  We have constructed a moderately large area (32 cm2), fine pixel (2.5 mm
pixel, 5 mm thick) CZT imaging detector which constitutes the first section of
a detector module (256 cm2) developed for a balloon-borne wide-field hard X-ray
telescope, ProtoEXIST1. ProtoEXIST1 is a prototype for the High Energy
Telescope (HET) in the Energetic X-ray imaging Survey Telescope (EXIST), a next
generation space-borne multi-wavelength telescope. We have constructed a large
(nearly gapless) detector plane through a modularization scheme by tiling of a
large number of 2 cm x 2 cm CZT crystals. Our innovative packaging method is
ideal for many applications such as coded-aperture imaging, where a large,
continuous detector plane is desirable for the optimal performance. Currently
we have been able to achieve an energy resolution of 3.2 keV (FWHM) at 59.6 keV
on average, which is exceptional considering the moderate pixel size and the
number of detectors in simultaneous operation. We expect to complete two
modules (512 cm2) within the next few months as more CZT becomes available. We
plan to test the performance of these detectors in a near space environment in
a series of high altitude balloon flights, the first of which is scheduled for
Fall 2009. These detector modules are the first in a series of progressively
more sophisticated detector units and packaging schemes planned for ProtoEXIST2
& 3, which will demonstrate the technology required for the advanced CZT
imaging detectors (0.6 mm pixel, 4.5 m2 area) required in EXIST/HET.
