Monte-Carlo study of High-Purity Germanium Detectors for the BL3 Experiment

The Beam-Lifetime 3 (BL3) experiment is a next-generation neutron lifetime experiment designed to measure the neutron lifetime by observing the decay of cold neutrons using a beam in the NIST reactor. To support BL3, a simulation is being developed to study the Alpha-Gamma device designed to calibrate the efficiency of the neutron fluence monitor. This device consists of three High-Purity Germanium (HPGe) detectors and six silicon detectors, which are used to count the alpha and gamma particle decay products of the neutron capture reaction at a centrally located target consisting of Boron-10. We use the framework of Geant4, a Monte-Carlo based simulation tool, to track the particles and their interactions with various geometries. To validate the detector performance, we start with simulations of energy depositions in the HPGe detectors using calibration sources, like Cobalt-60 and Cesium-137. Features on these energy spectra give insight to various signal and background physics processes. By comparing the energy spectra from detectors of varying dimensions, we choose a detector geometry that optimizes the signal efficiency while minimizing the sensitivity to background interactions. We study this background by simulating common sources of gamma emitters in the reactor environment. The plan is to develop a background model and use this simulation tool to design a shielding package, using lead or other high-Z elements, necessary to reduce and control the background. The data from these two simulations will be used to optimize the placement, dimensions, and shielding of the HPGe detectors in the Alpha-Gamma device for the BL3 experiment.