Radiation Physics

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Radiation Physics

• Ya-yun Hsiao

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Outline

• Radiodecay
• Interactions of radiations with matter

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Example

• A 6.2 mg sample of 90Sr (half-life 29.12 year) is
  in secular equilibrium with its daughter 90Y
  (half-life 64.0h). (a) how many Bq of 90Sr are
  present? (b) how many Bq of 90Y are present? (c)
  what is the mass of 90Y present? (d) what will
  the activity of 90Y be after 100 years?

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Secular equilibrium ?SrNSr?YNY
NSr NY NTOTAL
NY
NSr1.04x1016 atoms
/3985.84.147x 1019 atoms A(t)
?N(t)
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• (a)
• (b)
• (c)
• (d)

mY
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Example

• Consider the following beta decay chain with
  half-lives indicated,
• 210Pb (22y)? 210Bi (5d)? 210Po
• A sample contain 30 MBq of 210Pb and 15M Bq of
  210Bi at t0 (a) calculate the activity of 210Bi
  at time 10d (b) If the sample were originally
  pure 210Pb, how old would it have been at t0?

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Interactions of photons with matter

• Photoelectric Effect
• Compton Effect
• Pair production

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Photoelectric effect

• Most of this energy goes to an atomic electron
  (with a small amount to the nucleus) resulting in
  a free electron and an ionized atom.
• The photoelectric effect may only occur if the
  incident photon has an energy higher than the
  binding energy of the atomic electron.

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Top insert to illustrate the interaction of a
photon with an atom to eject an electron from the
K shell to produce a photoelectron. When the
hole in the K shell is filled, characteristic
radiation is emitted. Main graph Photoelectric
attenuation coefficients for water and lead
plotted on a log-log scale.
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Compton Effect

• An interaction between a photon and an atomic
  electron
• An inelastic process
• Some of the kinetic energy of the photon is
  required to overcome the binding energy of the
  atomic electron.

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Compton Effect

• And the recoil electron energy is given by
• T h? h?'
• High energy incident photons suffer a large
  energy change, but low energy incident photons do
  not. This is important in radiotherapy since
  when the incident energy of the photon is large
  most of the available energy goes into kinetic
  energy of the recoil electron, which goes on to
  deposit energy in tissue, and very little energy
  goes to the scattered photon

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Pair Production

• Pair production is an interaction between a
  photon and a nuleus in which the photon is
  transformed into an electron-positron pair. In
  the center of mass systm, the energy threshold
  for this e-p pair creation is 2moC2 1.022 MeV.

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Pair Production
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Mass Attenuation Coefficient
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Mass Energy Transfer Coefficient
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Mass Energy Transfer Coefficient

• Photons do not deposit energy in a material. It
  is the fast electrons that they produce following
  interaction that deposit energy. Over many
  interactions there will be an average energy
  transfer, , to the electron.
• The mass energy transfer coefficient is ?tr/?

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Mass Energy Absorption Coefficient

Letting g represent the average fraction of the
initial kinetic energy transferred to electrons
that is subsequently emitted as bremsstrahlung
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Mass Energy Absorption Coefficient
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