Characterization of two extraordinary AmLi neutron sources

Abstract

Two legacy AmLi sources have been actively used across several projects within Pacific Northwest National Laboratory, however, applicability of source measurement data has been substantially limited due to the lack of original manufacturer documentation and cross comparison to other sources. This paper summarizes measurements conducted to better quantify source radionuclide content, neutron energy spectrum and emission rate of the AmLi sources. The measured neutron energy spectra of AmLi sources produced by Monsanto Research Corporation have not been found in open literature before and therefore the focus of the study was to obtain the high definition energy distribution of the sources. The initial gamma spectrometry measurements, aimed to estimate the radionuclide content and potential impurities, revealed a significant contamination of the source material with beryllium. Consequent neutron spectrum measurements confirmed beryllium contamination through the presence of a high energy tail contributing up to 17% to the total neutron emission rate. The obtained lithium-only portion of the measured source energy distribution was compared against other AmLi spectra discussed in the past and recent literature. Determination of the total neutron yields of the sources was made with measurements in a neutron well counter. Detection efficiency of the well counter was calculated specifically for these sources via detailed Monte Carlo simulations based on the obtained neutron energy distribution.