BNCT beam quality sensitivity to a source neutron spectrum

It has been shown that the clinical quality of the neutron beam available for boron neutron capture therapy (BNCT) depends strongly on the design of the beam shaping assembly (BSA). The same study showed that a neutron source based on the {sup 7}Li(p,n) reaction, which is induced by protons accelerated to 2.5 MeV, could provide treatment beams having better depth-dose characteristics than those attainable using existing reactor sources. The neutron flux spectrum from the former source peaks closer to the optimal treatment energy and has a narrower distribution. This poses a question: Is the superior shape of the treatment flux spectrum from the accelerator-based source mainly due to its softer source spectrum-its maximum neutron energy is below 800 keV-or to nonoptimal design of the BSA for the reactor-based source?