The Polarized Gamma-ray Observer (PoGOLite) is a balloon-borne instrument that will measure gamma-ray polarization in the energy range 25-80 keV from astronomical sources such as pulsars, accretion discs and jets from active galacticnuclei. The two additional parameters provided by such observations, polarizationangle 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 closepacked array of phoswich detector cells (PDCs) based on coincident detection of Compton scattering and photoelectric absorption. Each PDC comprises three different scintillating components and combines photon detection, active collimation and bottom anticoincidence into one single unit. The three parts are viewed by a photomultiplier tube (PMT) and pulse shape discrimination is used to identify signals from dierent parts. Surrounding the detector array is a segmented side anticoincidence shield (SAS) made of BGO crystals.
The detector elements of the instrument (PDCs, SAS units, 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, suchas threshold settings and PMT voltages, have also been dened.
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 were accurately recreated i nsimulations, demonstrating that the treatment of neutron interactions in Geant4 is reliable.
A Pathnder version of the PoGOLite instrument has been constructed and tested with unpolarized and polarized photon beams, and results have been compared with simulations. The Pathnder 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 about 20 days. The main observational targets for this flight will be the Crab system and Cygnus X-1.
Stockholm: KTH Royal Institute of Technology , 2011. , vi, 182 p.
2011-04-01, Sal FB53, Roslagstullsbacken 21, AlbaNova, Stockholm, 13:00 (English)
Kanbach, Gottfried, Dr.