ENHANCEMENT OF THE EPITHERMAL NEUTRON BEAM USED FOR BORON NEUTRON-CAPTURE THERAPY

Purpose: This report describes a study to enhance the epithermal neutr on beam at the Brookhaven Medical Research Reactor by increasing the e pithermal neutron flux and/or reducing contamination by fast neutrons. Methods and Materials: The beam was reevaluated using Monte Carlo cal culations and flux and dose measurements in air and in an ellipsoidal head phantom at the patient irradiation port. Changes in its geometry and materials were considered, including rearranging the fuel elements in the reactor core and redesigning the moderator and the patient irr adiation port. Results: Calculations of the new fluxes and doses at th e patient irradiation port showed that the epithermal neutron flux can be increased by 100%, while the fast neutron dose per epithermal neut ron can be reduced by 38%. In 1992, some of the proposed changes were made. In June 1992, measurements were made after one additional fuel e lement was added to replace a graphite spacer block on the epithermal beam side of the reactor core. The results show that the epithermal ne utron flux increased by 18%, as predicted by the Monte Carlo calculati ons. In October 1992, the fuel elements in the reactor core were rearr anged by placing four new fuel elements in the first row facing the ep ithermal port; the intensity of the epithermal neutron beam increased by 50% and the fast neutron and gamma doses per epithermal neutron may have decreased slightly. Conclusion: The epithermal neutron beam at t he Brookhaven Medical Research Reactor has gained a 50% increase in th e epithermal neutron flux and the fast neutron and gamma doses per epi thermal neutron are reduced slightly after the rearrangement of the fu el elements in the core.