Measurement of the thick target 7Li(p,n) neutron source spectrum using a 3He ionization chamber

Neutron spectra from the thick target 7Li(p,n)7Be reaction were measured using a 3He ionization chamber in the proton energy region of 1.95–2.30 MeV. A digital pulse processing system was employed for pulse height analysis to achieve an enhanced pulse pile-up rejection. For analyses of measured spectra, an unfolding code was written according to the van Cittert iterative algorithm with Janssen constraint. When testing the unfolding with an ideal fluence spectrum, cross-correlation filtering was found to be useful for stabilizing the result with noisy data. Only the high-energy part of the measured spectra (above 794 keV) was used in unfolding due to the interference from the thermal peak tail. Theoretical neutron spectra were also calculated by combining the analytical spectra from the 7Li(p,n) cross-section data with Monte Carlo simulation. A significant spectral modification was introduced by the lithium target holder assembly and the cooling water. The theoretical spectra were compared with the unfolded spectra and their patterns were consistent.