A High Relative Biological Efficiency of 650-keV Neutrons and 250-kVp X-rays in Somatic Mutation Induction

Abstract

THE mean linear energy transfers of 650-keV neutrons and of 250-kVp X-rays are so markedly different that it is not usually possible to establish a unique value for their relative biological efficiency. For example, it is well known that the densely ionizing neutrons induce linear dose-response relationships for chromosome aberrations, whereas the dose-response curves for X-rays are often non-linear. Moreover, the former radiation but not the latter is almost independent of environmental factors such as oxygen tension during irradiation. Under certain conditions, and with certain biological test systems, however, some of these difficulties are minimized. For example, Neary et al.¹ compared the relative biological efficiency, in terms of chromosome aberration induction, of 0.7- and 3.0-MeV neutrons with cobalt-60 γ-rays administered at low dose rates. There the dose-response relationships were almost linear, and relative biological efficiency values of 35 were obtained; they also derived ultimate maximum relative biological efficiency values from a comparison of the one-hit components of the neutron and γ-dose-response curves and found that these could be as high as 100. Similarly, Smith et al.², using a test system (somatic mutation induction in dry seed) in which the dose-response relationship for X-rays was linear, obtained an average relative biological efficiency value of about 75 for mono-energetic neutrons of 0.43–1.80 MeV.