The survival of Chinese hamster cells in culture was used as a test system to determine the RBE of neutrons over a wide energy range. The Radiological Research Accelerator Facility (RARAF) at Brookhaven National Laboratory was used for experiments involving nine neutron energies between 110 keV and 15 MeV; for all but the lowest energy the beams were essentially monoenergetic. Additional experiments were performed with high energy cyclotron-produced neutrons at the Naval Research Laboratory, Washington, D. C., and at the Texas A&M Variable Energy Cyclotron (TAMVEC). In both cases broad neutron energy spectra were involved. In each experiment, survival curves were obtained for one neutron energy and compared with 250 kVp X rays, using cells from the same suspension and common controls. In this way a detailed study was made of the relation between RBE and neutron absorbed dose for each neutron energy. At any given cell survival level, RBE varies with neutron energy. Neutrons at 350 keV are biologically the most effective, the RBE falling off for both higher and lower neutron energies.

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