Double Scatter Telescope for Medium Energy Gamma Ray Astronomy from a Satellite

A large area (1 m2) medium energy (1-30 MeV) telescope for gamma ray astronomy is discussed. This telescope utilizes the double scattering of gamma rays between two scintillator arrays with directionial discrimination by means of time-of-flight. The first and second arrays consist of a series of plastic and NaI(Tl) scintillators, respectively, in the shape of long (1 m) linear elements viewed by photomultiplier tubes at both ends. The lateral position of the interaction in the plastic is determined by timing and in the NaI(Tl) by pulse height. Neutron-induced background is eliminated by using a pulse shape discrimination plastic scintillator. At 6 MeV, the telescope has an area-efficiency factor of 300 cm2, an energy resolution of 8% (FWHM), an angular resolution of 3° (HWHM) and a sensitivity of 5 × 10-6 γ/cm2sec for line emission. Discrete sources can be located to 0.5°.