NEWLY DEVELOPED LOW-BACKGROUND HARD X-RAY GAMMA-RAY TELESCOPE WITH THE WELL-TYPE PHOSWICH COUNTERS

We have developed a low background hard-X-ray/gamma-ray telescope (Welcome-1) for balloon-borne experiments in the energy range from 40 keV to 800-1000 keV. The detector is based on newly developed well-type phoswich counters. In the first well-type phoswich counter, GSO(Ce) (Gd2SiO5 doped with Ce) is used as the detection part and CsI(Tl) as the shielding part. Welcome-1 consists of 64 GSO/CsI well-type phoswich counters assembled in the ''compound-eye configuration and the effective area is 740 cm2 at 122 keV and 222 cm2 at 511 keV line. The well-type phoswich counter and the compound eye configuration reduces background significantly both external and internal and allows us a high signal-to-noise ratio in balloon-borne experiments. The data from the detector is taken by the versatile data acquisition system based on the VME computer specially designed for balloon-borne experiments. In the system, the arrival time of each event is recorded by the clock system which utilizes GPS satellite. The background levels at an altitude of 4.5g/cm2 are 1 X 10(-4)/cm2/s/keV at 122 keV. Crab nebula has been observed with the signal to background ratio better than unity between 100 and 200 keV. In this paper, the desine of the telescope and the flight performace are presented.