Relative biological effectiveness of neutron radiation and its implications for quality factor and dose limitation

Abstract

The biological effectiveness of fission neutrons relative to low dose rate photon radiations with energies in excess of 400 keV lies in the range from 10 to possibly in excess of 100 depending upon the effect studied; in contrast to the quality factor used in radiological protection for those neutrons of about 10. However, for the majority of genetic effects, tumour induction and life shortening in rodents, the value would appear to lie in the region of 20 to 30. One consistent biological observation is that the effectiveness of photon radiation is decreased at low dose rates. Bearing in mind that human risk estimates have almost all been derived from high dose, high dose rate exposures, it does not seem likely that the current risks to humans from either photon or neutron radiation at low doses and low dose rates have been grossly underestimated. This conclusion is reached after making allowances for the possible effects of a revision of the dosimetry of the Japanese atomic bomb survivors. The recent ICRP recommendation that the quality factor for neutrons should be increased by a factor of two has to be seen in a context in which the concept used in the measurement and control of individual exposures to neutron radiation overestimates the dose limiting quantity also by a factor of about two or three. Therefore, there does not appear to be any urgency in altering the current techniques of controlling exposures to neutron radiation to accommodate the ICRP recommendation, particularly since the ICRP may revise the physical basis for the specification of quality factor in the not too distant future.