Some Experiments upon the Biological Effects of Fast Neutrons

Abstract

A quantitative comparison has been made of the action of fast neutrons (2·8 MEV) and γ rays in inhibiting mitosis in tissue cultures of chick embryo fibroblasts and in the root of the broad bean. The method of dose measurement used makes it possible to compare the effects of equal amounts of γ-ray and neutron energy absorbed per unit volume of tissue. In the case of both tissue cultures and bean roots, the curve relating residual mitosis and dose was found to be approximately exponential, and in the former case the shape of the curve was significantly different from that of the corresponding γ-ray curve, which is of the usual sigmoid type. No intensity dependence was observed. The exponential nature of the neutron curve suggests that the inhibition of mitosis is brought about by one, or at most a few, recoil protons, and the magnitude of the neutron dose which reduces mitosis to 37 per cent. then shows that the volumes within which these particles must pass to be effective, if spherical, have radii of about 2 μ or 3 μ respectively for the two tissues studied. The results indicate that in each case the same number of ions have to be produced within these volumes whichever radiation is used. The difference between both shape and magnitude of the mitosis-dose curves for γ rays and neutrons is then accounted for quantitatively by the fact that in the former case the ionisation would be produced by several hundred secondary electrons, and in the latter by a single recoil proton. Other quantitative studies of the biological effects of fast neutrons are reviewed, and an hypothesis is advanced to account for the large relative efficiency of neutron compared with γ-ray energy which has been observed in some cases. The hypothesis is elaborated in some detail for the case of the lethal action of neutrons on broad bean roots (γ/N = 11), and it is shown that both the high efficiency and the shape of the mortality curve may be accounted for by simple and plausible assumptions.