WELL-TYPE PHOSWICH COUNTER FOR LOW-FLUX X-RAY GAMMA-RAY DETECTION

We have developed a new kind of phoswich counters that are capable of detecting low flux hard X-ray/gamma-ray 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 (PSD), 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 cm2) was flown on board a baloon, setting a limit to the Co-57 line flux from SN1987A at around 10(-4) cm-2 s. The sensitivity for continuum flux was around a few x 10(-6) cm-2 s-1 keV-1 between 100 and 200 keV. Efforts to lower the sensitivity further are underway. In the second model, CsI(Tl) was replaced by BGO and radioactive contaminations in GSO were reduced. More improvements are to be made the in near future to reach a sensitivity (3sigma) around 5 x 10(-7) cm-2 s-1 keV-1 for continuum.