Neutron interactions in AGATA and their influence on [formula omitted]-ray tracking

Abstract

A GEANT4-based Monte Carlo simulation of the future 4 π γ -ray spectrometer AGATA has been performed. Emphasis was put on studies of neutron interactions, how they affect the γ -ray tracking performance of the array, and if AGATA could be used as a neutron detector. The obtained results show that there is a large probability to detect neutrons in AGATA. Using an energy threshold of 5 keV on the HPGe detector signals, which is the goal for AGATA, the neutron detection efficiency is 40– 50 % and the number of interactions per detected neutron is 3–11, for neutrons with energies in the range 0.5–18 MeV. The depth profile and the distance between nearest-neighboring interaction points have been investigated and were found to be rather similar for neutron and γ -ray-induced events. For neutrons detected in coincidence with γ -rays, the photo-peak efficiency of AGATA was found to be reduced by about 1 % per emitted neutron using existing γ -ray tracking algorithms, while the peak-to-background ratios were reduced to a much larger extent. The possibility to discriminate between detection of neutrons and γ -rays using the time-of-flight technique was evaluated. In addition two new neutron- γ -ray discrimination methods were developed and tested. The first one is based on tracking of γ -rays following inelastic neutron scattering, and the determination of the direction of emission of such γ -rays, which is randomly oriented compared to γ -rays originating from the center of AGATA. The second method makes use of the excess of low-energy interactions produced by the recoiling Ge ions, assigning such interaction points, which are left over after γ -ray tracking has been performed, as being due to neutrons. None of the methods have so far given satisfactory results, which means that further developments regarding the neutron- γ discrimination in AGATA are needed.