PhD thesis defense: Pre-flight development of the PoGOLie Pathfinder

by Mr Mózsi Kiss (KTH Particle and Astroparticle Physics)

Friday 1 Apr 2011, 13:00 → 16:00 Europe/Stockholm

FB53

The Polarized Gamma-ray Observer (PoGOLite) is a balloon-borne instrument that will measure the gamma-ray polarization in the energy range 25-80 keV from astronomical sources such as pulsars, accretion discs and jets from active galactic nuclei. The two additional parameters provided by such observations, polarization angle and degree, will allow these objects to be studied in a new way, providing information about their emission mechanisms and geometries. The instrument measures azimuthal scattering angles of photons within a close-packed array of phoswich detector cells (PDCs) based on coincident detection of Compton scatterings and photoelectric absorption. Each PDC comprises three different scintillation components and combines photon detection, active collimation and bottom coincidence into one single unit. The three parts are viewed by a photomultiplier tube (PMT) and pulse shape discrimation is used to identify signals from different parts. Surrounding the detector is a segmented side anticoincidence shield (SAS) made of BGO crystals. The detector elements of the instrument (PDCs, SAS units and PMTs) have been characterized, resulting in a placement scheme which details where within the detector array each element should be placed in order to maximize the instrument sensitivity and response uniformity. Suitable operating parameters for flight, such as threshold settings and PMT voltages, have also been defined. Geant4 Monte Carlo simulations have shown that a polyethylene shield is needed around the detector array in order to sufficiently reduce the background from atmospheric neutrons. To validate these simulations, a simple detector array with four plastic scintillators and three BGO crystals shielded with polyethylene was irradiated with 14 MeV neutrons. Measured results weere accurateley recreated in simulations, demonstrating that the treatment of neutron interactions in Geant4 is reliable. A Pathfinder version of the PoGOLite instrument has been constructed and tested with unpolarized and polarized photon beams, and results have been compared with simulations. The Pathfinder is being prepared for a maiden flight from northern Sweden in mid-2011. A circumnavigation is foreseen at an altitude of up to 40 km, whereby the instrument travels westwards over Greenland and Canada and returns over Russia after a period of 20 days. The main observationsal targets for this flight will be the Crab system and Cygnus X-1.