Optimized accelerator based epithermal neutron beams for boron neutron capture therapy
Current accelerator based neutron source concepts for boronneutron capture therapy (BNCT) are centered on the lithium (p,n) reaction which has a reaction threshold energy of 1.881 MeV. Bombarding energies several hundred keV above the reaction threshold are used (2.3 MeV–2.5 MeV), resulting in neutrons having energies approaching 1 MeV. Large inefficient moderator reflector assemblies are required to reduce the energies of these neutrons to the desired therapy beam energy (1 eV to 10 keV). Operating with proton energies near the reaction threshold decreases the neutron yield but produces neutrons with energies less than 200 keV. In addition, all the neutrons emitted from the lithium target are kinematically collimated in the forward direction. This allows for a smaller more efficient moderator reflector arrangement, increasing the useful epithermal neutron flux available at the irradiation position and reducing gamma and fast neutron contamination. This concept has been proved by comparing Monte Carlo (MCNP) calculations with benchmark experiments. Neutronproperties (energies, yields and angular distribution), moderator reflector design concepts, optimum accelerator proton energy, which maximize performance for BNCT, have also been studied. The results indicate that for a lithium target, 3 mA of 1.91 MeV protons can produce an epithermal flux of 109 n/cm2/s at the patient irradiation position.