Studies of PoGOLite performance and background rejection capabilities

Abstract: The Polarized Gamma-ray Observer (PoGOLite) is a balloon-borne instrument capable of measuring as low as 10% polarization from a 200 mCrab source in a sixhour °ight. A wide array of sources can be studied, including pulsars, neutron stars, accretion discs and jets from active galactic nuclei. The two new observational parameters provided by such measurements, polarization angle and degree, will allow these objects to be studied in a completely new way, providing information both about the emission mechanisms and the geometries of the emitting objects. The instrument measures anisotropies in azimuthal scattering angles of gammarays with a close-packed array of 217 well-type phoswich detector cells (PDCs) by coincident detection of Compton scattering and photoelectric absorption. Each PDC comprises a \slow" plastic scintillator tube, a \fast" plastic scintillator rod and a BGO crystal. The fast scintillator is the main detector component, whereas the slow scintillator and the BGO crystal act as an active collimator and a bottom anticoincidence shield, respectively. The three parts are viewed by a single photomultiplier tube (PMT) and pulse shape discrimination is used to identify signals from each part. The detector array is surrounded by a 54-segment side anticoincidence shield (SAS) made of BGO crystals. Each segment is 60 cm long and consists of three crystals. A total of 187 crystals have been procured and tested for light yield, energy resolution, dimensions and surface ¯nish. All crystals have been found to be of excellent quality and measured characteristics have been within speci¯ed limits. The performance of the instrument has also been evaluated in several beam tests with polarized synchrotron photons irradiating a prototype detector array. Front-end electronics have been tested and a modulation in the observed scattering angles has been observed in line with expectations. Geant4-based Monte Carlo simulations of the instrument performance have shown that a 10 cm thick polyethylene shield is required around the detector array in order to su±ciently reduce the background from atmospheric neutrons. To validate these simulations, a simple detector array with four plastic scintillators and three BGO crystals was irradiated with 14 MeV neutrons. The array was shielded with polyethylene, mimicking the PoGOLite instrument design. Measured results could be accurately recreated in Geant4 simulations, demonstrating that the treatment of neutron interaction processes in Geant4 is reliable.

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