by aluminium about 3 original volume
mm thick.
 Gamma rays can penetrate most materials and Gamma rays have neither
are only stopped by a block of lead about 5 cm mass nor charge.They are
thick or very thick concrete wall. similar to X-rays, but
have a generally shorter
wavelength.
The main difference
between X-rays and
gamma rays is that
gamma rays originate from
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energy changes in the
nucleus of atoms while X-
 RADIOACTIVITY 5

Ionising Effect of the Radiations

Ionisation occurs when alpha, beta or gamma radiations pass through air, knocking off
electrons from air molecules, resulting in the formation of positive ions.
Alpha particle have the highest ionising power since they are heavy and slow.
Gamma rays have the least ionising power.

Radiation Detectors

Photographic Emulsions
All the three radiations affect photographic emulsion or plate.

Cloud Chamber
The ionisation of air molecules by radiations is investigated by a cloud chamber.
Saturated vapour (water or alcohol) is made to condense on air ions caused by
radiations. Whitish lines of tiny liquid drops show up as tracks when illuminated.

Expansion Cloud Chamber

When a radioactive element emits
radiations into the chamber, the air
inside is ionised.

If the piston is now moved down

suddenly, air in the chamber will
expand and cooling occurs. When this
happens, the ions formed act as nuclei
on which the saturated alcohol or
water vapour condenses, forming tracks.
 6 Modern Physics

Diffusion Cloud Chamber

It is made up of a cylindrical
transparent container partitioned
into two compartments by a
blackened metal plate.
The upper compartment is fitted
with a transparent perspex lid and
its top is lined with a thin strip of
felt ring soaked in alcohol or water.
The bottom compartment is fitted
with a sponge and closed with
removable cover. The upper
compartment contains air, which is at the room temperature at the top.
The air at the bottom is at a temperature of about –78 ° C due to a layer of dry ice
placed in the lower compartment. The felt ring at the top is soaked in alcohol. This
alcohol vaporises in the upper warm region, diffuses down and is then cooled.

At a certain height above the base of the chamber, the air contains a layer of saturated
alcohol vapour. Here, alcohol droplets form on the air ions produced by the radiation.
These are seen as tracks along the path of radiation. The tracks are well defined if an
electric field is created by frequently rubbing the perspex lid of the chamber with a
piece of cloth.
(a) The tracks due to alpha particles are short, straight
and thick.
This is because:
(i) alpha particles cause heavy ionisation, rapidly
losing energy, hence their short range.
(ii) they are massive and their path cannot therefore
be changed by air molecules.
(iii) alpha particles cause more ions on their paths as they knock off more
electrons.
(b) The tracks formed by beta particles are generally thin and irregular
in direction.
This is because beta particles, being lighter and faster, cause less
ionisation of air molecules. In addition, the particles are repelled by
electrons of atoms within their path.

(c) Gamma rays produce scanty disjointed tracks.

The rays eject electrons from their molecules. These
electrons behave like weak beta particles, which are
responsible for the tracks seen.

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 RADIOACTIVITY 7

Geiger-Muller Tube
The Geiger-Muller (G-M) tube is a type of ionisation chamber.
The tube consists of a thin mica (or aluminium) window at one end of a closed glass
tube which contains argon gas and a little bromine gas at low pressure. A thin wire runs
through the centre of the tube and is connected to the positive terminal of a high
voltage supply. The walls of the tube are coated with a conductor and connected to the
negative terminal of the power supply.

When a radioactive substance is placed in front of the window, the emitted radiations
enter the tube through the window and ionise the argon gas. The negative ions move
towards the central wire (anode), while the positive ions move towards the wall
(cathode). As the ions accelerate, they collide with more particles on their paths,
resulting in further ionisation. This secondary ionisation ultimately results in an
avalanche of electrons. A pulse current therefore flows. A single ionising particle is
able to produce as many as 10 8 electrons due to the pressure of the argon gas. The
process is referred to as gas amplification.
A corresponding pulse voltage is registered across the high resistance R. These
currents can be amplified and if passed through a loudspeaker, clicks are heard. The
loudspeaker can be replaced by a scaler or ratemeter to give the exact count rate of
the radiation. The amplification increases the sensitivity of the tube, making it possible
to register very small currents from beta and gamma radiations. During this process,
the positive ions are supposed to move to the cathode. However, because of their
mass, the movement is slow. They produce a shielding effect on the anode, reducing
the electric field between anode and cathode. Any ionisation caused by an incoming
radioactive emission will therefore not be detected. The time taken by the positive ions
to move away from the anode (reducing the shielding effect) so that the field comes to
normal is called ‘dead time’.
The bromine molecules inside the tube absorb the kinetic energy of the positive ions as
they move towards the cathode. If the positive ions were to collide with the cathode,
electrons would be produced which would cause a second electron avalanche, resulting
in a false pulse. Bromine gas acts as a quenching agent.

Background Radiation
This is encountered when carrying out experiments using radioactive materials, where
the counter registers some readings even in the absence of a radioactive source. This
implies presence of radiation.
The count registered in the absence of the radioactive source is called background
count. Some sources of these background radiation include:
(i) cosmic rays from outer space.
 8 Modern Physics

(ii) radiations from the sun.

(iii) some rocks which contain traces of radioactive material, e.g., granite.
(iv) natural and artificial radioisotopes.

Half-life and Decay law

Half-life is the time taken for half the number of nuclides initially present in a
radioactive sample to decay.
It is usually abbreviated by t1/2.
In general, if the original activity is A o, it reduces to Ao/2 in 1 half-life period, A o reduces
to Ao/4 in another half-life period as shown below.

Consider 2 g of radium, whose half-life is 1 600 years. In 1600 years 1 g will have
1
decayed. In the next 1 600 years, g of the sample will be the remaining, as in the
2
table below.

It can be shown that the number of nuclides remaining undecayed, N, after time T is
given by;

( ) , where N is the original number of nuclides and t the half-life.

T
1 t
N = No o
2
A graph of number of radioactive nuclides remaining against the time T is a curve as
shown below.

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 RADIOACTIVITY 9

Decay Law
They decay law states that the rate of disintegration at a given time is directly
proportional to the number of nuclides present at that time. This can be expressed as;
dN
α -N where N is the number of nuclides present at a given time.
dt
It follows that;
dN
= -λN, where λ is a constant known as the decay constant.
dt
The negative sign shows that the number N decreases as time increases.
dN
is referred to as the activity of the sample.
dt

Worked Examples
Example 1
The half-life of a certain radioactive element is 16 years.
(a) What fraction of the element will be remaining after 48
years?
(b) What fraction of the element will have decayed after 64
years?

Solution

()
T
1 t
(a) From the formula N = No ,
2

()
T
N 1
Fraction remaining = ¿ t
No 2

= ( ) ¿( ) =
48 3
1 16 1 1
2 2 8
64
(b) Number of half-lives after 64 years = =4
16

()
4
1 1
Fraction remaining after 4 half-lives = =
2 16
−1 15
Fraction decayed = 1 =
16 16

Example 2
 10 Modern Physics

Give practice on graph-drawing to determine half-life.

Artificial Radioactivity
Some naturally occurring nuclides can be made artificially radioactive by bombarding
them with neutrons, protons or alpha particles.
For example, when nitrogen-14 (147 N ) nuclide, which is stable, is bombarded with fast
moving alpha particles, radioactive oxygen is formed. This is represented by;
17 1
4
2 He +
14
7 N 8 O + 1H
→

Other artificially radioactive nuclides are silicon-27 (274Si ) , sulphur-35 (1635S ) and chlorine-
36 ( 36
17 Cl ).

Nuclear Fission
Nuclear Fission is the splitting of heavy nucleus when bombarded, producing a lot of
energy. When uranium-235 is bombarded with a neutron, it becomes uranium-236,
which is more active than uranium-235. Uranium-236 splits into barium-144 and
krypton-90, as shown below.
90
Kr + 2 ( 0n )+ Energy
235 1 236 144 1
92 U + 0n 92 U 56 Ba + 36
→ →

The energy released during the splitting is called nuclear energy.

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 RADIOACTIVITY 11

The emitted neutrons may encounter other uranium nuclides, resulting in more splitting
with further release of energy. The produced neutrons are called fission neutrons.
When this occurs, it is called a chain reaction.

Nuclear Fusion
Nuclear fusion is the combination of two or more light nuclei to form a heavier
nucleus and in the process releasing a lot of energy. An example of nuclear fusion is
the formation of alpha particles when lithium fuses with hydrogen;
7 1 8 4 4
3 Li + 1H 4 Be 2 He + 2He
→ →

This fusion is accompanied by the release of a lot of energy.

Applications of Radioactivity
Carbon Dating
Living organisms take in small quantities of radioactive carbon-14, in addition to the
ordinary carbon-12. The ratio of carbon-12 to carbon-14 can be used to determine the
age of the organisms.

Medicine
Gamma rays, like X-rays, are used in the control of cancerous body growths. The
radiation kills cancer cells when the tumour is subjected to it. Gamma rays are also
used in the sterilisation of medical equipment, and for killing pests or making them
sterile.

Detecting Pipe Bursts

Underground pipes carrying water or oil many suffer bursts or leakages. If the water or
oil is mixed with radioactive substances from the source, the mixture will seep out
where there is an opening. If a detector is passed on the ground near the area, the
radiations will be detected.

Determining Thickness of Metal Foil

In industries which manufacture thin metal foils, paper and plastics, radioactive
radiations can be used to determine and maintain the required thickness.

Trace Elements
The movement of traces of a weak radioisotope introduced into an organism can be
monitored using a radiation detector. In agriculture, this method is applied to study the
plant uptake of fertilisers and other chemicals.

Detection of Flaws
Cracks and airspaces in welded joints can be detected using gamma radiation from
cobalt-60. The cobalt-60 is placed on one side of the joint and a photographic film on
the other. The film, when developed, will show any weakness in the joint.
 12 Modern Physics

Hazards of Radiation
The effect of the radiation on humans depends on its nature, the dose received and the
part irradiated.
Gamma rays present the main radiation hazard. This is because they penetrate deeply
into the body, causing damage to body cells and tissues. This may lead to skin burns
and blisters, sores and delayed effects such as cancer, leukaemia and hereditary
defects. Extremely heavy doses of radiation may lead to death.

Precautions
Radioactive elements should never be held with bare hands. Forceps or well
protected tongs should be used when handling them. For the safety of the users,
radioactive materials should be kept in thick lead boxes. In hospitals and research
laboratories, radiation absorbers are used.

Review Exercises
1. The following is part of a decay series of uranium-238:
238 234 234 234 230
92 U Th
90 94 Pa 92 U Th
90
→ → → →

State the particles emitted during each decay.

2. (a) Define the terms radioactivity and half-life.
(b) Describe briefly how you would distinguish between alpha, beta and gamma
radiations.

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 RADIOACTIVITY 13

3. A radioactive source emits α, β and γ-radiations of the same intensity. Describe how
you would show that these radiations have different
penetrating powers.
4. (a) The figure below shows the tracks left in a cloud chamber
by α and β-particles of the same energy. Explain why the α-
particle has a shorter track than the β-particle.
(b) An alpha particle of energy 5 × 10 –13 J travels a distance of 5
cm in air before coming to rest. Each air molecule needs 2 × 10 –18 J to be ionised.
How many ions does the alpha particle create in each centimetre of its travel?
5. What is meant by chain reaction in radioactivity?
6. Beta particles can be used to determine the thickness of cloth. With the help of a
diagram, explain how this is done.
7. Briefly describe three uses of radioisotopes.
8. (a) What is meant by background radiation?
(b) Thorium-234 is a radioactive element with half-life of 24 days. Calculate the
1
time taken by 1 g of it to decay to g.
8
9. Give the numerical values of a, b, c, d, e and f in the nuclear equation:
a 206 c e
b Po 82 Pb+ d α + f γ
→

10. Radon-219 has half-life of 4 seconds. What fraction of it will remain after 20
seconds?

11. When carrying out experiments using a cloud chamber, thin white lines are always
observed. How are these lines formed and what do they consist of?
12. (a) State two precautions that have to be observed when using radioactive
substances.
(b) A radioactive material has half-life of 20 minutes. Calculate :
(i) the number of half-lives in one hour.
(ii) the fraction of the original mass remaining after one hour.
(iii) the fraction that would decay in two hours.

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