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

Novel phoswich counters have been developed that are capable of detecting low flux hard X-rays gamma -rays from localized sources. 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 well-shaped shielding part acts as an active collimator as well as an active shield. The whole assembly is viewed by a phototube from the exterior bottom surface of the shielding part. By using an appropriate pulse-shape discriminator, hard X-rays/ gamma -rays that have deposited energy only in the detection part can be selected. The first model counter was built by using a new scintillator, GSO, in the detection part and CsI(Tl) in the shielding part. A detector system consisting of 64 such phoswich counters (total area approximately 740 cm/sup 2/) was flown on board a balloon, setting a limit to the /sup 57/Co line flux from SN 1987A at around 10/sup -4//cm/sup 2/-s. The sensitivity for continuum flux was around a few*10/sup -6//cm/sup 2/-s-keV between 100 and 200 keV. >