Interactions of radiation with Matter

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.

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

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

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

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

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

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

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.

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

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.

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)

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

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;

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.