  The Energetic X-ray Imaging Survey Telescope (EXIST) is a mission design
concept that uses coded masks seen by Cadmium Zinc Telluride (CZT) detectors to
register hard X-rays in the energy region from 10 keV to 600 keV. A partially
active or fully active anti-coincidence shield/collimator with a total area of
between 15 and 35 square meters will be used to define the field of view of the
CZT detectors and to suppress the background of cosmic-ray-induced events. In
this paper, we describe the use of a sodium activated cesium iodide
shield/collimator to detect gamma-ray bursts (GRBs) and to measure their energy
spectra in the energy range from 100 keV up to 10 MeV. We use the code GEANT4
to simulate the interactions of photons and cosmic rays with the spacecraft and
instrument and the code DETECT2000 to simulate the optical properties of the
scintillation detectors. The shield collimator achieves a nu-F-nu sensitivity
of 3 x 10^(-9) erg cm^(-2) s^(-1) and 2 x 10^(-8) erg cm^(-2) s^(-1) at 100 keV
and 600 keV, respectively. The sensitivity is well matched to that of the coded
mask telescope. The broad energy coverage of an EXIST-type mission with active
shields will constrain the peak of the spectral energy distribution (SED) for a
large number of GRBs. The measurement of the SED peak may be key for
determining photometric GRB redshifts and for using GRBs as cosmological
probes.
