Effects of Small Daily Doses of Fast Neutrons on Mice

Abstract

The potential danger of overexposure to radium rays and roentgen rays is generally recognized. Likewise, it is known that even low doses of these radiations will, if received at frequent intervals over a long period, eventually produce recognizable damage. The advent of the cyclotron and the nuclear reactor has presented another radiation to be dealt with, namely, the neutron. This radiation is biologically effective, and in some respects is much more dangerous than x-rays or radium. The maximum permissible daily exposure for radium and x-radiation is set at 0.1 r per eight-hour day for a five-and-a-halfday week (1). It is now necessary to establish a safe limit for daily exposures to neutron radiation. It has been the purpose of the present investigation to study the effects of small daily doses of fast neutron radiation to the whole body of laboratory mice. Materials and Methods Animals: In the preliminary experiments, male and female mice of the Swiss strain (Rockland Farms) were used. In the main experiments, animals of the CF1 strain (Carworth Farms) were also used. These animals were from four to six weeks of age at the beginning of the experiments. They were fed on Rockland mouse diet, with water available at all times. The sexes were kept separate. The animals were not crowded, and care was taken to keep the same original groups of males together, as the introduction of a new animal had been found to encourage fighting. Examinations: Blood counts were taken twice a week (in the long-term experiments) on animals of each experimental and control group. Each individual was numbered and animals were taken serially for this examination. Since there were 25 animals in each group, there was a long lapse of time between counts on the same individual, except toward the end of the experiment. Small blood pipettes2 were used (volume about one-quarter that used for human samples) and were found to be reliable. Blood counts consisted of a red cell count, white cell count, hemoglobin determination (photo-electric hemoglobinometer), and a differential blood count made from a smear. The animals were examined closely every day for general condition, for signs of epilation, etc., visible or palpable tumors, infection, and eye defects. Each animal was weighed when its blood count was made, and once a week all were weighed. No animal was sacrificed until over half of the individuals in a particular group had died. Autopsies were done as soon after death as possible and, iftissues were fresh enough, they were preserved and prepared for microscopic examination. Tissues usually saved were bone marrow (sternum), spleen, gonad, lungs, kidney, adrenal, liver, and intestine. If a tumor or inflammatory condition was found in some other organ, it was, of course, added to the tissues to be studied microscopically. Radiation: The neutron radiation employed was produced by the 36-inch cyclotron in the Pupin Laboratories of Columbia University.