Optimizing the Search for High-z GRBs: The JANUS X-ray Coded Aperture
  Telescope
  We discuss the optimization of gamma-ray burst (GRB) detectors with a goal of
maximizing the detected number of bright high-redshift GRBs, in the context of
design studies conducted for the X-ray transient detector on the JANUS mission.
We conclude that the optimal energy band for detection of high-z GRBs is below
about 30 keV. We considered both lobster-eye and coded aperture designs
operating in this energy band. Within the available mass and power constraints,
we found that the coded aperture mask was preferred for the detection of high-z
bursts with bright enough afterglows to probe galaxies in the era of the Cosmic
Dawn. This initial conclusion was confirmed through detailed mission
simulations that found that the selected design (an X-ray Coded Aperture
Telescope) would detect four times as many bright, high-z GRBs as the
lobster-eye design we considered. The JANUS XCAT instrument will detect 48 GRBs
with z > 5 and fluence Sx > 3 {\times} 10-7 erg cm-2 in a two year mission.
