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
(1) The efficiency of ∼3 mev fast neutrons and cobalt-60 gamma-rays in inducing mitotic delay in the primary root-tip meristematic cells of Vicia faba has been studied. Fast-neutron doses of 2·02, 4·95 and 9·94 rads given under aerated and anoxic conditions, and gamma-ray doses of 46 and 114 rads given under conditions of aeration were used. (2) The results show that there is no qualitative difference between neutron- and gamma-ray-induced mitotic delay, but marked quantitative differences were observed. The relative biological efficiency of fast neutrons to gamma-rays was about 10 : 1, a value comparable to those obtained for inhibition of root growth and the production of micro-nuclei. (3) The presence or absence of oxygen had little influence on the amount of mitotic delay induced by a given dose of neutron radiation. This oxygen enhancement factor of 1 for neutrons is in marked contrast to the oxygen factor of 2 found for mitotic delay induced by gamma-radiation. The factors for both the radiations appear to be a little smaller for this effect than for inhibition of root growth and for chromosome damage. (4) The radiobiological implications of the values of r.b.e. and oxygen factor are discussed. In spite of the similarity of these factors for chromosome damage and mitotic delay, it is concluded that these effects do not stem from the same cellular lesion.