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.
Stanford Linear Accelerator Center, Menlo Park, CA, USA;
This paper appears in: Nuclear Science Symposium Conference Record, 2003 IEEE
Publication Date: 19-25 Oct. 2003
Volume: 3,
On page(s): 1708- 1713 Vol.3
ISSN: 1082-3654
ISBN: 0-7803-8257-9
INSPEC Accession Number: 8143886
Current Version Published: 2004-11-08
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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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