1. Describe how radiation interacts with matter.
2. Define radiation dose in units of rads and rems.

Interaction of Radiation with Matter. In passing through matter alpha, beta, and gamma rays lose energy chiefly by interaction with electrons which make up most of the volume of matter. Alpha and beta particles, on colliding with electrons, forcefully eject these electrons from atoms and molecules, and thereby produce ions. These particles lose only a small fraction of their energy in a single collision with an electron. Because alpha and beta particles are extremely energetic, thousands of collisions are required to bring them to rest. These particles produce "tracks" of ionization. Alpha, beta, and gamma rays are known as ionizing radiation.

Most devices for detecting radioactivity depend on the formation of ions. The best known instrument for detecting radiation is the Geiger counter in which ions produced by a particle trigger a pulse of electricity that is counted electronically. Darkening of photographic plates, discharging of electroscopes, and damage to biological tissue all involve ionization.

Units of Radiation Dose. Two units used to measure radiation dose are the rad and the rem. The rad (radiation absorbed dose) is defined as 0.01 joule of energy absorbed per kilogram of any absorbing material. The millirad is one-thousandth of a rad. Because beams of different radiations cause very different biological damage even when the body absorbs the same amount of energy from each type, it is necessary to define a unit specifically for biological tissue. The unit of biologically effective dose is the rem (radiation equivalent in man), which is the absorbed dose in rads multiplied by the relative biological effectiveness factor, RBE. The millirem is one thousandth of a rem. For beta and gamma rays RBE = 1.0; for fast neutrons and alpha particles RBE = 10. Thus, a dose of one rad of alpha radiation is equivalent to 10 rem.

dose (in rem) = RBE x dose (in rad)