Design and Performance Of A Thermal Neutron Beam for Boron Neutron Capture Therapy At The University Of Missouri Research Reactor

The University of Missouri Research Reactor has designed and constructed a thermal neutron beam line for use in small animal boron neutron capture therapy (BNCT) experiments. Details of the facility have been previously reported by Brockman et al.[1] and Pozzi et al.[2]. The facility utilizes a 15.24 cm diameter beam tube that extends from the beryllium reflector to the biological shield, a distance of approximately 3.95 m. This design incorporates a neutron filter approach that is similar to that previously reported by Kim et al. but without cryogenic cooling [3]. The design combines single crystal silicon and single crystal bismuth at room temperature to remove fast neutrons and reactor gamma rays. The silicon crystal provides the bulk of the neutron spectral filtering while the bismuth crystal effectively reduces the reactor gamma ray component. A parameter study was undertaken to find the optimum combination of silicon and bismuth that maximizes thermal neutron flux while maintaining fast neutron and gamma ray components within an acceptable range. Both DORT and MCNP5 computations indicated that the optimal silicon filter is 50-55 cm in length and the optimal bismuth filter is 8 cm in length. The neutron spectrum of the facility has been experimentally characterized three times since 2009 using flux foils and an unfolding method developed by Idaho National Laboratory. The most recent measurements were made in 2012 and indicate that silicon and bismuth filter performance has not degraded. Before the beam enters the irradiation chamber it is collimated to 12.7 cm in diameter using enriched Li polyethylene. The thermal neutron flux profile of the beam varies by less than 10% over the active 12.7 cm diameter. A gantry system was built to allow simultaneous irradiation of 1-4 mice at a time. Each irradiation position incorporates a shield constructed of Li2CO3 that minimizes the neutron dose to healthy tissue.