The Relative Biological Efficiency of Single Doses of Fast Neutrons and Gamma-rays on Vicia Faba Roots and the Effect of Oxygen

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Abstract
Primary roots of V. faba have been irradiated with fast neutrons of mean energy 3 MeV in air-saturated or air-free water at about 22[degree] C, or by cobalt-60 gamma-rays in air-saturated water at about 19[degree] C. The reduction in root growth in terms either of total growth in 10 days, or of minimum growth rate, has been taken as the measure of the biological effect of the radiation. The dose-response curves with neutrons and gamma-rays in air-saturated water are of different shapes, so that there is no single value of relative biological efficiency. The r.b.e. was about 15 for small doses and fell to a minimum of about 7 as the dose was increased. The form of the variation was consistent with what would be expected if reduction of root growth is principally due to chromosome damage. The presence of O2 increases the radiosensitivity to fast neutrons; the ratio of equivalent doses in air-free and air-saturated water at 22[degree] C is about 14. There is some indication that the O2 factor is highest for small doses and falls steadily with increasing dose.