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 atom
1. 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.
69
Result Conclusion
Most 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 )
r2

Physics Form 5
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 H2
1 and H3
1
70
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
Nucleon
Number,
A
Proton
number
Z
p n e
Hydrogen
1
1 H
1 0 1 1 1
Carbon
14
6 C 6 8 6 14 6
Beryllium
9
4 Be 4 5 4 9 4
N = A - Z

Physics Form 5
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.
71
Element Stable
isotopes
Unstable
isotopes
Carbon 12
6 C
14
6 C
Oxygen 16
8 O
15
8 O,
19
8 O
Lead 206
82 Pb,
208
82 Pb
210
82 Pb,
214
82
Pb

Physics Form 5
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.
72

Physics Form 5
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 4
2 He 0
1− 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
73

Physics Form 5
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 plate
α 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 ;
U238
92 → Th234
90 + He4
2
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 ;
Sr90
38 → Y90
39 + e0
1−
74

Physics Form 5
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 → Co60
27 + γ
Example 1
Balance the following equations:
(a) Po214
84 → Pb82 + He4
(b) Bi83 → Po214
84 + e0
+ γ
Solution
Example 2
How many alpha particles and beta particles are
emitted when Th232
90 decays into Pb208
82 ?
Solution
A decay series
1. 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
75

Physics Form 5
The number of atoms , mass or activity of a
radioactive substance decreases with time.
Half-life
The half-life of a radioactive material is the time
taken for half of the unstable atoms to decay.
Or
The half-life of a radioactive material is the time
taken for the activity of radioactive fall to half its
original activity.
76
Half-life T0 1T1/2 2T1/2 3T1/2
Number of
undecayed atoms. N (½)1
N = ½N (½)2
N = ¼ N (½)3
N=
1
8 N
% atoms Undecayed 100 % 50 % 25 % 12.5 %
Mass 64 g 32 g 16 g 8 g
Activity(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–
1
8 N =
7
8 N
Mass haveDecayed 0 g 32 g 48 g 56 g
% atoms
Decayed
0 % 50 % 75 % 87.5 %

Physics Form 5
Example :
234
91 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 4
The 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
77

Physics Form 5
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.
78
Radioisotopes Half life Uses Process
Sodium-24 15 hours
Emit β and γ
detect 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.

Physics Form 5
Radioisotope in Industry
79
Radioisotope Half life Uses
Strontium-90 28 years
emits β
the 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 hours
Emit 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 years
Emit 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.

Physics Form 5
Radioisotopes in Agriculture
Radioisotopes Halflife Uses
Phosphorus-
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-14
1. Carbon-14 has a half-life 5 700 years
2. 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
80

Physics Form 5
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 1
The 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 reaction
1. 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 mass
1. 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.
81

Physics Form 5
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
82
Nuclear Fusion Nuclear Fisson
Definition 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

Physics Form 5
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 Function
Graphite 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 Function
Steam 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
83

Physics Form 5
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
84
Harmful effect Examples
Somatic 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

Physics Form 5
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 possible
8. Radioactive substances are kept in thick lead
containers
9. 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.
85

Radioactivity: physics form 5.

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