Well-type phoswich counters 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/γ-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 counter was built by using a new scintillator (GSO) for the detection part and CsI(Tt) for the shielding part. A detector system consisting of 64 such phoswich counters (total area ~740cm 2 ) was flown three times on board a balloon, setting a limit to the 57 Co line flux from SN1987A at around 10 -4 cm -2 s, determining the pulsating hard X/γ-ray flux of PSR15O9-58, determining the hard X/γ-ray spectra of CenA and GX339-04. Analyses have revealed the fact that background counts due to the Compton scattering, nuclear reactions, and β-γ radioactivities in the detector are largely suppressed because they are likely to register at least one extra count in the shielding part. The ultimate sensitivity of the detector will then be determined by the level of radioactive contamination. Other scintillator combinations such as GSO/BGO, NaI(Tl)/CsI(Tl), and YAIO 3 /BGO have also been studied. Efforts to reduce the radioactive contamination in scintillators have also been actively pursued. In near future we expect to reach a sensitivity (3σ) around a few times 10 -7 cm -2 s -1 keV -1 for continuum in a 10 4 s balloon observation.