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The Energetic X-ray Imaging Survey Telescope (EXIST) is a mission designconcept that uses coded masks seen by Cadmium Zinc Telluride (CZT) detectors toregister hard X-rays in the energy region from 10 keV to 600 keV. A partiallyactive or fully active anti-coincidence shield/collimator with a total area ofbetween 15 and 35 square meters will be used to define the field of view of theCZT detectors and to suppress the background of cosmic-ray-induced events. Inthis paper, we describe the use of a sodium activated cesium iodideshield/collimator to detect gamma-ray bursts (GRBs) and to measure their energyspectra in the energy range from 100 keV up to 10 MeV. We use the code GEANT4to simulate the interactions of photons and cosmic rays with the spacecraft andinstrument and the code DETECT2000 to simulate the optical properties of thescintillation detectors. The shield collimator achieves a nu-F-nu sensitivityof 3 x 10^(-9) erg cm^(-2) s^(-1) and 2 x 10^(-8) erg cm^(-2) s^(-1) at 100 keVand 600 keV, respectively. The sensitivity is well matched to that of the codedmask telescope. The broad energy coverage of an EXIST-type mission with activeshields will constrain the peak of the spectral energy distribution (SED) for alarge number of GRBs. The measurement of the SED peak may be key fordetermining photometric GRB redshifts and for using GRBs as cosmologicalprobes.Comment: 20 pages, 10 Figures, Accepted May 19, 2006 A&

Using the active collimator and shield assembly of an EXIST-type mission as a gamma-ray burst spectrometer