Investigation of neutron shielding materials for low-background gamma spectrometry

In gamma spectrometers located at ground-level or in shallow-underground laboratories, muon-induced neutrons contribute significantly to the background. We studied several neutron-shielding materials including water, pure polyethylene (PPE), borated polyethylene (BPE) and lithiated polyethylene (LPE) positioned inside the lead shield. Neutrons are scattered and absorbed inside these materials, however, sequential (n,n′γ), (n,γ) and (n,α) reactions generate gamma emissions. The resulting background increased by 35.9% for water and 37.6% for PPE. For BPE, the background increased by 11.3% only and it decreased by 9.4% for LPE, owing to the absorption of neutrons by boron and lithium, respectively.