7: Atomic and Nuclear Physics

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7 Atomic and Nuclear Physics

• 7.2 Radioactive Decay

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Radioactivity Radioactive Decay 1895 Wilhelm
Roentgen creates and discovers X-ray radiation,
using an electrical energy source. 1896
Henri Becquerel discovers that some Uranium
compounds emit radiation without the need for an
external energy source. In 1898 this effect was
termed radioactivity by Marie
Curie. Radioactivity The spontaneous emission
of radiation by a material
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Radiation Radiation is emitted by elements with
unstable nuclei in an attempt to loose energy and
so gain stability. When radiation is emitted and
the nucleus of the atom changes we say it has
decayed. There are three types of radiation -
Alpha (a) - Beta (ß) - Gamma (?) Electromagnetic
wave
Involve particles
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• Alpha decay - When a nucleus emits an alpha
  particle, it loses two protons and two neutrons.
  So the nucleon number decreases by four and the
  proton number by two.
• - Energy of a particle is around 6 MeV. This is
  very high for such a small particle and so if
  contact is made with human cells, much damage can
  be done due to the high degree of ionisation that
  will be caused.

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• Beta decay - When a nucleus emits a beta minus
  particle, a neutron turns into a proton plus an
  electron and an anti-neutrino (nu - ? a
  virtually massless particle with no charge). The
  electron is emitted (at high speed) as the beta
  minus particle.
• - nucleon number is unchanged
• - proton number goes up by one
• - Although they move much faster than alpha
  particles, ß particles create less ionisation due
  to their small mass.

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3. Gamma emission After an alpha or beta
particle have been emitted, the new nucleus may
be left in a very excited state due it to having
excess energy. This energy may be emitted as a
burst of gamma radiation- a gamma photon. Gamma
does not cause a change in the structure of the
nucleus. - Gamma produces very little ionisation
but is very penetrating.
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Nuclear Stability Q. Write down the neutron
number (N) and proton number (Z) for these stable
nuclides For Helium N Z For Iron N gt Z
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• Nuclear Stability
• For nuclear stability
• Very light nuclei N Z
• Heavier nuclei N gt Z
• The green region shows nuclides with too many
  neutrons. Beta particle emission causes N to
  decrease by one and Z to increase by one. This
  changes the nuclide and moves it into the band of
  stability.

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• Q.
• Nitrogen17 is a radioactive isotope of
  nitrogen.
• Would it become stable if it emitted a Beta
  particle?
• What element would the new daughter nuclide be?

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Decay Chains A nucleus may undergo a series of
decays before it becomes stable. This is known as
a decay chain.
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Biological effects of radiation
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Some of the new compounds formed may be toxic,
contributing to destruction of cells (e.g
hydrogen peroxide H2O2)
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High dose of radiation - Organs and central
nervous system fail to function. Death within
days. Medium dose of radiation - Some organs
dont function properly including parts of
digestive system. Sickness and possibly death in
weeks. Low dose of radiation - Hair loss,
internal bleeding and some sickness. Low chance
of immediate death but high chance of cancer
later in life. Safe dose of radiation - Very
low doses carry little risk e.g from X-rays or
background radiation. There is still some chance
of ionisation.
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Half Life Radioactive decay is a completely
random process thus it is not possible to predict
which nucleus will decay next. However, due to
the large number of nuclei involved, a
statistical pattern is established. Task
Use dice or the radiation lab computer model of
radioactive decay to take results, plot a decay
curve and determine the half life of the sample.
The half life of a material is the average time
taken for half of the radiocative nuclei within a
sample to decay.
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Text
(An asymptotic curve)
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Activity The activity of a sample is a measure of
how many counts of radiation are emitted per
second. Thus the activity is proportional to the
number of radioactive nuclei remaining. So a
graph of activity against time would also enable
us to determine half life. Q. Sketch a graph to
show how a decay curve may look if the background
count was not compensated for.
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