Comparative Mortality after 24-Hour, Whole-Body, Exposures of Mice to Fission Neutrons and Cobalt-60 Gamma-Rays

Abstract

Effects of increasing the time of irradiation from 1.5 to 24 hours, in order to test the dose-rate dependence of fission neutrons and Co60 [gamma] rays were studied. CF No. 1 female mice were exposed to fission neutrons (mean energy, 1.7 Mev) obtained by means of a uranium converter plate at the thermal column of a reactor (CP-3). Thirty-day mortality curves were constructed following a single whole-body exposures. Comparisons were made between the probit regression lines (30-day mortality on dose) following exposure to each radiation for 1.5 and 24 hours. Dose rate dependency following [gamma]-irradiation was shown; the LD50/30 days, which was 929 [plus or minus] 5r following 90-minute exposures, was increased (43%) to 1324 [plus or minus] 11 r following 24-hour irradiation. The two [gamma] probit regression lines are parallel. Mortality of mice exposed to fission neutrons was independent of dose rate. This difference in dose rate dependence may aid in determining possible different mechanisms of mortality following these two radiations. The relative biological effectiveness of these 2 radiations was determined to be 6.3 for the 24-hour exposures as compared to 4.4 for the 1.5-hour irradiations. Daily probability of death was calculated for comparable doses of the 2 radiations. The earlier mode of death following fission neutrons, with a peak around days 5-10, was apparent over the entire range of doses utilized. Similarly, [gamma]-rays produced a later mortality peak (days 13 to 16). Mixtures of the 2 radiations (50% - 50%) also showed these 2 distinct mortality models.