Development of a High Sensitivity PMT assembly for the balloon-borne Soft Gamma-ray Polarimeter

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The Polarized Gamma-ray Observer-Light Version (PoGOLite) is a new balloon-borne experiment designed to measure polarization from astronomical objects, such as pulsars, black holes, and Active Galactic Nuclei, in the 20-100 keV energy range. It is under construction for the first flight expected in 2009. The original purpose for this research was to develop a high sensitivity light sensor (PMT-ASSY) which can be used in the balloon (i.e., low-pressure) environment, and to construct more than 150 units of them for the flight PoGOLite sensors between 2004 and 2006. Now we are happy to report that we have completed our research, with a number of fuits than what we had anticipated. Our results are summarized as follows:(1)Production of more than 150 PMT assembly for the first flight of the PoGOLite.(2)Construction of prototype polarimeter which consisted of 7 phoswich units, and detailed performance test at the synchrotron radiation facility in the KEK photon factory.(3)Development of a "Monte-calro simulator" which enables realistic simulations of PoGOLite sensitivity, under various background events such as cosmic X-ray background (CXB), charged particles, and atmospheric gamma-ray.(4)Detailed test of charged particle response under 400 MeV proton beam at the cyclotron facility in the RCNP, Osaka University(5)Balloon-fight of 1 unit PMT-Assembly as a monitor counter of the PHENEX (Polarimeter for High ENErgy X-rays) experiment in 2006.Through above detailed performance tests, we confirmed that the detector system (PMT-ASSY, and associated electronics, high voltage etc) developed in this research works pretty well under serious conditions expected in balloon orbit. In particular, we showed that the PMT-ASSY monitors count rate of signal/background events for 16 hours in the PHENEX experiments. Dramatically, the count rate of the monitor counter increased significantly when the Crab-nebular/pulsar was locked in the detector field of view.