Rat fibrosarcoma cells have been irradiated with fast neutrons of mean energy 7.5 MeV, in the presence and absence of secondary charged-particle equilibrium. In the latter condition the biological effect was produced only by the more densely ionizing secondary particles (α particles and heavy recoils) which gave an absorbed dose about 20% of the Kerma. This component of the radiation was found to give an oxygen enhancement ratio (OER) of 1.0 and an RBE of 7. The OER of the recoil proton component was almost the same as that of X-rays, but the RBE was about 2.5. It is concluded that, if the dose is separated into two components, namely, recoil protons and more densely ionizing particles, both contribute about equally to the biological effect under aerobic conditions but the latter alone is responsible for the reduced OER.

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