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

Abstract

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 anticoincidence shield/collimator with a total area of between 15 m 2 and 35 m 2 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 GEANT 4 to simulate the interactions of photons and cosmic rays with the spacecraft and the instrument and the code DETECT2000 to simulate the optical properties of the scintillation detectors. The shield/collimator achieves a νF ν -sensitivity of 3 × 10 −9 erg cm −2 s −1 and 2 × 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.

Yost-Swift Gamma-ray Burst Science and Capabilities Needed to EXIST

“…The active shields (1cm CsI on 4 sides of each sub-telescope around the CZT array; 2cm planar shield beneath the CZT array) are read out for GRB spectroscopy. Simulations [25] show that their sensitivity for GRB spectra nicely extends that of the CZT array to much higher energies, as shown in Figure 3. E peak values, and thus constraints on GRB luminosities, could be measured up to 5-10 MeV.…”

EXIST’s Gamma‐Ray Burst Sensitivity

“…Next we simulate the spectra that the EXIST suite of detectors will observe. Figure 9 shows a count spectrum (counts s −1 keV −1 ) for a moderately strong burst as it might be observed by the LETs (lefthand set of curves), HETs (middle set) and the CsI active shields for the HETs (righthand set; based on Garson et al 2006a). The solid curves show the signal count rate, while the dashed curves provide the estimated background.…”

“…The active CsI shields behind the HET detector plane and in the lower part of the HET collimators will be instrumented to provide 64 channel spectra between ∼300 keV to ∼10 MeV (see Garson et al 2006a). The current plan is that spectra accumulated every 1 s will be downlinked.…”

HX-POL—A Balloon-Borne Hard X-Ray Polarimeter

“…The Si and CZT detectors reside above an active shield (CsI(Na), 2 cm×25 cm×25 cm) that reduces the atmospheric and internal backgrounds (e.g. events initiated by neutron capture in the CZT detectors) [11].…”