  One of the key aspects of a detector material for space-borne hard X-ray and
gamma-ray telescopes is the rate of prompt and delayed background events
generated inside the material by charged and neutral particles striking the
detector. These particles are Cosmic Rays, particles trapped in Earth's
magnetic field, and secondaries from Cosmic Ray interacting with the atmosphere
and the spacecraft. Here, we present a preliminary study of Cadmium Zinc
Telluride (CZT) and its behaviour in space environments. We have used the
simulation package MGGPOD to estimate the background of the CZT detectors in
the proposed Energetic X-ray Imaging Survey Telescope (EXIST) for possible
orbital parameters. The EXIST mission will make use of 6 square meters of 0.5
cm thick CZT detectors to record cosmic X-rays in the energy range from 10 keV
to 600 keV. The detectors will be shielded by a fully or partly active shield.
For the specific detector and shielding geometry considered here and an orbit
with a low (7 deg) inclination, the background rate is dominated by the diffuse
extragalactic below 200 keV. Prompt and delayed hadronic backgrounds grow
increasingly important above this energy, becoming the main contributors to the
total background above 1 MeV. A fully active shield performs slightly better
than a half active/half passive shield.
