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Numerical simulations are used to predict the cosmic-ray- induced background in a passively-shielded gas scintillation proportional counter. A pair of these detectors will be flown as focal plane instruments for a hard x-ray telescope balloon- borne experiment. The investigation begins with one- dimensional transmittance studies to determine optimum thickness and composition for additional passive shielding.These simulations suggest, within weight and other design constraints, 0.3 cm of lead would reduce shield leakage within the detector by an order of magnitude over the approximately 40 - 200 keV range while adding only negligibly to photon production within the shielding mass by hadronic interactions. Simulations of the entire as-built detector, on the other hand, predict this added shielding reduces shield leakage by only approximately 40% and the total background rate (including shield leakage and production but ignoring aperture flux) by only approximately 27%. The discrepancy between one-dimensional and full detector results is attributed to multiple Compton scattering of unattenuated hard x-rays within the pressure vessel which reduces initial photon energies to within detectable bounds and to leakage and production in the attached, unshielded, electronics housing. The aperture flux can be reduced by 90% by adding an aperture collimator for a final (shielded and collimated) detector total background in the 15 - 50 keV operating range of approximately 0.0043 cts-s-1-cm-2- keV-1; a 65% reduction compared to the as-built detector. The dominant source of background remains cosmic diffuse and atmospheric gamma-ray leakage through the radiation shields and thin pressure vessel walls with a minor photon production contribution. Although this rate is higher than typically attained using active shielding techniques, a high S/N ratio is achieved by the combined telescope-detector system.
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