An imaging low energy gamma ray telescope

An imaging low energy gamma ray telescope (ZEBRA) has been designed to operate in the energy range 200 keV to 10 MeV. The telescope makes use of an array of one dimensional sodium iodide position sensitive detectors, which can locate the position of interacting gamma ray photons of energy 662 keV to a precision of u2.35 cm (FWHM). The energy resolution is 11% at this energy. The geometry of these detectors makes it possible to have a large, sensitive two dimensional position sensitive plane. A coded aperture mask is used in conjunction with the focal plane to cast a shadowgram onto the imaging plane. A reconstructed image of the sky may be produced from the recorded data by a suitable deconvolution procedure. The technique of coded aperture imaging allows a large field of view to be observed whilst still maintaining a high angular resolution and as a consequence many gamma ray sources can be studied during a single observation. The space multiplexing of the image and shadow over the position sensitive plane also has the practical advantage that continuous real time measurements of the background are made. A satellite low energy gamma ray imaging telescope is also presented (SIGMA) which employs an Anger camera as the position sensitive plane. Coded aperture imaging is employed to generate high angular resolution pictures of the gamma ray sky. The predicted performances of the ZEBRA and SIGMA telescopes have been evaluated and are presented.