Prompt sub-MeV neutron production from the 7Li(p,n)7Be reaction on mercury

Proton-beam-generation experiments have been conducted on the NRL Mercury pulsed-power generator operating in positive polarity with a lithium metal target embedded in the cathode. The accelerating voltage was limited to below 2.7 MV in order to limit the energy of neutrons produced in the ⁷Li(p,n)⁷Be reaction (Q = 1.88 MeV) to below 1 MeV. Analyses based on published results¹ and calculations presented here are used to predict the angular distribution of neutron yield and spectrum for each shot. Predicted neutron yields are compared to Rh-counter and Al-activation measurements. The results of these comparisons are quite encouraging, showing better than factor-of-2 agreement between the two sets of measurements and the analysis over the voltage range of the shot series. In order to achieve this level of agreement, a series of MCNPX computations has been carried out to determine the spectral contribution of neutrons reflected from the Mercury test-cell environment² and the associated changes in detector calibrations. The agreement between measurements and modeling provides a check on the voltage calculated using a positive-polarity ion-diode model. For operation at 2.5-2.6 MV, on-axis neutron yields from the p-Li reaction are in the 10¹¹ neutrons/steradian range.