The High Energy X-ray Imager Survey (HEXIS) Coded Mask balloon instrument will test the performance of
the electronics and the detector for the proposed MIRAX satellite mission, and measure the background in a
near space environment. HEXIS is a Coded Mask Imager based upon a 100 x 100mm Tungsten MURA mask
and a set of four Cadmium-Zinc-Telluride (CZT) crossed strip detectors assembled as one detector module with
40 cm2 detector area and 0.5mm pitch strips creating an effective 126 x 126 grid of 0.5 x 0.5mm2 pixels. Each
detector strip can be read out individually using Readout Electronics for Nuclear Application (RENA)-ASICs
developed by NOVA R&D. The system has an operating energy range of <10 to 200 keV. The telescope has a
passive shield as part of the instrument structure, which is surrounded by an active anti-coincidence shield of
plastic scintillators with embedded wavelength shifting and light transmitting fibers. The first HEXIS balloon
flight is planned for Spring 2007. We present the lab performance for one module using RENA ASICs and for
the scintillator shield. The MIRAX Hard X-ray Imager (HXI) will contain two cameras with 9 detector modules
each.
We present results of detailed characterization of the spatial properties of large area crossed strip CZT detectors now under development at CASS/UCSD for use in coded aperture astrophysical hard X-ray instrumentation. We address the volumetric uniformity of spectral response for these detectors as determined by charge sharing and loss, diffusion, and electron trapping. Results are presented for our prototype detector having 500 μm pitch and collecting area 32×32 mm2; however, we also explore these characteristics as they effect performance of larger and smaller pitch detectors of similar design.
KEYWORDS: Sensors, Gamma radiation, Electrodes, Imaging spectroscopy, Time metrology, Signal attenuation, Signal detection, Kinematics, Gamma ray imaging, 3D modeling
We are developing 10 mm thick CZT detectors with 3-D readout for ~100 keV to ~1.5 MeV gamma-rays. Multiple-site gamma-ray interactions are fully measured, i.e., the energy and 3-D position of each site are determined. Spatial resolution is 1 mm FWHM. Anode pixel readout with 1 mm pitch is used for x- and y-positions and charge drift times for z-positions. Drift time measurements are triggered by the cathode signal and end when each interaction site's charge cloud reaches an anode pixel. Post-event processing corrects for signal loss due to charge trapping and accurately determines gamma-ray energies, with a goal of 1% energy resolution at 662 keV. Compton kinematic analysis can identify the initial interaction site in most cases as well as constrain the incident gamma-ray direction. Tests were made with a prototype detector, measuring 10 x 10 x 10 mm3 and operated at 1000 V bias. The measured drift time resolution of 25 nsec FWHM at 662 keV and 60 nsec at 122 keV corresponds to z-position resolution of 0.25 and 0.60 mm FWHM, respectively. The technique is described and results of modeling and tests are presented.
A large area (40 cm2) CZT module for space-borne X-ray astronomy applications has been under development at UCSD. This
module employs four 32mm x 32mm x 2mm CZT crossed strip
detectors with 0.5 mm pitch which are sensitive in the 10-200 keV range. The compact design includes readout and control electronics below the detector plane, which allows for efficient tiling of modules to form large detector planes for wide field of view coded mask imagers, or for efficient packaging within an anticoincidence shield at the focus of a hard X-ray telescope. The module has successfully been shaken on all three axes at 14 g rms to validate the mechanical design for spaceflight applications. Spectral, spatial, and imaging performance is presented.
Our collaboration is characterizing a prototype detector module designed for high energy X-ray astrophysics research covering the 20 - 250 keV energy range. The module consists of a three dimensional position sensitive CdZnTe detector, 25 mm X 25 mm X 2 mm, with 1 mm pitch crossed strip electrodes, an interleaved steering electrode, and an Application Specific Integrated Circuit (ASIC) for individual electrode readout. The newly developed readout system is compact, lightweight, has low power consumption and will lead to reduced system electronic noise. The detector is surrounded by a plastic anti-coincidence system for charged particles, and passive shielding that has been optimized based on results from two previous balloon flights. The first balloon flight test of the new detector module is scheduled for Fall 2000. In addition to our continuing balloon studies, we are investigating proton radiation damage effects and present preliminary results. After proton irradiation, the energy resolution is not significantly degraded, calibration photopeaks are down shifted by less than 10% in energy, and the depth of interaction dependence is nearly eliminated.
HEXIS is a MIDEX-class mission concept for x-ray astronomy. Its objectives are to improve our knowledge of the high energy x-ray sky by increasing the number of sources above 20 keV to > 2,000, discovering transient sources such as x-ray novae and gamma-ray bursts, and making spectral and temporal studies of the sources. With mission life > 3 years, a 1-year all-sky survey sensitivity of approximately 0.3 mCrab, and continuous monitoring of the entire visible sky, HEXIS will provide unprecedented capabilities. Source positions will be determined to accuracies of a few arcmin or better. Spectra will be determined with an energy resolution of a few keV and source variability will be studied on time scales from < 1 sec to years. In addition, 10 times more sensitive studies of limited fields will be performed at the same time. Gamma-ray bursts will be detected about 4 times/week at about the same sensitivity as BATSE and the sensitivity to nova-like x-ray transients will be approximately 6 mCrab in one day. HEXIS contains a set of coded mask imagers that use position-sensitive CZT detectors operating from approximately 5 keV to 200 keV. Detector planes are built with 41 cm2 CZT detector modules which employ crossed-strip readout to obtain a pixel size of 0.5 mm. Nine modules are grouped in a 369 cm2 array for each imager. In the past 2 years significant progress has been made on techniques requires for HEXIS: position-sensitive CZT detectors and ASIC readout, coded mask imaging, and background properties at balloon altitudes. Scientific and technical details of HEXIS are presented together with result form tests of detectors and a coded mask imager.
The scientific objectives, status, and future instrumental requirements of high energy X-ray astronomy (20 to 200 keV) are discussed. Two particularly compelling requirements are: (1) an improvement in sensitivity to a level of about 5 microCrab and (2) a survey of the sky at a sensitivity of about 0.1 milliCrab, which will discover and characterize about 10,000 new sources. The first requirement can be fulfilled by imaging telescopes that use large-area focusing X-ray mirrors, which are effective over 5-30 arcminute fields, and the second requirement can be met by arrays of large area coded mask imagers with wide fields, about 50 deg. Multilayer mirror and CdZnTe detector technology now in development offers the potential to meet these objectives. Position-sensitive CdZnTe detectors are well-suited to both of these imaging techniques, and instrument concepts that use these detectors are described. Detectors with pixel readout are better suited for focusing telescopes, and those with crossed-strip readout are better suited for coded mask imagers. Technical aspects of these detectors are discussed. Recent work at UCSD and WU on CdZnTe strip detectors is described in detail. Studies with small, 40 micron, X-ray beams have mapped a crossed-strip detector's spatial response with fine spatial resolution.
The scientific objectives and future requirements of high energy x-ray astronomy are discussed and concepts for imaging instruments based on CdZnTe detectors and coded masks are reviewed. An instrument concept based on CdZnTe strip detectors, HEXIS, is described in detail. Technical requirements for large area CdZnTe strip detectors are discussed and recent work at UCSD and WU on the capabilities of CdZnTe strip detectors is described in detail. Studies with small, approximately 50 micron beams demonstrate that crossed strip detectors have good properties for both spatial and spectral measurements.
The preliminary design for the 15 - 250 keV, 200 cm2, phoswich detectors for the High Energy X-ray Timing Experiment (HEXTE) for NASA's X-ray Timing Explorer mission has been completed, and the first engineering model has been fabricated. This unit has undergone extensive environmental and performance testing, including extended vibration, thermal range, resolution, uniformity, and pulse shape, and is within specifications for all tests. Broad beam energy resolution of better than 15% at 60 keV and clear separation of NaI and CsI pulse shape peaks are seen. The design and test results will be presented.
Michael Pelling, Paul Feffer, Kevin Hurley, Sharad Kane, Robert Lin, Steven McBride, J. Primbsch, David Smith, K. Youseffi, G. Zimmer, F. Cotin, J. Lavigne, G. Rouaix, Said Slassi-Sennou, Gilbert Vedrenne, Richard Pehl, Chris Cork, Paul Luke, Norman Madden, Don Malone
The elements of a high resolution gamma-ray spectrometer, developed for observations of solar flares, are described. Emphasis is given to those aspects of the system that relate to its operation on a long duration balloon platform. The performance of the system observed in its first flight, launched from McMurdo Station, Antarctica on 10 January, 1992, is discussed. Background characteristics of the antarctic balloon environment are compared with those observed in conventional mid-latitude balloon flights and the general advantages of long duration ballooning are discussed.
Results are reported from an effort to define a passive magnetic field concept for the High Energy X-ray Timing Experiment (HEXTE), in the interest of reducing the detector-gain variations due to 0.5-1.0-sec timescale magnetic field variations. This will allow a sensitivity of the order of 1 percent of the HEXTE background. While aperture modulation and automatic gain control will minimize effects on timescales of tens of seconds and longer, passive magnetic shielding of the photomultiplier tubes will address 1-sec timescale variations due to aperture motions.
The High Energy X-Ray Timing Experiment (HEXTE), currently under development for the X-Ray Timing Explorer (XTE) mission, employs a closed loop gain control system to attain 0.5 percent stabilization of each of eight-phoswich detector gains. This Automatic Gain Control (AGC) system utilizes a split window discriminator scheme to control the response of each detector pulse height analyzer to gated Am-241 X-ray events at 60 keV. A prototype AGC system has been implemented and tested within the gain perturbation environment expected to be experienced by the HEXTE instrument in flight. The AGC system and test configuration are described. Response, stability and noise characteristics are measured and compared with theoretical predictions. The system is found to be generally suitable for the HEXTE application.
The HEXTE, part of the X-Ray Timing Explorer (XTE), is designed to make high sensitivity temporal and spectral measurements of X-rays with energies between 15 and 250 keV using NaI/CsI phoswich scintillation counters. To achieve the required sensitivity it is necessary to provide anticoincidence of charged cosmic ray particles incident upon the instrument, some of which interact to produce background X-rays. The proposed cosmic ray particle anticoincidence shield detector for HEXTE uses a novel design based on plastic scintillators and wavelength-shifter bars. It consists of five segments, each with a 7 mm thick plastic scintillator, roughly 50 cm x 50 cm in size, coupled to two wavelength-shifter bars viewed by 1/2 inch photomultiplier tubes. These segments are configured into a five-sided, box-like structure around the main detector system. Results of laboratory testing of a model segment, and calculations of the expected performance of the flight segments and particle anticoincidence detector system are presented to demonstrate that the above anticoincidence detector system satisfies its scientific requirements.
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