Interactions of radiation with Matter

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Interactions of radiation with Matter
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Interaction with beta
Electrons excited or kicked off.
ionization Energy dissipated as heat. As Z of
material increases, so does bremsstrahlung. Note
that range is different from path.
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Interaction with gamma
Photon travels until it hits something, either an
electron or a nucleus. Several types of
interactions have been observed.
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Interaction of gamma with matter

• Photoelectric effect
• Photon hits electron, all of energy is
  transmitted, electron is ejected.
• Most likely with low energy photons, high Z
  material

http//www.faqs.org/docs/qp/images/peeffect.gif
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Gamma interaction-2

• Compton scattering
• Not all energy transmitted to electron.
• Electron ejected, secondary photon emitted
• With low energy photons, independent of Z

http//www.phys.jyu.fi/research/gamma/publications
/akthesis/img220.png
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Raleigh scattering and nuclear magnetic resonance

• Both involve impact of gamma on nucleus
• Raleigh gamma is deflected (elastic collision),
  keeps going.
• occurs when particles are very small compared to
  the wavelength of the radiation. (10-15 vs 10-10)
• NMR absorbed, emitted in a new direction

hosting.soonet.ca/.../scattering.gif
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Pair production and annihilation
Two gamma collide, convert to a positron and a
negatron. Complete energy to matter
conversion These two betas collide, converting to
2 gammas with equal energy of 511 kev. Complete
matter to energy conversion.
www.mhhe.com/.../fix/ student/images/26f14.jpg
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Summary of interactions

• Alpha
• Penetrates short distance into matter, giving up
  its energy by ionizing matter and releasing heat.
• Beta
• Bounces around, giving up energy by ionizing
  matter and dissipating kinetic energy as heat.
• Gamma
• Penetrates, colliding with electrons
• Photoelectric effect, Compton scattering
• Collides with nuclei (Raleigh scattering, NMR)
• Collides with another gamma

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About interactions

• Radiation is moving energy
• All types have kinetic energy
• Alpha and beta particles have charge
• Energy cannot be created or destroyed
• Energy is transferred
• Dose is a measure of how much energy is deposited
  in an absorber
• Absorber could be inanimate or could be flesh
• Energy left as heat, electrical potential, etc.

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Bragg Effect

• As particles (alpha, beta) slow down, ionizations
  increase near the end of their paths.
• Proton anti-cancer therapy relies on this.

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About Dose

• Linear Energy Transfer
• Average energy deposited in absorber per unit
  distance traveled by charged particle.
• RAD radiation absorbed dose
• The amount of energy absorbed per unit of
  absorbing material. (new units Gray)
• RBE Relative Biological Effectiveness
• Depends directly on the LET, a quality factor Q
  used in determining the effect of LET on the
  absorbed dose, i.e. how much damage.

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More on dose

• REM roentgen equivalent man
• Effective dose resulting from the RAD and the RBE
• REM Q x dose (in RAD)
• Q is a measure of RBE as determined from LET.
• New unit is sieverts
• Slowly moving, greatly ionizing alpha particles
  have a much higher LET, so Q will be gt1, and the
  energy absorbed will have a bigger biological
  effect (if absorbed by living tissue)

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More on calculating REM
LET (keV per µm) Q example
3.5 and less 1 X-rays,ß, ?
7 2 neutrons
23 5
53 10
175 and over 20 alpha
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Comparing old, SI units
Old SI
Radioactive material curies becquerels
Deposited energy Rads Grays
Dose to humans Rems Sieverts
Units of energy in air Roentgens none
Rad 100 ergs/gram Rem rad x Q 1 Gray
100 Rads, 1 j/kg 1 Sievert 100 rem
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Radiation Safety Rules of Thumb

1. Alpha particles up to 7.5 MeV are stopped in the
   dead layer of normal skin.
2. Beta particles will penetrate about 4 meters in
   air per MeV of energy.
3. Beta particles will penetrate about 0.5 cm in
   soft tissue per MeV of energy.
4. Beta particles up to 70 KeV are stopped in the
   dead layer of normal human skin.