Interactions of radiation with Matter

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 http://www.faqs.org/docs/qp/images/peeffect.gif

5. 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

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. www.mhhe.com/.../fix/ 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 • 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

14. Comparing old, SI units 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.