1) The document discusses radioactivity and nuclear physics, including the structure of the atom's nucleus, radioactive decay, uses of radioisotopes, nuclear energy, and management of radioactive substances. 2) It explains key concepts such as alpha and beta particles, half-life, radioactive tracers, and the three main types of radioactive emissions. 3) Applications of radioactivity covered include uses in medicine, agriculture, archaeology, industry, and the production of nuclear energy through fission.

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Chapter5:Chapter5:
RadioactivityRadioactivity
Form 5Form 5
1
PhysicsPhysics
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The study o f m atte rThe study o f m atte r

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Objectives:Objectives:
((what yo u willle arnwhat yo u willle arn))
1) understanding nucleus of atom
2) radioactive decay
3) uses of radioisotopes
4) nuclear energy
5) management of radioactive
substances
Physics: Chapter 5Physics: Chapter 5
2
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Nucleus of atomNucleus of atom
An atom consists of a central nucleus where most of
the mass of the atom is concentrated.
Orbiting around nucleus are electrons.
The nucleus is composed of protons that are
positively charged, and neutrons that are neutral.
Nucleons = protons + neutrons
n
n
+
+
–
– + proton
n neutron
– electron
A helium atom
4 He
2

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Nucleus of atomNucleus of atom
proton number(Z) = the number of protons in nucleus
nucleon number(A) = the number of nucleons (protons & neutrons)
in nucleus
nuclide = a nucleus species with a certain proton number & certain
nucleon number
4 He
2
represents nucleus with proton number Z & nucleon AA
X
Z
represents nuclide with 2 protons & 4 nucleons
The number of neutrons is 4 – 2 = 2
Isotopes = nuclides with same proton number, different nucleon numbers
Isotopes of an element have the same chemical properties but
different physical properties, such as mass.
1H 2H 3H
Exam ple
iso to pe s:
(tritium)(deuterium)(hydrogen)

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Nucleus of atomNucleus of atom
Rutherford’s alpha-particle (α-particle) scattering experiment
Rutherford bombarded gold foil with α-particles.
•Most α-particles go through gold foil undeflected as the nucleus is
very tiny (occupies a small fraction of the volume of atom).
•Some α-particles are slightly deflected, others are deflected through
large angles. The positive α-particles are repelled by a massive,
positively charged nucleus.
α-particle
source
α-particle
deflected
Gold foil
Fluorescent screen
Telescope
vacuum

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Radioactive decayRadioactive decay
Radioactivity = spontaneous disintegration of unstable nuclei
accompanied by emission of energetic particles or radiations
(photons).
Spontaneous disintegration = emissions of the particles or
photons are not planned in advance
Radioactive decay is random because it is not possible to predict
• which nuclei
• the number of nuclei that would decay at a particular instant
Radioactive decay is not affected by
• physical conditions such as temperature and pressure,
• chemical composition
The particles emitted in radioactive decays are α-particles and β-
particles, and the radiation emitted is gamma-ray (γ-ray).
Apparatus used to detect radioactive emissions include cloud
chamber and Geiger-Muller tube (GM tube).

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Radioactive decayRadioactive decay
The tracks of radioactive emissions can be
observed in a cloud chamber.
α-particles’ tracks:
• thick because of their high ionizing power
• straight because of the comparative large mass
• all of same length because they are emitted with the same speed
β-particles’ tracks:
• thin because of their weak ionizing power
• wavy because of the comparative small mass
• long because of its relative long range in air
γ-rays’ tracks:
• identical to β-particles’ tracks but are short
• the tracks are those of electrons produced from ionisation of air

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doctronics
Radioactive decayRadioactive decay
Geiger-Muller tube
When connected to a counter, it will count the number of
β-particles or γ-ray photons that enters it.
When connected to a ratemeter, it will give the number
of particles per seconds that enter the GM-tube.
The GM-tube is unable to detect α-particles which
cannot penetrate the window of the tube.

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Radioactive decayRadioactive decay
Changes to structure of nucleus during radioactive decay.
Alpha-decay
A
X
Z
A-4
Y
Z-2
+ 4
He
2
(α-particle)
Beta-decay
A
X
Z
A
Y
Z+1
+ 0
e
-1
(β-particle)
Gamma ray
No changes in the proton number and nucleon number.
Proton number increases by 1.
Nucleon number unchanged.
Proton number decreases by 2.
Nucleon number decreases by 4.

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Radioactive decayRadioactive decay
The half-life, T½ of a radioisotope is the time taken for half
of the numberof nuclei in a sample to decay.
It is also the time taken for the rate of decay of a sample to
become half.
N0 ½N0 ¼N0
T½ T½
A0 ½A0
¼A0
T½ T½

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Radioactive decayRadioactive decay
Radioisotope = an isotope that is radioactive
Uses in medicine
(a) γ-rays from cobalt-60:
- radiotherapy to destroy cancerous cells
- sterilization to destroy bacteria or germs
(b) Radioactive tracers:
- iodine-131 to evaluate function of thyroid gland
- sodium-24 to estimate volume of blood in patient
Uses in agriculture
(a) Radioactive tracers used in plant nutrient research.
(b) γ-rays used to sterilize insects, destroy
pests/bacteria in food/fruits.
Uses in archaeology
(a) Carbon-14 dating: Proportion of C-14 to C-12 in living
organism is the same as that of the atmosphere. When
an organism dies, its proportion decreases. Its age is
estimated by measuring its proportion in sample.

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Radioactive decayRadioactive decay
Uses in industry
(a) Gauge control
GM-tube connected to ratemeter measures thickness of
paper by its constant count rate.
(b) Leaktracer
Sodium-24 used as tracer to locate damaged
underground pipes. GM-tube is used to detect high
count rate from leaks in the pipe.
(c) Quality control
γ-rays (Cobalt-60) used to detect flaws in joints between
pipes carrying natural gas.
(d) Smoke detector
Americium-241 emits α-particles which ionizes air
particles, allowing current to flow between charged
plates. Smoke particles which reduces current flow by
deflecting α-particles can then be detected.

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Nuclear EnergyNuclear Energy
The unit of mass used for measuring the mass of atoms,
Atomic mass unit (a.m.u.),
u = (mass of an atom of carbon-12)
1 u = 1.66 x 10-27
kg
1
12
Einstein’s energy-mass relation
The energy equivalent E of mass m is given by
Energy, E = mc2
where c = 3.0 x 108
m s-1
Nuclear fission = splitting of a nucleus into two nuclei
Slow neutrons are used to split the nucleus.

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ManagementManagement
2 negative effects of radioactive materials
- Somatic damage: near-term death of cells of sensitive
organs such as eyes.
- Genetic damage: long-term effect; mutation of cells in
subsequent generations
α-particles:
Quite harmless outside body due to short range and weak
penetration power. Inside body, they are the most damaging due to
their strong ionizing power.
β-particles:
Harmful both outside and inside body due to stronger penetration
power, but moderate ionizing power.
γ-rays:
Harmful outside body due to strong penetration power.

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SummarySummary
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What you have learned:What you have learned:
1. understanding nucleus of atom
ThankYouThankYou
2. radioactive decay
3. uses of radioisotopes
4. nuclear energy
5. management of radioactive
substances