Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.4606h&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #46.06; Bulletin of the American Astronomical Society, Vol. 24, p.1195
Astronomy and Astrophysics
Astronomy
Scientific paper
Prometheus I is a very low background 0.03 - 6.5 MeV gamma-ray balloon - borne imager. It eliminates the activation component of instrumental background through use of: (a) A gamma ray detector that is segmented in two dimensions; (b) An active collimator fabricated from low - Z matter , and which has a 10 keV threshold; (c) An active collimator that vetoes prompt events due to fast neutrons; (d) A radiationless absorber of slow neutrons. The segmented detector rejects beta rays emitted by internal NaI activated nuclei because a beta causes only one segment to pulse, but a gamma ray interacting through Compton scattering causes several segments to simultaneously pulse. It also serves as a position sensitive gamma - ray detector for the coded mask that fills the collimator's aperture. The low - Z matter is plastic scintillator, which has only a few spallation products. Gamma rays from them are vetoed because they are accompanied by charged particles that are all vetoed. The plastic moderates all fa st neutrons down to slow speeds, through (n,p) scatters by the hydrogen in the plastic. It is surrounded, outside and inside, by LiF or Li foil, which absorb slow neutrons. Because of its low background, the 3 sigma spectral - line sensitivity at 1 MeV of Prometheus II is expected to be 5 x10 (-5) photons cm (-2) sec (-1) , given six hours observing time at midlatitude balloon altitudes. Prometheus will be used to produce a 0.5 - degree map of the entire sky, in an energy band above that of the hard X-ray band, but below that of the COMPTEL sky map. The map will be the principal scientific objective of two Long Duration Balloon flights that are planned for 1994 and 1995. Prometheus has an in-flight selectable field of view (FOV) that ranges from 4 deg x 4 deg to 38 deg x 38 deg, and 30 arc-minute angular resolution. The localizability of bright sources is about 3 arc minutes. Since there is no unvetoed activation gamma radiation from the collimator, the energy spectrum of the aperture radiation may be unambiguously determined, by measuring the dependence of counting rate upon size of the FOV. This has implications for an independent means of measuring the "MeV bump" in the spectrum of the diffuse extragalactic background. This research was supported in part by NASA, through Grant NAGW-2256.
Haymes Robert C.
Moss Michael Jamieson
Walker P. W.
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