Detection of background thermal neutrons in a modified low-background germanium gamma-ray spectrometer

The paper presents the detection of background neutrons using the 558.4 keV gamma line emitted from excited 114Cd nuclei after neutron induced processes. Stable cadmium and 60 L water moderator were placed inside low background shield of high purity germanium (HPGe) based spectrometer. The method was applied to study diurnal mean variation of background neutron flux. For this purpose 1 h gamma background spectra of modified HPGe spectrometer were collected subsequently for 50.5 days and then summed in 24-h-cycle. Results for the 558.4 keV line are presented along with those of main background lines. The total gamma-ray spectrum was also analysed and results are presented and discussed. No cyclic structure was noticed for all examited lines but 2223 keV, for which some day-night effect is suggested to exist. Test of another moderator made of 108 kg of graphite was done, but the results obtained with water are more promising.