Performance of different phoswich configurations in a balloon flight experiment

Four different phoswich units were included in a modular hard X-ray balloon experiment launched on August 10, 1982 in order to check the dependence of background level and rejection efficiency of NaI(Tl)/CsI(Na) phoswich detectors on scintillator thicknesses. The results concern the dependence of the background intensity on both the thickness of the active shield, for a fixed primary detector thickness, and on the thickness of the primary detector, given the active shield thickness. A direct comparison of phoswich detectors with passively shielded NaI(Tl) crystals is also given. As a consequence practical hints for designing new phoswich detectors are derived and the limiting sensitivity of these detectors for hard X-ray observations of celestial sources is inferred.