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.

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.

Each detector module (DM) is surrounded by graded-Z passive shields (Pb-Sn-Cu) and CsI rear active or passive rear shields.

Parameters ProtoEXIST1 EXIST HET
Energy 20 - 600 keV 10 - 600 keV
Mask (Tungsten) 2.5/5mm thick, 5 mm pixel (1) 5 mm thick, 2.5 mm pixel
Detector (CdZnTe) 5mm thick, 2.5 mm pixel (1) 5 mm thick, 1.25 mm pixel
Detector Area (Module/Total) 16x16 cm2 / 0.1 m2
56x56 cm2 / 5.6 m2
Modules 2x2 3x6
Fully Coded FoV (Module/Total) 10°x10° / 10°x10° 21°x21° / 65°x131°
Angular Resolution
/5σ localization
17.2'/3.4' 5.7'/1.2'
Mask detector sep. 0.9 m or 0.6 m 1.5 m
Temporal resolution < 1 ms < 1 ms
Shields CsI + Passive CsI + Passive
Sensitivity (5σ) 5 mCrab ( < 100 keV)
~3h scan
0.05 mCrab (< 150 keV)
0.5 mCrab ( > 150 keV)
~1yr survey
(1) For ProtoEXIST2, 1.25 mm detector pixel and 2.5 mm mask pixel. See CZT & ASIC development.