Tadahisa Tamura

Use of a large area photodiode in CsI(Tl) scintillation counters

Abstract A newly developed large area photodiode has been studied as the photon sensor for medium-size CsI(Tl) scintillation counters. The photodiode has a sensitive area of 18 × 18 mm 2 and a thickness of 500 μm. When combined with a CsI(Tl) crystal of 2.5 × 2.5 × 5.0 cm 3 , it gives an energy resolution of around 5.1% (FWHM) for 1.33 MeV gamma-rays. Characteristics of the low noise preamplifier designed for the CsI(Tl)-photodiode counter will also be given.

Hard x‐ray observation of Cen A and a break in the energy spectrum

We studied hard X‐ray/γ‐ray emission from Cen A (NGC 5128) in a balloon experiment with a low background detector (Welcome‐1) in Brazil. The energy spectrum of CenA is obtained from 40 keV to 600 keV. We combined the energy spectrum obtained by the Ginga satellite in a similar state. The combined spectrum indicates that there is a break at 185±22 keV. The spectrum is fitted to the broken power law model with the photon index of α1=−1.79 up to 188 keV, and α2=−3.7−1.7+0.9 above the break.

Detector to study low-flux hard X-ray/gamma-ray sources

Abstract We have developed a new kind of phoswich counters that will be capable of detecting low flux hard X-rays/gamma-rays from astronomical objects. The new phoswich counter consists of a small inorganic scintillator with a fast decay time (the detection part) glued to the interior bottom surface of a rectangular well-shaped block of another inorganic scintillator with a slow decay time (the shielding part). Here, the well-shaped shielding part acts as an active collimator as well as an active shield. We have built a detector system consisting of 64 such phoswich counters: newly developed scintillator (GSO) is used for the detection part and CsI( Tl ) is used for the shielding part. The total geometrical area of the 64 detection parts is about 740cm 2 and its 3σ sensitivity is expected to reach below 10 −5 cm −2 s −1 keV −1 up to 700keV. With several improvements such detectors will be able to detect hard X-rays/gamma-rays at a flux level around 10 −6 cm −2 s −1 keV −1 upto 2 MeV.