Flight Performance of an advanced CZT Imaging Detector in a Balloon-borne Wide-Field Hard X-ray Telescope - ProtoEXIST1

摘要

We successfully carried out the first high-altitude balloon flight of awide-field hard X-ray coded-aperture telescope ProtoEXIST1, which was launchedfrom the Columbia Scientific Balloon Facility at Ft. Sumner, New Mexico onOctober 9, 2009. ProtoEXIST1 is the first implementation of an advanced CdZnTe(CZT) imaging detector in our ongoing program to establish the technologyrequired for next generation wide-field hard X-ray telescopes. The CZT detectorplane in ProtoEXIST1 consists of an 8 x 8 array of closely tiled 2 cm x 2 cm x0.5 cm thick pixellated CZT crystals, each with 8 x 8 pixels, covering a 256cm^2 active area with 2.5 mm pixels. A tungsten mask, mounted at 90 cm abovethe detector provides shadowgrams of X-ray sources in the 30 - 600 keV band forimaging, allowing a fully coded field of view of 9 Deg x 9 Deg with an angularresolution of 20 arcmin. To reduce the background radiation, the detector issurrounded by semi-graded (Pb/Sn/Cu) passive shields on the four sides all theway to the mask. On the back side, a 26 cm x 26 cm x 2 cm CsI(Na) active shieldprovides signals to tag charged particle induced events as well as >~ 100 keVbackground photons from below. The flight duration was only about 7.5 hours dueto strong winds (60 knots) at float altitude (38-39 km). Throughout the flight,the CZT detector performed excellently. The telescope observed Cyg X-1 for ~ 1hour at the end of the flight. Despite a few problems with the pointing andaspect systems that caused the telescope to track about 6.4 deg off the target,the analysis of the Cyg X-1 data revealed an X-ray source at 7.2 sigma in the30-100 keV energy band at the expected location from the optical images takenby the onboard daytime star camera. The success of this first flight is veryencouraging for the future development of the advanced CZT imaging detectors(ProtoEXIST2, with 0.6 mm pixels).