Neutron induced background gamma activity in low-level Ge-spectroscopy systems

Two high purity germanium (HPGe) detectors were located in two different passive shields: one in pre-WW II iron and the second in commercial low background lead. Gamma lines emitted after neutron capture, as well as after inelastic scattering on the germanium crystal were detected and then analyzed. The thermal and fast neutron fluxes were calculated and their values were compared for the two different kinds of detector shield. Several materials having different neutron slowing-down properties were packed in Marinelli geometry, positioned on the lead shielded detector and measured for around 106 s. The main goal was to estimate a possible influence of the sample on the intensity of the neutron induced Ge gamma lines. On the iron-shielded detector a massive (3 in. thick) NaI Compton suppression system showed a measurable activity from neutron capture and inelastic scattering on sodium and iodine nuclei.