- conditions. 14 - Water or soft tissues exposed to one roentgen (r) receive an absorbed dose of very nearly one rad. defined, As the roentgen cannot be applied to radiations other than X rays or gamma rays. To compare the effectiveness of absorbed doses of radiation delivered in different ways, the term relative biological effectiveness (RBE) is used. is the rem. The unit of RBE By definition, one rad of X rays, gamma rays or beta rays is equivalent to one rem but one rad of alpha rays is equivalent to about 10 rem. Dose in rad x RBE = dose in rem. Biological Effects of Ionizing Radiations The ionizations that occur when nuclear radiations pass through a gas or a phosphor make it possible for radioactivity to be detected and measured, tions, and the same ioniza- when they occur in tissuessof living organisms, are primarily responsible for the biological effects that occur as a result of irradiation. The answer to the question, "Why are radioisotopes harmful?", is that they emit tonizing radiations which are capable of injuring or destroying cells. However, precise information on the exact manner in which