Chapter 5: Radioactivity

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Chapter 5 Radioactivity
Form 5
Physics
Next gt
The study of matter
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Objectives (what you will learn)
1) understanding nucleus of atom2) radioactive
decay3) uses of radioisotopes4) nuclear
energy5) management of radioactive substances
Physics Chapter 5
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Nucleus 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
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Nucleus 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
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.
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Nucleus of atom

• Rutherfords alpha-particle (a-particle)
  scattering experiment
• Rutherford bombarded gold foil with a-particles.
• Most a-particles go through gold foil undeflected
  as the nucleus is very tiny (occupies a small
  fraction of the volume of atom).
• Some a-particles are slightly deflected, others
  are deflected through large angles. The positive
  a-particles are repelled by a massive, positively
  charged nucleus.

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Radioactive 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
a-particles and ß-particles, and the radiation
emitted is gamma-ray (?-ray). Apparatus used to
detect radioactive emissions include cloud
chamber and Geiger-Muller tube (GM tube).
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Radioactive decay

• The tracks of radioactive emissions can be
  observed in a cloud chamber.
• a-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

• ß-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

• ?-rays tracks
• identical to ß-particles tracks but are short
• the tracks are those of electrons produced from
  ionisation of air

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Radioactive decay
Geiger-Muller tube When connected to a counter,
it will count the number of ß-particles or ?-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 a-particles which cannot
penetrate the window of the tube.
doctronics
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Radioactive decay
Changes to structure of nucleus during
radioactive decay. Alpha-decay
Proton number decreases by 2. Nucleon number
decreases by 4.
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Radioactive decay
The half-life, T½ of a radioisotope is the time
taken for half of the number of nuclei in a
sample to decay.
It is also the time taken for the rate of decay
of a sample to become half.
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Radioactive decay
Radioisotope an isotope that is radioactive
Uses in medicine (a) ?-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) ?-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 decay
Uses in industry (a) Gauge control GM-tube
connected to ratemeter measures thickness of
paper by its constant count rate. (b) Leak tracer
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 ?-rays (Cobalt-60) used to detect flaws
in joints between pipes carrying natural
gas. (d) Smoke detector Americium-241 emits
a-particles which ionizes air particles, allowing
current to flow between charged plates. Smoke
particles which reduces current flow by
deflecting a-particles can then be detected.
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Nuclear Energy
Einsteins 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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Management

• 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

a-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.
ß-particles Harmful both outside and inside body
due to stronger penetration power, but moderate
ionizing power.
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?-rays Harmful outside body due to strong
penetration power.
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Summary
What you have learned

1. understanding nucleus of atom

2. radioactive decay
lt Back
3. uses of radioisotopes
4. nuclear energy

1. management of radioactivesubstances

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Thank You