Well-type phoswich counter for low-flux X-ray/γ-ray detection

Phoswich counters that can detect low-flux hard-X-ray/γ-ray from localized sources are described. The counter consists of a small inorganic scintillator with a fast decay time (the detection part) glued to the interior bottom surface of a well-shaped block of another inorganic scintillator with a slow decay time (the shielding part). The first model counter was built by using a new scintillator, gadolinium silicate (GSO), in the decision part and CsI(Tl) in the shielding part. A detector system consisting of 64 such phoswich counters (total area ~740 cm²) was flown onboard a balloon, setting a limit to the ⁵⁷Co line flux from SN 1987A, at ~10^-4 cm^-2 s The sensitivity for continuum flux was around a few ×10 ^-6 cm^-2 s^-1 keV^-1 between 100 and 200 keV. In the second model, CsI(Tl) was replaced by bismuth germanate (BGO), and radioactive contaminations in GSO were reduced. Improvements are planned to reach a sensitivity (3σ) around 5×10^-7 cm^-1 s^-1 keV^-1 for the continuum