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Instrument
The high sensitivity and temporal coverage needed to achieve the EXIST
and ProtoEXIST science demand a very large area, wide-field
coded aperture imager. Broad-band (~10 - 600 keV) hard X-ray imaging over
a wide FOV is best conducted with a coded aperture
telescope with an imaging detector capable of fine position resolution
(to record the coded mask shadow), high Z stopping power for good
high energy sensitivity in moderate detector thickness, and compact
mounting and tiling capability for extension to a very large area total
detector array. ProtoEXIST consists of two telescopes, employing
a large array of CdZnTe detectors with Tungsten masks and covering
20° x 20° (50% coding). The figure on the left illustrates
ProtoEXIST gondola with the pressure vessel capable of
accomodating four coded-mask telescopes, star
camera and support electronics. The ProtoEXIST telescopes
are mounted in an offset elevation pivot that allows direct zenith
observation without interference from the balloon rotator or flight
train.
Each detector module (DM) is surrounded by graded-Z passive shields (Pb-Sn-Cu) and CsI rear active or passive rear shields.
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The two telescopes - ProtoEXIST1/2 will demonstrate the progress of
the CdZnTe detector developments. Under the current APRA program,
we have developed 256 cm2 of 2.5 mm
pixel (5 mm thick) CdZnTe detectors for ProtoEXIST1 and plan to develop
~256 cm2 of 0.6 mm pixel (5 mm thick) CdZnTe
detectors for ProtoEXIST2, which will meet the requirements of the
EXIST detectors. The mask pixel size will be 5 mm for ProtoEXIST1
and 1.2 mm for ProtoEXIST2 accordingly and their thickness will
be 5 mm and 0.3 mm respectively.
With the 1.0 m and 0.7 m separation between the mask and detector,
ProtoEXIST1/2 will have 20' and 5' angular resolution respectively.
The figure on the right shows the pressure vessel
for housing the detector planes and the four mask towers
are mounted on the top for possible four telescopes.