Court, Andrew John (1987) Hard X-ray astronomy from balloon altitudes. University of Southampton, Doctoral Thesis.
Abstract
A large area (3200 cm2) hard x-ray (15 to 300keV) telescope, was built as a collaborative project between Southampton University and two Italian Institutes. The telescope was flown on a stratospheric balloon from Trapani, Sicily on the 29th of July 1986. A number of astronomical observations were made by the telescope from an altitude corresponding to a residual atmospheric pressure of 4mb. The sensitivity of the instrument is such that during an observation of the Crab the signal-to-noise ratio was found to be approximately 4 sigma in one second. The background levels in the NaI(Tl) hard x-ray detection units, were found to be approximately 6x10-4 photons cm-2 s-1 keV-1. A previous flight of this instrument from Palestine, Texas had a rate of about 1x10-3 photons cm-2 s-1 keV-1 reflecting the difference in the geomagnetic cut-off rigidity of 4.7 GeV/c for Palestine and 8.3 GeV/c for Trapani. The background levels in the x-ray detection units are a critical factor in determining the sensitivity of the overall telescope. A Monte Carlo model was used to examine the noise contributions from various hard x-ray photon sources for a satellite-borne detector. Analytical calculations of neutron and spallation induced background counting rates were combined with this data to assess potential detector/veto configurations. As a further refinement, an experiment was constructed to evaluate the background counting rates associated with a number of veto materials. For this experiment scintillation crystals of Bismuth Germanate and Barium Fluoride were compared with a standard NaI(Tl) crystal during a stratospheric balloon flight from Trapani, Sicily in July, 1985. The results indicated that Barium Fluoride is potentially the most suitable for use in x-ray veto systems. (D75972/87)
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