Comparative mortality following single whole-body exposures of mice to fission neutrons and Co60 gamma rays.

Abstract

During the past several years experiments have been planned and undertaken to test the radiobiological additivity of fast neutrons and Co60 gamma rays in their acute lethal action on mice (1). Since high-energy gamma rays and fission neutrons are often encountered together, the question of whether they are additive in producing certain biological effects has practical as well as theoretical significance. As a basis for these studies, complete dose-survival curves for the mouse were constructed following exposure to each of the two radiations alone. The object of this paper is to present these survival data for the thirty-day acute period following such exposures. Material and Methods Animals: The mice used in these experiments were CF #1 females, shipped to our Laboratories at five to six weeks of age. Upon arrival each week, they were divided into groups of eight, by means of a table of random numbers. Each group was housed in a plastic cage (2) and placed in isolation for two to four weeks before being used in any experiment. The animals were maintained on a diet of Rockland pellets and water ad libitum. After the isolation period, the cages were randomly divided into experimental (to be irradiated) and control groups. The animals varied from 20 to 28 gm. in body weight at the time of irradiation. Methods of Irradiation: All exposures were carried out in a gamma-neutron radiation chamber (3). Fast neutrons were obtained by Zirkle's method (4), in which the thermal column of a reactor is used in conjunction with a plate of uranium to convert slow neutrons to fission neutrons. The experiments were carried out at the CP-3′ heavy-water reactor at the Argonne National Laboratory. The slow neutron flux from this reactor was approximately 6 × 107 n∕cm.∕sec. at the uranium converter plate of the radiation chamber. The fission neutrons reaching the mice were appropriately filtered against contamination either with pile and fission gamma rays or with thermal neutrons. The slow neutron component reaching the mice was less than 0.5 per cent of the fast neutron component and was therefore of negligible importance in these exposures (5). Contamination from gamma radiation was estimated, by means of carbon chambers and film, to be approximately 7 per cent of the total dose (6). The spectrum of fast neutrons used in these animal exposures was essentially the fission spectrum (7), slightly modified by transmission through 9 cm. of lead. Studies were made of ionization tracks produced in films irradiated at the animal positions. The spectrum, measured by this technic, extended from the lower limits obtainable with the method (0.5 MEV) to approximately 8 MEV, with a peak between 0.5 and 0.6 MEV and a mean energy of 1.70 MEV.