Connected with this physical half-life of the isotope is another kind of half-life called the "biological half-life" which is the amount of time it takes for one-half of the radioisotope to be removed from the body, through elimination of waste matter. A further aspect of radiation that is of interest in the present context is that charged particles lose energy along their path to the mediums they are penetrating. even produce radioactivity. This energy may break up molecules or For the energetic particles of interest here, thousands of molecules may he affected along the track of the particle. Because of the positive-negative electrical charges, tear electrons from atoms they strike. these particles can It takes anywhere from ten to one hundred electron volts of energy to tear an electron from an atom. the radiation energy of particles is measured in the thousands volts -- kev) or millions (million electron volts -- mev). Usually (kilo electron Consequently, particles in the kev or mev can disrupt or ionize thousands of other atoms before their energy is used up and they stop. Measurement The amount of radiation may be measured in a number of different ways. Of particular interest here are measures that indicate the interaction of the radiation with various The "Roentgen" types of material. is a unit for exposure which indicates the ability of the X or gamma radiation to produce ionization in air. The "Rad" is a unit of absorbed dose which indicates the amount of energy absorbed in a material per unit mass of material. The "Rem" is the unit of dose equivalent. The dose cquivalent is for radiation pretection purposes and provides a common frame for assessing the wy combined effects of different kinds of radiation on 0209 the human body. For