FRACTIONATED, LOW DOSE‐RATE IRRADIATION OF MICE WITH FISSION NEUTRONS AND CO⁶⁰ GAMMA‐RAYS*

Abstract

In this session on radiation dose-rate we should like to present some data on the comparative effects of fission neutrons and Cow gamma-rays on lethality in C F No. 1 female mice. All animals were exposed in the gamma-neutron radiation chamber at the CP-5 reactor.' Both radiations were delivered at a dose rate of approximately 1 rad/min. The mice exposed to y-rays received 13 weekly doses of 300 rads, delivered in one, three, or six equal fractions per week. Under the conditions of exposure a t CP-5, fission neutrons have been found to be almost three times as effective as Corn y-rays for acute 30-day lethality in this mouse. There is cvidence, confirmed in several laboratories, that the relative biological effectiveness of neutrons over y-rays is increased from about two or three for single acute exposures to as high as five to fifteen for irradiation with small doses over long periods of time.* I n the experiment described in this paper, an RBE value of 5 was arbitrarily chosen as a basis for the selection of the gamma-ray dose. Thus the total neutron weekly dose was 60 rads (total delivered in three fractionation patterns). The irradiations were continued for 13 consecutive weeks, hence the mice were exposed to almost 4,000 rads of gamma-rays. This experimental plan is summarized in TABLE 1.The experiment has not yet been completed, since there are a few neutronirradiated mice still alive. Although statistical analysis will not be made until all the irradiated animals are dead, several general conclusions are already clear.The mean survival times of the gamma-irradiated mice are not significantly different whether they are exposed once or three times a week. There is an indication that daily exposure (six times a week) increases the mean survival time, but the numbers are small and the difference may not be significant. There is no such indication for the neutron-irradiated mice.Since the three subgroups for each radiation appeared similar in their survival f,imes, the data were pooled and cumulative mortalities for the two radiations were compared. The results are shown in FIGURE 1.It is clmr from the data that the figure of 5 is too high for the neutron/ gamma 11BE under the conditions of this experiment. The RBE for *This work performed under the auspices of the U. S. Atomic Energy Commission. 185Survival data are listed in TABLE 2.