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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
36° x 18° (fully coded). The figure on the left illustrates
ProtoEXIST gondola, showing two 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.
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The two telescopes - ProtoEXIST1/2 will demonstrate the progress of
the CdZnTe detector developments. Under the current SR&T program,
we plan to develop ~1000 cm2 of 2.5 mm
pixel (5 mm thick) CdZnTe detectors for ProtoEXIST1 and >
256 cm2 of 1.25 mm pixel (5mm 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 2.5 mm for ProtoEXIST2 accordingly and their thickness will
be 5 mm. With the same 1.2 m separation between the mask and detector,
ProtoEXIST1/2 will have 17' and 8' angular resolution respectively.
The figure on the right shows detector planes for two telescopes inside
the pressure vessel with the kevlar dome.
The detector plane of ProtoEXIST1 will divide into the four
identical detector module (DM). Each DM is surrounded by shields.
Four telescopes have the identical rear CsI active shield (blue).
In the case of the side shields, we employ two types of shielding
configurations: two telescopes (A, C) with the combination of CsI active
(blue) and Pb-Sn-Cu passive shields (light brown), and the other two (B, D)
with Pb-Sn-Cu passive shields. We also employ two types of FoVs: narrow
FoV (A, B) and wide FoV (C, D). This 2x2 configuration allow us to identify
each background component. For instance, A and B are expected to have
the identical diffuse X-ray background due to the identical FoV, but they
will have different internal background due to the different shielding
configurations. Similar, A and C will have the similar internal background,
but not the diffuse X-ray background.