Large-area balloon-borne polarized gamma ray observer (PoGO)
BIanford, R.; Chen, P.; Kamae, T.; Madejski, G.; Ng, J.; Mizuno, T.; Tajima, H.; Thurston, T.; Barbier, L.; Bloser, P.; Cline, T.; Hunter, S.; Harding, A.; Krizmanic, J.; Mitchell, J.; Streitmatter, R.; Tueller, J.; Groth, E.; Fernholz, R.; Marlow, D.; Bogaert, G.; Gunji, S.; Sakurai, H.; Saito, Y.; Takahashi, T.; Kataoka, J.; Kawai, N.; Fukazawa, Y.; Carlson, P.; Klamra, W.; Pearce, M.; Bjornsson, C.-I.; Fransson, C.; Larsson, S.; Ryde, F.
Nuclear Science Symposium Conference Record, 2003 IEEE
Volume 3, Issue , 19-25 Oct. 2003 Page(s): 1708 - 1713 Vol.3
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Summary:We are developing a new balloon-borne instrument (PoGO), to measure polarization of soft gamma rays (25-200 keV) using asymmetry in azimuth angle distribution of Compton scattering. PoGO will detect 10% polarization in 100mCrab sources in a 6-8 hour observation and bring a new dimension to studies on gamma ray emission/transportation mechanism in pulsars, AGNs, black hole binaries, and neutron star surface. The concept is an adaptation to polarization measurements of well-type phoswich counter technology used in balloon-borne experiments (Welcome-1) and AstroE2 Hard X-ray Detector. PoGO consists of close-packed array of 397 hexagonal well-type phoswich counters. Each unit is composed of a long thin tube (well) of slow plastic scintillator, a solid rod of fast plastic scintillator, and a short BGO at the base. A photomultiplier coupled to the end of the BGO detects light from all 3 scintillators. The rods with decay times < 10 ns, are used as the active elements; while the wells and BGOs, with decay times ∼ 250 ns are used as active anti-coincidence. The fast and slow signals are separated out electronically. When gamma rays entering the field-of-view (fwhm ∼3deg²) strike a fast scintillator, some are Compton scattered. A fraction of the scattered photons are absorbed in another rod (or undergo a second scatter). A valid event requires one clean fast signal of pulse-height compatible with photo-absorption (> 20 keV) and one or more compatible with Compton scattering (< 10 keV). Studies based on EGS4 (with polarization features) and Geant4 predict excellent background rejection and high sensitivity.

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