1. Interactions of radiation with Matter

2. 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.

3. Interaction with gamma
Photon travels until it hits something, either an electron or a nucleus.
Several types of interactions have been observed.

4. 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

5. Gamma interaction-2 Compton scattering
Not all energy transmitted to electron.
Electron ejected, secondary photon emitted
With low energy photons, independent of Z

6. 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

7. 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.
student/images/26f14.jpg

8. 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

9. 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.

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

11. About Dose Linear Energy Transfer RAD: radiation absorbed dose
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.

12. 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 >1, and the energy absorbed will have a bigger biological effect (if absorbed by living tissue)

13. More on calculating REM
LET (keV per µm) Q
example
3.5 and less 1
X-rays,β,  7 2
neutrons 23 5 53 10
175 and over 20
alpha

14. 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;

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