The Basic Unit of the Imaging Plane of the ZEBRA Low Energy Gamma Ray Telescope

The mechanical, elecrtical, and performance characteristics of one element of the ZEBRA telescope's position sensitive aetection plane are presented. One unit comprises a NaI(Tl) bar 55×5,8×5 cm thick and two photomultiplier tubes one mounted on each end. The surface of the bar has been treated to enhance the exponential attenuation of the scintillation light produced by an event. The signal from the two tubes are taken in coincidence and, by the application of two simple algoritms, the energy and position of the event along the bar can be calculated. The tubes are powered by two programmable low consumption high tension supplies. Their digital control makes automatic gain correction feasible during the flight.