Newly developed low background hard X-ray/gamma-ray telescope with the well-type phoswich counters

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 is discussed. The detector is based on newly developed well-type phoswich counters. In the first well-type phoswich counter, GSO(Ce) (Gd ₂SiO₅ 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 reduce background significantly both externally and internally, and allow a high signal-to-noise ratio in balloon-borne experiments. The background levels at an attitude of 4.5 g/cm² are 1×10^-4/cm²/s/keV at 122 keV. Crab nebula is observed with the signal to background ratio better than unity between 100 and 200 keV. The design of the telescope and the flight performance are presented