The Polarized Gamma-ray Observer (PoGOLite) is a balloon-borne polarimeter based on measuring anisotropy in the azimuthal scattering angle distribution of photons in the energy range 25–80 keV. This is achieved through coincident detection of Compton scattering and photoelectric absorption within a close-packed array of phoswich detector cells (PDCs). Each PDC contains a plastic scintillator rod (main detector component), a plastic scintillator tube (active collimator) and a BGO crystal (anticoincidence shield).

A significant in-flight background is expected from atmospheric neutrons as well as from neutrons produced by interactions of cosmic rays with mechanical structures surrounding the instrument. Although this background can be reduced by introducing suitable shielding materials such as polyethylene, the shield geometry must be optimized through simulations in order to yield sufficient shielding with an acceptable increase in weight.

Geant4-based Monte-Carlo simulations have shown that a 10 cm thick polyethylene shield surrounding the PoGOLite instrument is required to sufficiently reduce the background, i.e. fake polarization events from atmospheric neutrons. In order to validate these simulations, a beam test was carried out, at which 14 MeV neutrons were used to irradiate a simple detector array with four plastic scintillators and three BGO crystals. The array was configured to mimic the PoGOLite detector geometry and also featured a polyethylene neutron shield. Here, we present details of the neutron beam test and our simulation thereof, which demonstrate that the treatment of neutron interactions within the Geant4 framework is reliable.…