  We describe the detector development for a balloon-borne wide-field hard
X-ray (20 - 600 keV) telescope, ProtoEXIST. ProtoEXIST is a pathfinder for both
technology and science of the proposed implementation of the Black Hole Finder
Probe, Energetic X-ray Imaging Survey telescope (EXIST). The principal
technology challenge is the development of large area, close-tiled modules of
imaging CZT detectors (1000 cm2 for ProtoEXIST1). We review the updates of the
detector design and package concept for ProtoEXIST1 and report the current
development status of the CZT detectors, using calibration results of our basic
detector unit - 2 x 2 x 0.5 cm CZT crystals with 2.5 mm pixels (8 x 8 array).
The current prototype (Rev1) of our detector crystal unit (DCU) shows ~4.5 keV
electronics noise (FWHM), and the radiation measurements show the energy
resolution (FWHM) of the units is 4.7 keV (7.9%) at 59.5 keV, 5.6 keV (4.6%) at
122 keV, and 7.6 keV (2.1%) at 356 keV. The new (Rev2) DCU with revised design
is expected to improve the resolution by ~30%.
