The neutron component of background of an HPGe detector operating in a surface laboratory

Highlights

• Study of cosmic ray interactions with an HPGe detector was carried out.

• Many γ-lines visible in the spectra were induced by neutrons.

• Inelastic scattering and neutron capture were the most often occurring processes.

• GEANT4 simulations were in a very satisfactory agreement with the experiment.

• Inelastic neutron scattering on ²⁰⁸Pb nuclei contributes by 35% to the 2614.51 keV peak.

Abstract

Investigation of neutron-induced background was carried out by studying interactions of cosmic-ray neutrons with an HPGe detector inside its shield placed on a ground floor of a 3-storey building. The study was conducted experimentally and by Monte Carlo simulations using GEANT4 simulation tool. Detailed analysis of measured background γ-ray spectra showed that many γ-lines visible in the spectra were induced by neutrons. The majority of detected γ-rays originated in germanium, copper, lead and tin. Iron and aluminium components were less important background sources. Inelastic scattering and neutron capture were the most often occurring processes of neutron interactions with the detector and its shielding. The contamination by natural radionuclides, specifically of ⁴⁰K, ²¹⁴Pb, ²¹⁴Bi and ²⁰⁸Tl, was also present in the background spectra. Nevertheless, approximately 35% of ²⁰⁸Tl peak at the energy of 2614.51 keV was produced by inelastic scattering of neutrons on ²⁰⁸Pb nuclei. The experimental background was compared with GEANT4 simulations, which were carried out without and with the shielding layer of the building. The final integral counting rates for measured spectrum in the energy range from 50 keV to 2875 keV was 1.26 ± 0.07 s⁻¹ and for simulated one 1.25 ± 0.13 s⁻¹, indicating a good agreement with the experiment.