Energetic X-ray Imaging Survey Telescope (EXIST)

We have begun to study a mission to carry out the first high sensitivity imaging survey of the entire sky at hard x-ray energies (5-600 keV). The Energetic X-ray Imaging Survey Telescope (EXIST) would include 2-4 large area coded aperture telescopes with offset fields of view allowing total exposures of ≳500 ksec and flux sensitivities below 1 mCrab over the full sky in a year with time resolution from msec to months for each source as well as high spatial and spectral resolution for sources, transients and gamma-ray bursts. A pointed Observatory phase, with the telescopes co-aligned, would follow and achieve still greater sensitivities and temporal coverage, allowing the detailed study of virtually all classes of accretion sources (cataclysmic variables to quasars) as well as diffuse galactic emission. The baseline concept originally proposed for the detector is a modularized array (4 x 4) of Cd-Zn-Te crystals (6.25 cm^2 each, or 100 cm^2 /module). An array of 5 x 5 modules, or 2500 cm^2 total detector area with 1.25 mm spatial resolution, would constitute the focal plane readout of each of the 4 telescopes. A brief description of the proposed detector and telescopes and predicted backgrounds and sensitivity is given. An alternative detector concept, employing a hybrid (stacked) gas counter (2 atm Xe/TMA, optical avalanche chamber) and imaging phoswich scintillator (NaI/CsI), is also described and tradeoffs presented.