Optimal Neutron Source Beam Shaping Assembly For Boron Neutron Capture Therapy PDF Download

Boron Neutron capture therapy (BNCT) is a form of radiation therapy in which nuclides having a high tendency for capturing thermal neutrons, react by emitting charged particles of short range and leaving essentially no residual radioactivity. If these capture nuclides are selectively introduced into the tumor cells, it is theoretically possible to destroy only the tumor and in the process spare the neighboring healthy tissue. Currently, reactors are the only known source of neutrons being used for BNCT. A number of studies have been done using Monte Carlo computer codes such as MCNP to determine an optimum design of a moderator reflector configuration for an accelerator based neutron source utilizing the Li-7(p, n)Be-7 reaction. To confirm this, benchmark experiments were conducted using both a 2.0 MeV Radio Frequency Quadrupole (RFQ) accelerator and a 2.0 MeV Van de Graaff generator. All previous studies had concluded that 2.5 MeV protons would be optimum, which produces neutron having a maximum energy of 787 keV. For BNCT the desired neutron energies are between 1 eV and nominally 10 keV, i.e. in the epithermal region. These neutrons must hence be moderated to bring them down to the desired energy range. The moderator-reflector assemblies for reactor neutrons or those from 2.5 MeV protons on lithium require a little less than a meter of material to achieve the desired neutron spectrum. This research work focuses on using protons with energies just above the Li-7(p, n)Be-7 reaction threshold. These lower energy protons produce neutrons with a maximum energy of just a few hundred keV requiring much less moderation as compared to the 2.5 MeV proton lithium source. The penalty for the low energy neutron spectrum is the low yield of neutrons per proton as compared to the 2.5 MeV protons on lithium. But, less moderation implies that the patient can be moved closer to the lithium target (neutron source), thus making a more efficient use of the source neutrons. This would partially or wholly compensate for the lower yield per proton. In addition the smaller moderator-reflector assembly would result in less scattering occurring, thereby not degrading the forward directional quality of the beam as much as in current assemblies designed for use with the 2.5 MeV protons on a lithium target.