Epithermal and Fast Neutron Spectrum Measurement at the Brookhaven Medical Research Reactor (Bmrr)

Abstract

The main purpose of the Brookhaven Medical Research Reactor (BMRR) is for boron neutron capture therapy (BNCT). In particular, an epithermal neutron beam was developed to provide a neutron beam where most of the neutrons are in the epithermal range. It was concluded in earlier studies1 that the ideal beam for BNCT is in the energy range of 1–20 keV. Several Monte Carlo and other calculations of the neutron spectrum were made in the BMRR epithermal neutron beam.2 There are considerable differences between the various calculated spectra. One indirect spectrum measurement was published using foil activation.3 The measured spectrum did not agree well with any of the calculated spectra. All calculated and measured spectra show that in spite of the careful design of neutron and gamma filtering (made of bismuth, Al202, and D20),4 the high energy neutrons still make up a considerable part of the neutron spectrum. These high energy neutrons increase the dose to the irradiated sample. According to the ICRP 60,5 the quality factor of neutrons of energy between 200 keV and 2 MeV is 20 times higher than that of gamma radiation and 4 times higher than thermal neutrons. The importance of the fast neutron contribution to the RBE was demonstrated by Laster6 using a biological dosimeter. The conclusion of this introduction is that a direct reliable measurement of the epithermal and fast neutron spectra is needed.