Study of Neutron Induced Activity in Low-Background Gamma Spectroscopy Systems

Cosmic rays induced neutrons can produce measurable activity in carefully selected materials used for construction of low-background gamma spectroscopy systems. In order to improve shielding of low-background counting systems, flux of neutrons inside the shield and level of induced activity should be measured [1, 2]. Several gamma lines emitted after neutron capture as well as after inelastic scattering in Ge and couple different surrounding shielding materials were detected and analyzed in different conditions. Two HPGe detectors were located in two different passive shields: pre WW II iron and commercial low-background lead. Significantly lower presence of neutrons is observed in iron shielded detector then in detector shielded by lead, as can be expected. But introducing the massive (3 inch thick) NaI Compton suppression system in iron shield produced measurable activity obtained by neutron capture and inelastic scattering at sodium and iodine nuclei. Different materials were packed in Marinely geometry with the purpose of measuring the gamma lines produced by interaction of relatively abundant neutrons present in environment of lead shielded detector (Fig. 1). The flux of neutrons was calculated and the values obtained for different shields were compared.(author)