external radiation, the dose equivalent is obtained by multiplying the absorbed dose of a given type of radiation by the quality factor for that radiation and summing these products for all radiations-incident at a given point in the body. The quality factor is defined in terms of the energy deposition per unit path length of the charged particles at the point of interest in the human body. The 'Relative Biological Effectiveness" (RBE) is the ratio of the absorbed dose of two radiations producing the same biological effect. radiations, called the reference radiation, or gamma rays, One of these is frequently moderate energy X- This term is most often used in reporting the results of biological experiments and the reference radiation is often moderate energy, x or gamma rays. Thus, if the absorbed dose of the reference radiation is 20 times larger than the absorbed dose of the radiation of interest to produce the same effect, one would say that the RBE of the other radiation is 20. Lethal Dose: The LD-50 (LD equals lethal dose) that will kill 50 percent of the organisms exposed. used when the radiation is administered acutely. is the absorbed dose This term is usually One needs also to indicate the time span over which the observations are made following the irradiation. Also, it is usually used when the absorbed dose is more or less consistent throughout the organism and the magnitude is given for no pre- or posttreatment of the organism. The LD-50 in 30 days for man is about 300 rads. This is a numerical value stated in the Langham National Academy of Science report on space travel, Treatment for Acute Effects There are no methods of treatment which will cure the immediate effects of severe doses of radiation -- only treatment for the different kinds of FY 6 9010210