Variations in lithium target thickness and proton energy stability for the near-threshold 7Li(p,n)7Be accelerator-based BNCT

The usable range of thickness for the solid lithium target in the accelerator-based neutron production for BNCT via the near-threshold 7Li(p,n)7Be reaction was investigated. While the feasibility of using a 7Li-target with thickness equal to that which is required to slow down a mono-energetic 1.900 MeV incident proton to the 1.881 MeV threshold of the 7Li(p,n)7Be reaction (i.e., tmin = 2.33 ?m) has already been demonstrated, dosimetric properties of neutron fields from targets greater than tmin were assessed as thicker targets would last longer and offer more stable neutron production. Additionally, the characteristics of neutron fields generated by 7Li(p,n)7Be for Gaussian incident protons with mean energy of 1.900 MeV were evaluated at a 7Li-target thickness tmin. The main evaluation index applied in this study was the treatable protocol depth (TPD) which corresponds to the depth in an irradiated medium that satisfies the requirements of the adapted dose protocol. A maximum TPD (TPDmax) was obtained for each irradiation condition from the relationship between the TPD and the thickness of boron dose enhancer (BDE) used. For a mono-energetic 1.900 MeV proton beam, the deepest TPDmax of 3.88 cm was attained at the 7Li-target thickness of tmin and a polyethylene BDE of 1.10 cm. When the intended TPD for a BNCT clinical treatment is shallower than the deepest TPDmax, the usable 7Li-target thickness would be between tmin and an upper limit tupper whose value depends on the BDE thickness used. In terms of the effect of stability of the incident proton energy, Gaussian incident proton energies stable to within ?10 keV of 1.900 MeV were found to be feasible for the neutron production via the near-threshold 7Li(p,n)7Be reaction for BNCT provided that a suitable BDE is used.