Cell Inactivation by Tritium Decays at 37 and -196°C: Some Comparisons with X Rays

was found to be 3.0 relative to X rays, but at 37?C, the two radiations were of similar effectiveness (RBE = 0.6). The implications of this result regarding possible tritium transmutation effects at low temperatures are discussed. Unifilar and bifilar modes of tritiated thymidine incorporation gave similar results for cell killing at -196?C. In relation to absorbed dose, the biological effectiveness of tritiated thymidine was unchanged by a reduction of the irradiation temperature from 37 to -196?C, whereas a corresponding temperature reduction reduced the effectiveness of X rays by a factor of about 4. This latter result is in agreement with generally accepted views of direct and indirect action of radiation on mammalian cells.