Life-shortening in mice irradiated with either fission neutrons or cobalt-60 gamma rays.

Abstract

During the past decade the Neutron Toxicity Group has been investigating the biological effects of fission neutrons and Co60 gamma rays on a variety of organisms (1). During the early years of the program, efforts were devoted largely to the study of the acute effects of single exposures of neutrons and gamma rays on mammals (2). The problem of lack of additivity of the two types of radiation was investigated; different modes of mortality were found following each, in mice, and different mechanisms of death were postulated (3). The present experiments were carried out at the CP-5 research reactor to compare the results of earlier acute studies with those at lower dose levels. They were undertaken to evaluate the effects of a series of brief fractionated exposures ranging from the 30-day acute dose of both fission neutrons and Co60 gamma rays, through the subacute area, to low total-dose levels. Such information will be useful in planning experiments in which animals will be exposed for the length of their lives in the low-level neutron room of the new “Janus” reactor. This reactor, now being built in Argonne's Division of Biological and Medical Research, should be in operation in the near future. Experimental Studies 1. Mouse Longevity After 13 Brief Daily Exposures to Fission Neutrons and Co60 Gamma Rays: CF No. 1 female mice were received from Carworth Farms at five to six weeks of age and isolated for at least four weeks before irradiation. A total of 198 mice were used in the neutron experiment, shown in the left portion of Table I, while 158 were used in the gamma experiment, shown in the right half of the table. The mice were randomly distributed in 6 exposure groups with an unirradiated control group for each type of radiation. All mice were irradiated in the gamma-neutron radiation chamber (4) at the CP-5 reactor. Each group of irradiated mice was exposed briefly to thirteen daily doses, varying from 30 seconds to 16 minutes each day. The neutron intensity at the animal positions was approximately 4.4 rads per minute; the gamma-ray dose rate from the Co60 sources was approximately 13 rads per minute. Thus, although the gamma-irradiated and neutron-irradiated mice were exposed over equal periods, the former animals received a total dose three times that of the latter. The experimental plan was set up in this fashion, since the results of acute irradiation at the CP-5 reactor suggest that neutrons were approximately three times as effective as gamma rays, with the 30-day death being the criterion (5) In Table I the numbers of mice per group and the daily radiation doses are listed for each radiation; the unirradiated controls form Group 0. The exposure time for each successive radiation group was doubled (1/2, 1, 2, 4, 8, and 16 minutes, for Groups 1 to 6, respectively). Survivors of all groups were observed to determine their longevity; death checks were made throughout the animal colony twice a day.