UNIT 5.
2 RADIOACTIVITY
1 Radioactive Decay
Radioactive decay is the emission of an -particle or a -particles and/or -radiation from the unstable
nucleus of an atom. [2]
The isotopes of an element may be radioactive due to:
1 an excess of neutrons in the nucleus
2 and/or the nucleus being too heavy [2]
Radioactive decay is of three types.
(i) Alpha Decay (-decay)
Example
220
86 Rn 42 + 216
84 Po + energy
In -decay, the nucleon no. decreases by 4 and the proton no. decreases by 2. [1]
(ii) Beta Decay (-decay)
Example
1
14 0 14
6C + 7N + energy
In -decay, the nucleon no. remains the same, but the proton no. increases by 1. [1]
Explanation
Actually in -decay, a neutron of the parent nucleus splits into a proton and an electron: [1]
1
0n 11p + 0
1e
The proton formed stays in the nucleus (i.e. daughter nucleus); however, the electron is emitted as a
beta particle. So, beta radiation is actually a beam of fast-moving electrons.
Notes
During -decay, the stability of the nucleus increases as a result of reduction in the no. of
nucleons.
During -decay, the stability of the nucleus increases as a result of reduction in the no. of excess
neutrons.
(iii) Gamma Decay (-decay)
Example
( 147 N) * 14
7N + -radiation
In -decay, the nucleon no. and the proton no. both remain unchanged.
Note
The nucleus changes to that of a different element during alpha and beta decays only.
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MOMENTUM
2 Nature and Properties of Alpha, Beta and Gamma Radiations
Ionization by , and -Radiations
-particle
When an -particle passes an atom, it knocks the outer electrons from it (i.e. ionises the atom).
Ionisation requires energy; so every time an ionisation occurs, the -particle loses some of its KE
and eventually comes to rest (after causing several ionisations).
and -radiations can also cause ionisation.
-radiation is the least ionising radiation, as it has no charge.
-radiation is most ionising radiation, as an -particle:
1 has double positive charge (so interacts strongly with the atoms)
2 has the largest KE (so can cause more ionisations)
3 is relatively slow (so gets more time to interact with the atoms) [3]
Penetrating Abilities of , and -Radiations
. .
. .
. .
thin 2 mm 2 cm
paper aluminium lead
Note
-radiation is the least interacting radiation, and so it has the largest penetrating ability. [1]
Deflection of , and -Radiations by Electric Field
.
. . .
. .
. .
Note
An -particle is much more massive than a -particle, so it undergo smaller deflection.
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MOMENTUM
Deflection of , and -Radiations by Magnetic Field
The direction of magnetic force on a charged particle moving in a magnetic field is determined by
Fleming’s left-hand rule.
The rule states that if we stretch first two fingers and the thumb of our left hand such that they are
mutually perpendicular to each other, with the first finger pointing in the direction of the magnetic field
(i.e. N pole to S pole) and the second finger in the direction of the current, then the thumb points in the
direction of the magnetic force.
Note
For a positive charge, the direction of current is the same as that of the motion of the charge, and vice
versa (for a negative charge).
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MOMENTUM
The following table summarises the nature and properties of , and -radiations.
radiation alpha beta gamma
2 1 0
4
symbol and 42 He 0 0
nature
packet of electromagnetic
& 2 protons & 2 neutrons 1 electron
energy
composition
4u
mass u / 2000 0
(where 1u mass of proton)
+2e
charge 1e 0
(where 1e = charge on proton)
10 2.9 10 3.0 10
7 8 8
speed (m/s)
relative
4 2
ionising 10 10 1
ability
several several hundred metres in
penetration 4 cm to 5 cm in air
metres in air air
power/range thin sheet of paper
2 mm Al 2 cm lead
deflection
by
electric and yes yes no
magnetic
fields
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MOMENTUM
3 Detection of Ionising Radiations
electronic
counter
Geiger-Muller tube (GM tube) connected to an electronic counter is used to detect and count ionising
radiation. Every time an alpha particle, a beta particle or a gamma ray photon enters the GM tube, an
electronic pulse is generated in it. The electronic counter connected to the tube counts the pulses and
shows the total pulses generated (in a certain time).
A GM counter keeps giving the reading even when there is apparently no source around. This
happens due to the background radiation.
Background radiation is a low-level and almost constant radiation from the environment we are all
exposed to all the time. [1 or 2]
The sources that make a significant contribution to the background radiation include:
radon gas (in air)
rocks and building
(granite rocks emit radioactive radon gas)
food and drink
(e.g. those containing potassium-40)
cosmic rays
(cosmic rays are high-energy particles from outer space. They are mostly absorbed by the
atmosphere, but some reach the Earth’s surface)
4 Random Nature of Radioactive Decay
Radioactive decay is a spontaneous process, which means that it cannot be controlled by external
factors (such as temperature and pressure).
Radioactive decay is also a random process, which means that it cannot be predicted with certainty
how many nuclei will decay in a given period of time or when the next decay will happen. Likewise, it
cannot be predicted with certainty either which particular nucleus will decay next.
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MOMENTUM
5 Decay Curve and Half-Life
no. of undecayed
nuclei left
The half-life of a radioactive substance is the average time taken for half of the nuclei in its sample to
decay. [1 or 2]
After one half-life, the activity (i.e. the decay rate) of the sample also becomes half, and so does the
count rate (however, the background radiation remains almost constant).
Sample Problem
If initial count rate due to a radioactive sample is 1000 cpm and the background count rate is 15 cpm,
then determine the total count rate (cpm) after:
(a) one half-life
(b) two half-lives
Solution
(a) total cpm = cpm from source after one half-life + cpm from background
= 500 + 15
= 515 cpm
(b) total cpm = cpm from source after two half-lives + cpm from background
= 250 + 15
= 265 cpm
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MOMENTUM
6 Practical Applications of , , and -Radiations
1 -emitter is used in smoke detectors.
Explanation
-particles ionise the air in the smoke detector, and the subsequent motion of the ions towards
the electrodes in the smoke detector produces a small current. When smoke enters the smoke
detector, the motion of the ions is impeded (as they adhere to the smoke particles). As a result,
the current falls, and the alarm goes off.
Notes
-emitter is used in smoke detectors because:
1 -particles have high ionising power
2 -particles do not travel very far in air (so it’s safe to use -emitter in smoke detectors) [2]
A long half-life -radiation source is preferred so that (almost) constant activity is maintained
for a long time. [2]
2 -emitter is used in measuring and automatically controlling the thickness of paper, plastic and
metal sheets during manufacture. A long half-life source is preferred.
3 -emitter is used to diagnose cancer.
Explanation
A chemical containing -emitter is injected into the patient. The cancer cells absorb the chemical
strongly. The -radiation emitted travel outside the body. External imaging of the intensity of the -
radiation indicates where the chemical has been strongly absorbed and hence the location of
cancer cells.
Notes
-emitter is used because:
1 -radiation can easily penetrate skin and internal organs
2 -radiation has low ionising power (so it’s relatively safe to use -emitter here) [2]
A short half-life source is preferred so that the activity is low after the procedure is over and
so will not pose any ongoing radiation threat. [2]
4 -emitter is used to detect flaws (e.g. cracks) in underground water pipes.
Note
A short half-life source is preferred so that the activity is low after the detection procedure is over.
5 -emitter is used to kill the cancerous cells (i.e. tumour). A long half-life source is preferred.
6 -emitter is used to sterilise medical equipment (e.g. syringes) by killing bacteria. A long half-life
source is preferred.
7 -emitter is used to irradiate certain foods, again by killing bacteria to preserve the food for longer.
A long half-life source is preferred.
7 Effects of Ionising Radiation on Living Things
1 An intense dose of ionising radiation causes a lot of ionisation in the cell, which may kill the cell.
2 If DNA in the nucleus of a (somatic) cell is damaged, it may lead to cancer (i.e. the cell may start
multiplying uncontrollably and a tumour may form).
3 If DNA of a reproductive cell (i.e. gamete) is damaged, it may lead to genetic disorder in the future
generations.
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MOMENTUM
8 Precautions and Safety Measures
1 The exposure time to the radiation should be minimised.
2 The distance from the source should be kept as large as possible.
3 People (working with radioactive sources) should be protected by the use of lead shielding.
4 Radioactive sources should be stored in thick lead-made containers.
5 To avoid direct contact with -emitter, the user should use long tongs (or forceps) to hold it.
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