A spaceborne CsI(Tl) calorimeter for cosmic gamma rays

The study of the cosmic gamma rays in the GeV energy range has so far been performed with instruments with a rather poor energy resolution. Recently it has been suggested that there might be very sharp lines in the high-energy gamma-ray spectrum coming from the annihilation reactions of one candidate for the dark matter of the Universe. From general grounds it is important to accurately measure the shape of the gamma-ray spectrum. Some features of an instrument capable of measuring accurately gamma rays in the GeV energy range are discussed. The main component of this instrument called SUSY-SKY is an electromagnetic calorimeter based on rods of Cs(Tl). The expected energy resolution is about 1% at 3 GeV. The use of such a calorimeter in a spaceborne experiment poses some technical problems. The temperature variation of the scintillation light yield must be accurately known in order to correct the data. This has been studied previously but only using low-energy gamma-ray sources. The temperature dependence using cosmic ray muons and various radioactive sources is reported here. The calibration of individual rods is crucial to achieve the desired resolution. Various methods of performing calibration in space are discussed, such as radioactive sources and cosmic rays, and it is concluded that a calibration procedure utilizing cosmic rays is feasible. Space experiments require stable low-power electronics. Readout electronics are presented, estimates of noise and rates are given