Flight performance of an advanced CZT imaging detector in a balloon-borne wide-field hard X-ray telescope—ProtoEXIST1

Abstract

We successfully carried out the first high-altitude balloon flight of a wide-field hard X-ray coded-aperture telescope ProtoEXIST1, which was launched from the Columbia Scientific Balloon Facility at Ft. Sumner, New Mexico on October 9, 2009. ProtoEXIST1 is the first implementation of an advanced CdZnTe (CZT) imaging detector in our ongoing program to establish the technology required for next generation wide-field hard X-ray telescopes such as the High Energy Telescope (HET) in the Energetic X-ray Imaging Survey Telescope (EXIST ). The CZT detector plane in ProtoEXIST1 consists of an 8 × 8 array of closely tiled 2 cm × 2 cm × 0.5 cm thick pixellated CZT crystals, each with 8 × 8 pixels, mounted on a set of readout electronics boards and covering a 256 cm 2 active area with 2.5 mm pixels. A tungsten mask, mounted at 90 cm above the detector provides shadowgrams of X-ray sources in the 30 -600 keV band for imaging, allowing a fully coded field of view of 9 • × 9 • (and 19 • × 19 • for 50% coding fraction) with an angular resolution of 20 . In order to reduce the background radiation, the detector is surrounded by semi-graded (Pb/Sn/Cu) passive shields on the four sides all the way to the mask. On the back side, a 26 cm × 26 cm × 2 cm CsI(Na) active shield provides signals to tag charged particle induced events as well as 100 keV background photons from below. The flight duration was only about 7.5 hours due to strong winds (60 knots) at float altitude (38-39 km). Throughout the flight, the CZT detector performed excellently. The telescope observed Cyg X-1, a bright black hole binary system, for ∼ 1 hour at the end of the flight. Despite a few problems with the pointing and aspect 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 7.2σ in the 30-100 keV energy band at the expected location from the optical images taken by the onboard daytime star camera. The success of this first flight is very encouraging for the future development of the advanced CZT imaging detectors (ProtoEXIST2, with 0.6 mm pixels), which will take advantage of the modularization architecture employed in ProtoEXIST1.

Performance of pixelated CZT detectors as a function of pixel and steering grid layout

“…1). Top: Steering grid voltage V 0 with the highest change in rate of N ¼2 pixel events, derived from the fitted function (2). Middle: Saturation rate Rsat per surface area A ¼ p 2 (rates for DET2 were increased by a factor of 1.57 to account for the higher absorption thickness of DET5).…”

“…The distribution of pulses is expected to have an (unknown) spread itself, making the measured distribution an upper limit on the electronic noise. 2 The pixels were deposited with a layer of titanium followed by a layer of gold on top (for bonding to a ceramic carrier after finalizing the studies described in this paper). For a few pixels the gold layer peeled off, see Fig.…”

“…The Burst Alert Telescope (BAT) on board the Swift satellite (launched in 2004), for example, is equipped with 32,768 CZT detectors to measure X-rays in the energy range of 15-150 keV [1]. Other examples for astrophysics applications are the balloon-borne hard X-ray telescope ProtoEXIST [2] or the hard X-ray polarimeter X-Calibur [3] which is scheduled for a balloon flight on board the InFOCuS X-ray telescope in 2013. CZT detectors with steering grid pixel layouts are also increasingly used for energy resolved photon counting applications in diagnostic medicine [4][5][6].…”

Through-Silicon-Vias (TSVs) for 3D readout of ASIC for nearly gapless CdZnTe detector arrays

“…The full array will be read out and controlled by a Detector Mother Board (DMB) containing a FPGA (as on ProtoEXIST2). This architecture uses only two stacks of substrate backend electronics boards (DCA and DMB), which is simplified from a three-board stack system in ProtoEXIST2 [2]. Fig.…”

Tiled Array of Pixelated CZT Imaging Detectors for ProtoEXIST2 and MIRAX-HXI

“…In codedaperture imaging, 2.4% loss of pixels means about 1.2% reduction in SNR. A simple depth sensing cut using the neighbor pixel charges [5,6,10] also reduces the background by ~25 % at low energies (<40 keV) and ~50% at high energies (>60 keV). Fig.…”