An application of the CSSE code: analysis of pulse pile-up in NaI(Tl) detectors used for TNA

A systematic analysis of γ-ray spectra produced by NaI(Tl) detectors of different sizes employed in nuclear sensors for Humanitarian Demining has been performed by Monte Carlo calculations. The study has been undertaken in the field of the Thermal Neutron Analysis (TNA) technique, based on (n,γ) reactions. The sensor geometry and configuration as well as the detector response function have been simulated by the CSSE code, specifically developed and tested for these purposes (Nucl. Instr. and Meth., this issue; The EXPLODET Project Progress Report 1999, DEPD 1/NP/00, Report, Physics Department and INFN of Padova, 2000). The sensor is basically made of a moderated neutron source (a 252Cf source enclosed in a Pb sphere and a high-density polyethylene structure) and a γ-ray detectors array. Events coming from pile-up of simultaneous pulses in the detector have been investigated with particular care, since they could cause a significant background in the spectrum region where the explosive signature (10.83 MeV γ-ray emerging from neutron capture by 14N) is found. The study has been performed for different moderator sizes and shapes and for different detectors configurations.