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Thick CZT Detectors for Space-Borne X-Ray Astronomy

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Thick CZT Detectors for Space-Borne X-Ray Astronomy Thick CZT Detectors for Space-Borne X-ray Astronomy H. Krawczynskia, I. Junga, J. Perkinsa, A. Burgerb, M. Grozab aWashington University in St. Louis, Department of Physics, 1 Brookings Dr., CB 1105, St. Louis, MO 63130; bFisk University, Department of Physics, 1000 Seventeenth Ave. North, Nashville, TN 37208 ABSTRACT Cadmium Zinc Telluride (CZT) detectors are having a major impact on the ¯eld of hard X-ray astronomy. Without the need for cryogenic cooling they achieve good spatial and energy resolutions over the broad energy range from 10 keV to 600 keV. In this paper, we briefly review the historical development of detectors used in X- ray astronomy. Subsequen» tly, we present an evaluation of CZT detectors from the company Imarad. The standard 2 2 0.5 cm detectors, contacted with 8 8 In pixels and an In cathode, exhibit FWHM energy resolutions of 7 keV£ £at 59 keV, and 10 keV at 662 keV.£A direct measurement of the 662 keV photopeak e±ciency gives 67%. We have started a detailed study of the performance of Imarad detectors depending on surface preparation, contact materials, contact deposition, post-deposition detector annealing, and detector passivation techniques. We present ¯rst results from contacting detectors with Cr, Ag, Au, and Pt. Keywords: C ZT, X-ray and Gamma-ray detectors, space-applications, contact technology. 1. INTRODUCTION { DETECTORS IN X-RAY ASTRONOMY X-ray astronomy has come a long way since the ¯rst detection of X-ray emission from an extra-solar object in 1962.1 In the meantime, X-ray astronomy has emerged as one of the drivers of modern astronomy, as evident from the award of the Nobel prize in physics to X-ray pioneer Riccardo Giacconi in 2002, and the existence of two major X-ray observatories, Chandra and XMM-Newton, orbiting Earth since 1999. The most important results of X-ray astronomy have so far been obtained from observations in the \soft" X-ray band with photon energies between 0.1 keV and several keV. To mention several important milestones, the Einstein observatory2 achieved in 1978 a ¯rst breakthrough in the 0.2 keV to 4 keV band by combining a grazing incidence mirror with position sensitive detectors. The observatory carried a \High Resolution Imager" (HRI), consisting of two cascaded Micro Channel Plates (MCP) read out by a crossed-grid charge detector. Another main instrument was the Imaging Proportional Counter (IPC), a 4 cm deep, 800 Torr Ar-Xe-CO2 proportional counter which achieved a modest energy resolution of between 50% and 100%. The grazing incidence mirror and the HRI achieved an e®ective detection area of 10 cm2. The mirror and the IPC had a detection area of 100 cm2. The Roentgen Satellite (ROSAT) was launched in 1990 and used similar detectors as the Einstein observatory. Its HRI and IPC cameras collected respectively 8 and 2.4 times more photons per unit time than their Einstein counterparts.3 The Chandra and XMM-Newton satellites are equipped with powerful X-ray grazing incidence mirrors and state-of-the-art photon detectors. The Chandra satellite carries the Advanced CCD imaging spectrometer (ACIS)4 made of Si-CCDs with energy resolutions of between 2% and 10% over the energy range from 0.1 keV to 10 keV. The XMM-Newton satellite carries two conventional Metal Oxide Semiconductor (MOS) cameras, and a \PN" camera.5 The fully depleted pn CCDs (280 micron deep depletion region) can detect X-rays with energies up to 15 keV. The Chandra X-ray mirror and ACIS camera achieve a 1 keV detection area of 340 cm2 and an unique angular resolution of 0.5". The XMM-Newton mirrors and CCD cameras have a combined 1 keV detection area of 2,100 cm2 and an angular resolution of 15". Compared to the soft X-ray band, the hard X-ray band ( 20-200 keV) has progressed more slowly. Hard X-rays are more di±cult to focus, scintillation detectors had only» modest spatial and energy resolutions, and Ge Further author information: Send correspondence to H.K.: E-mail: [email protected], Telephone: 314 935 8553 detectors required resource intensive cryogenic cooling. The High Energy Astrophysics Observatory-1 (HEAO-1) was equipped with a 1.4± 20± fan collimator and 250 cm2 of NaI scintillator, and scanned the hard X-ray sky in the years 1977-1979.6 More£ recently, the Rossi X-Ray Timing Explorer (RXTE)7 launched in 1995 and Satellite per Astronomia X, \Beppo" (BeppoSAX)8 launched in 1996 used essentially the same detector technology with 1± pencil beam collimators. The wide bandgap semiconductors Cadmium Telluride (CdTe) and Cadmium Zinc Telluride (CZT) are now boosting the ¯eld of hard X-ray astronomy. The International Gamma-Ray Astrophysics Laboratory (INTEGRAL)9 launched in 2002 carries the Imager on Board the Integral Satellite (IBIS) that uses CdTe detectors with a detector area of 2,600 cm2. The SWIFT satellite,10 to be launched in October 2004, carries the Burst Alert Telescope (BAT)»with a CZT detection area of 5,240 cm2. The IBIS and the BAT use the approach of coded mask imaging. Embedded in a collimator assem»bly, the detectors see the sky through a patterned shadow mask. The detectors image the sum of all shadows cast by the X-ray sources in the ¯eld of view. A deconvolution algorithm is used to derive the X-ray surface brightness distribution from the detected image. In this approach, the e®ective detection area is roughly half of the active detector area (the other half of the detector area is covered by the coded mask), and is not given by the collection area of any focusing optics. The congressionally approved Beyond Einstein Program11 of NASA's \Structure and Evolution of the Uni- verse" theme plans for two major X-ray missions. In both missions, CZT plays an important role. The Constellation-X observatory12 will perform high throughput X-ray spectroscopy with high energy and angu- lar resolution over the energy range from 0.25 keV to 10 keV employing X-ray gratings with Si-CCDs, as well as non-dispersive microcalorimeters. The design includes a hard X-ray telescope that uses CZT detectors to cover the energy range from 6 keV to 40 keV or higher. Constellation-X is currently receiving NASA funds for Pre-Phase A development, the \advanced concepts" phase. Given the current budget plan, the mission will be launched in 2016.13 A second recommended mission is the \black hole ¯nder probe", a large ¯eld of view, hard X-ray telescope. The Energetic X-ray Imaging Survey Telescope (EXIST) design14 proposes an assembly of coded mask imagers. Hard X-rays are detected with 0.5 cm thick CZT detectors with a total detector area of 80,000 cm2. EXIST has been selected for a 2-year NASA» concept study, along with the Coded Aperture Survey » 15 Telescope for Energetic Radiation (CASTER), a competitor design which employs the new scintillators LaBr3 or LaCl3 as primary detectors. There is no de¯nitive time schedule for the black hole ¯nder probe, as its funding is presently beyond the budgetary horizon of 2009.13 In this paper, we emphasize thick CZT detectors and their application in EXIST-type hard X-ray telescopes. In Sect. 2 we will discuss key requirements for CZT detectors in this application. In Sect. 3 we scrutinize the performance of In contacted standard detectors from Imarad. Furthermore, we present detailed detector simulations and compare simulated data with experimentally measured data. In Sect. 4 we give ¯rst results of contacting the detectors with Cr, Ag, Au, and Pt. Finally, in Sect. 5, we summarize our ¯ndings and describe work in progress. 2. THICK CZT FOR HARD X-RAY TELESCOPES 2.1. CZT substrates Presently, major vendors of CZT substrates are eV Products,16 Imarad,17 and Bicron.18 eV Products and Bicron use the High-Pressure Bridgman (HPB) process to grow the substrates, while Imarad uses the modi¯ed High-Pressure Bridgman (mHPB) process. HPB substrates are more expensive than mHPB substrates, typical prices for individual crystals being $2000 cm¡3 and $350 cm¡3, respectively. While HPB and mHPB CZT achieve similar performances at high energies, mHPB detectors give poorer results at low energies. The latter can in part be explained by the higher resistivity of HPB crystals ( 2 1011 ohm-cm) compared to mHPB crystals ( 1010 ohm-cm) that results in lower leakage currents. An alternativ» ¢ e to CZT might be CdTe.19 An advantage of»CdTe is that large uniform single crystals can be grown. Although CdTe detectors require cooling to temperatures of -20±C to achieve good performances, such temperatures can easily be maintained in space. 2.2. Requirements for thick CZT detector in space-borne X-ray telescopes In the following, we discuss the requirements for 0.5 cm thick CZT detectors used for the detection of 10 keV to 600 keV photons in space-borne X-rays telescopes. Batteries 4 Channel 16 Channel ASIC Fast Amplification High Voltage Test−In Water Cooling CZT Detector Figure 1. Test set-up for evaluation of the CZT detector performance (Washington University). The hybrid read-out system consists of a low-noise ASIC readout for 16 pixels and a fast (100 MHz analog bandwidth) readout for 3 pixels and the cathode; pixels which are not read out are held at ground potential. Photopeak e±ciency: A crucial detector parameter is the photopeak e±ciency ². The detector area required to achieve a certain flux sensitivity scales with 1=²; the mass and cost of the full satellite can depend even more strongly on ².
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