In recent years, the biological effects of repeated small doses of ionizing radiation have been the subject of considerable study. Scientific investigation has shown that when certain critical organs are irradiated, changes may occur which impair organ function and alter organ morphology. Irradiation of the reproductive system, for example, may cause mutations in succeeding generations, and irradiation of the hematopoietic system may lead, after the lapse of a few years, to leukemia. Similar qualitative relationships exist between radiant exposure of the thyroid gland and thyroid cancer, between irradiation of the orbit and cataract, and between irradiation of bone and bone sarcoma. In addition to the foregoing, it is known that the life span of an organism may be reduced by radiation exposure. The precise mechanism of this effect, however, is poorly understood. Although much has been written about the biological effects of radiation, it is noteworthy that current knowledge is more qualitative than quanti...