  We report our in-depth study of Cd-Zn-Te (CZT) crystals to determine an
optimum pixel and guard band configuration for Hard X-ray imaging and
spectroscopy. We tested 20x20x5mm crystals with 8x8 pixels on a 2.46mm pitch.
We have studied different types of cathode / anode contacts and different pixel
pad sizes. We present the measurements of leakage current as well as spectral
response for each pixel. Our I-V measurement setup is custom designed to allow
automated measurements of the I-V curves sequentially for all 64 pixels,
whereas the radiation properties measurement setup allows for interchangeable
crystals with the same XAIM3.2 ASIC readout from IDEAS. We have tested multiple
crystals of each type, and each crystal in different positions to measure the
variation between individual crystals and variation among the ASIC channels. We
also compare the same crystals with and without a grounded guard band deposited
on the crystal side walls vs. a floating guard band and compare results to
simulations. This study was carried out to find the optimum CZT crystal
configuration for prototype detectors for the proposed Black-Hole Finder
mission, EXIST.
