The Radiological Research Accelerator Facility (RARAF) has the capability of producing essentially monoenergetic neutron beams, ranging in energy from 15.4 MeV down to 220 keV. In addition, two lower energy neutron beams are available which consist of a wide spectrum of energies and are described as the 110 keV and 60 keV spectra. Seedlings of Vicia faba have been used to measure the oxygen enhancement ratio (OER) and the relative biological effectiveness (RBE) of each of these neutron beams. The OER decreases as the neutron energy is reduced between 15.4 MeV and 220 keV, but does not appear to decrease further for lower energy neutrons. RBE increases as the neutron energy is reduced from 15.4 MeV to 440 keV; the curve then goes through a maximum at around 350 keV, and for lower energies the RBE falls again.

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