Cadmium Zinc Telluride (CZT) is the detector material of choice for the detection of X-rays in the 10 keV-1MeV
energy band with excellent spatial and energy resolutions and without cryogenic cooling. In this contribution,
we report on recent results of the CZT detector development program and several astrophysical experiments
which make use of CZT detectors. In the first part of the paper, we discuss the performance of pixel and
cross-strip CZT detectors read out with an ASIC developed at the Brookhaven National Laboratory. Our pixel
detectors achieve some of the best energy resolutions reported in the literature. Cross-strip detectors are found
to give an inferior performance and we investigate the reason for this performance difference. We also present
results from a precision measurement of the effect of a steering grid on multi-pixel events obtained with a
200 micrometer collimator. In the second part of the paper, we describe the design and performance of the
hard X-ray polarimeter X-Calibur. The polarimeter uses a 14 cm long scintillator scatterer, surrounded by an
assembly of 32 2-5 mm thick CZT detectors. We discuss the sensitivity of the polarimeter to measure the linear
polarization of 10 keV-80 keV X-rays on short and long balloon flights and results from testing the polarimeter
in the laboratory.
We report here on the optimization of 0.5 cm thick pixelated Orbotech CZT detectors with regards to the
best contacting materials and the use of steering grids. We evaluated the performance of different contacting
materials. Our study differs from earlier ones in that we investigated the performance of different anode and
cathode materials separately. We obtain the best performance with Au cathodes. For different anode materials
Ti and In give the best energy resolutions. The detector (2.0×2.0×0.5 cm3, 8×8 pixels) shows excellent 59 keV,
122 keV and 662 keV energy resolutions of 1.4 keV, 1.9 keV, and 7.4 keV, respectively. Furthermore, we report
on using steering grids to improve on the performance of the pixelated detectors. Previously, the benefit of
steering grids had been limited by additional electronic noise associated with currents between the negatively
biased steering grids and the anode pixels. We are currently exploring the possibility to isolate the steering grid
from the CZT substrates by a thin layer of Al2O3. We performed a series of measurements to determine by
how much the isolation layer reduces the grid-pixel currents. Comparing the currents between two Au contacts
before and after isolating one of the two contacts from the CZT with a 700 nm thick layer of Al2O3, we measure
that the isolation layer reduces the currents by a factor of about 10 at 500 V. We present some results from
a detector before and after deposition of an isolated steering grid. The grid indeed improves on the detectors
energy resolution and detection efficiency. We show that simulations can be used to model the anode to cathode
charge correlation in excellent agreement with the experimental results.
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.
Monitor e Imageador de RAios-X (MIRAX) is a Brazilian high energy astronomy mission dedicated to monitoring the central 1000 sq. deg. of the Galactic plane to observe unpredictable transient phenomena from compact objects in the 2-200 keV range through nearly continuous imaging with good spatial/temporal/energy resolution. The strength of MIRAX lies in the departure of its observing strategy from traditional pointed programs and scanning monitors. MIRAX will achieve two major advantages over previous and existing missions. First, it will detect, localize, and study transient phenomena, which last on all timescales from milliseconds to years, and are very likely to be missed by traditional observing strategies. Second, MIRAX will study longer lived phenomena in exquisite detail. The mission elements and science will be presented.
We describe the hard x-ray mission MIRAX - jointly proposed by teams from Brazil, the USA, Germany and the Netherlands. The scientific objective is to provide continuous 2-200 keV imaging of the central 1000 square degrees of our Galaxy for 9 months per year over up to 5 years. Durign times when the sun crosses the Galactic Center other areas like the Cygnus-, Vela- and the Magellanic Cloud-regions can be observed. MIRAX will detect, localize, identify and study sources of medium to hard x-ray emission, with special emphasis on short-lived, rare and unpredictable events, including weak x-ray transients and fast x-ray movae. MIRAX will reach in a one day observation a sensitivity of 1mCrab in 2-10 keV and 2.5mCrab in 10-100keV. MIRAX will provide a unique capability to study compact galactic objects - notably accreting neutron stars and black holes. It will:- Probe neutron star and x-ray burst theory wiht 20,000 type I x-ray bursts and 50 'superbursts' - Measure spin frequencies of neutron stars from 10-100 burst oscillations - Observe explosive flares and x-ray light curves during ejections in superluminal jets - Study soft gamma-ray repeaters, fast x-ray novae and new types of phenomena yet to be discovered. We describe the science and the instrumentation.
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.
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.
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.
We have studied charge splitting among the anodes of a CdZnTe (CZT) strip detector using a 30 micrometer collimated X-ray beam. The results indicate that charge splitting is a consequence of photoelectron range and diffusion. We also studied charge loss to the inter-anode regions. We showed that the charge loss increases as depth of interaction increases for interactions occurring above the inter-anode region.
The FAR.XITE is a proposed balloon payload. After a test flight, our initial goal is to fly 10 nested mirror modules, but then even more modules can be added until the mass limit of the pointing system and balloon gondola are reached. These limits are yet to be determined. In our design, Wolter I mirrors are coated with multilayers that allow FAR.XITE to reach 100 keV with better than 1 arc minute angular resolution. We summarize the science objectives, optical design and specifications that were previously reported, and present our recent results of advances in X-ray mirror and detector.
Kimberly Slavis, Paul Dowkontt, Fred Duttweiller, John Epstein, Paul Hink, George Huszar, Philippe Leblanc, James Matteson, Robert Skelton, Edwin Stephan
Cadmium Zinc Telluride (CZT) is a room temperature semiconductor detector well suited for high energy x-ray astronomy. We have developed a CZT detector with 500 micron crossed strip readout and an advanced electrode design that greatly improves energy resolution. We conducted two balloon flights from Fort Sumner, NM, to study the cross strip detector and a standard planar detector both sensitive in the energy range of 20-350 keV. The flights utilized a total of seven shielding schemes: 3 passive, 2 active and 2 hybrid passive-active. In the active shielding modes, the anti- coincidence shield pulse heights were telemetered for each CZT event, allowing us to study the effect of the shield's energy threshold on the spectral shape and magnitude of the background. We are also developing an energy-dependent background rejection technique based on the charge collection properties of the CZT detector. This technique employs the depth of interaction, as inferred by the ratio of cathode to anode pulse height, to reject events inconsistent with incident source x-rays. The long duration of the May flight enabled us to study activation effects. We present result of the effectiveness of each of the shielding schemes on both detectors, the rejection power of depth of interaction technique on the crossed strip detector, inferred aperture background flux and the level of activation after 22 hours as float.
MARGIE will be a large-area, wide field-of-view, hard x- ray/gamma-ray imaging telescope capable of providing accurate positions for faint gamma-ray bursts in near-real- time and of performing a sensitive survey of both steady and transient cosmic sources. The instrument is designed to image faint bursts at the low-intensity end of the log N - log S distribution. MARGIE was recently selected by NASA for a mission-concept study for an Ultra Long Duration Balloon flight. We describe a program to develop an instrument based on the new detector technology of either cadmium zinc telluride room-temperature semiconductors or pixelated cesium iodide scintillators viewed by fast timing charge- coupled devices.
The FAR_XITE balloon payload concept contains 10 co-aligned, hard x-ray telescopes, each containing a set of nested multilayer mirror modules and an actively shielded CZT strip detector at each focal plane. The 500 micron strip pitch provides 26 arcsecond pixels at the 4m focal length of FAR_XITE. The active shielding and advanced CZT detector techniques reduce the background at float altitudes to a few times 10-4 counts/cm2 keV. We describe these advanced detectors and how they allow us to meet the scientific objectives of the FAR_XITE program.
Position-sensitive CZT detectors for research in astrophysics in the five - several hundred keV range are being developed by several groups. These are very promising for large area detector arrays in coded mask imagers and small-area focal plane detectors for focusing x-ray telescopes. We have developed detectors with crossed-strip readout and optimized strip widths and gaps to improve energy resolution. A 'steering electrode' is employed between the anode strips to improve charge collection. A model of charge drift in the detectors and charge induction on the electrodes has been developed to allow us to better understand these types of detectors and improve their design. The model presently accounts for the electric field within the detector, the charges' trajectories, mobility and trapping of holes and electrons, and charge induction on all electrodes including their time dependence. Additional effects are being added. The model is described and its predictions are compared with laboratory measurements. Results include (1) the dependence of anode, cathode and steering electrode signal on interaction depth, transverse position, electron and hole trapping, strip width and gap, and bias, (2) trajectories of charges for various anode and steering electrode bias voltages, (3) a method to improve energy resolution by sign depth of interaction information, and (4) an electrode geometry and bias optimized for the improved energy resolution. In general, the model provides good agreement with the measurements.
The FAR_XITE is a proposed Balloon Payload that consists of 10 nested mirror modules. The mirrors are coated with multilayers that allow FAR_XITE to reach 100 keV with better than 1 arcminute angular resolution. We describe the science objectives, optical design and specifications, and present out recent result of advances in X-ray mirror fabrication techniques.
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.
Kimberly Slavis, Paul Dowkontt, Fred Duttweiller, John Epstein, Paul Hink, George Huszar, Philippe Leblanc, James Matteson, Robert Skelton, Edwin Stephan
CdZnTe (CZT) is a room-temperature semiconductor well suited for high energy x-ray astronomy. Knowledge of its background properties is essential for optimizing CZT based instruments. To study its background, we flew two CZT detectors on dedicated high altitude balloon flights from Fort Sumner, NM, the first in October 1997 and the second in May 1998. The first detector is a 12 by 12 by 2 mm3 detector with orthogonal crossed strips and the second is a standard 12 by 12 by 2 mm 3 planar detector. The cross strip detector has 500 micron pitched electrodes plus anode 'steering electrodes' to improve the anode charge collection. The energy range for these flights was 20 to approximately 350 keV. We have found a preliminary background level in 20-40 keV of 8.6 by 10-3 cts/cm2-s-keV using passive 2 mm PbSnCu shielding and 6 by 10-4 cts/cm2-s-keV using active collimated schemes for the first position-sensitive CAT detector at balloon altitudes.
Coded mask imagers for future high energy x-ray astronomy missions will require detector planes with areas of hundreds to thousands of cm2 and position resolutions < 1 mm. Such detectors will enable coded mask imagers to discover and study thousands of high energy x-ray sources. The UCSD/WU/UCR/NOVA collaboration has been developing CZT detector systems with crossed-strip readout to meet these requirements. We report progress on a compact detector module with 41 cm2 area and 0.5 mm spatial resolution. The design includes the bias network and ASIC readout electronics, and allows modules to be combined in large area arrays with very high live-area factors. Results from laboratory and balloon flight tests are presented.
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.
Giselher Lichti, Volker Schoenfelder, Roland Diehl, Robert Georgii, Thomas Kirchner, Gilbert Vedrenne, Pierre Mandrou, Peter von Ballmoos, Pierre Jean, Francis Albernhe, Philippe Durouchoux, Bertrand Cordier, Nene Diallo, Filomeno Sanchez, Pierre Leleux, Patricia Caraveo, Bonnard Teegarden, James Matteson, Robert Lin, Gerald Skinner, Paul Connell
INTEGRAL is ESA's high-energy astrophysics mission to be launched into a high eccentric orbit early in the next decade. One of the two mission's main telescopes is the gamma-ray spectrometer SPI. This instrument features a compact array of 19 high-purity germanium detectors shielded by a massive anticoincidence system. A coded aperture of the HURA type modulates the astrophysical signal. We present the spectrometer system and its characteristics and discuss the choices that led to the present design. The instrument properties like imaging capability, energy resolution and sensitivity have been evaluated by extensive Monte-Carlo simulations. The expected performance for narrow-line spectroscopy is characterized by an energy resolution of approximately 1.6 keV at 1 MeV, an angular resolution of approximately 2 degrees within a totally coded field of view of approximately 15 degrees, and a sensitivity of (2 - 5) multiplied by 10-6 gamma/(cm2 s) for 4 multiplied by 106 s observation time in the nominal energy range from approximately 20 keV and approximately 8 MeV. With these characteristic features it will be possible for the first time to explore the gamma-ray sky in greater depth and detail than it was possible with previous gamma- ray telescopes like SIGMA, OSSE and COMPTEL. In particular the field of nuclear astrophysics will be addressed with an unprecedented combination of sensitivity and energy. Especially the high-energy resolution allows for the first time measuring gamma-ray line profiles. Such lines are emitted by the debris of nucleosynthesis processes, by the annihilation process near compact objects and by the nuclear interaction between cosmic rays and interstellar matter. Lines of all these processes have been measured so far, but, owing to the relatively poor energy resolution, details of the emission processes in the source regions could not be studied. With the high-resolution spectroscopy of SPI such detailed investigations will be possible opening a wealth of astrophysical investigations.
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.
CHIP -- the continuous hard x-ray imager for astrophysics) -- is a powerful hard x ray imaging spectroscopy mission, an order of magnitude more sensitive to hard x rays than XTE, comprised of the large uniform coverage imager (LUCY) for all-sky coverage and the deep extragalactic survey imager (DESI) for more sensitive pointed observations. Both instrument complements will utilize the room temperature semiconductor CdZnTe in mosaiced arrays of position sensitive devices in conjunction with coded masks. Each unit will provide imaging from 2 - 100 keV with approximately 1 keV energy resolution throughout the entire energy range and few arcminute intrinsic spatial resolution. LUCY will provide (1) a complete flux- limited sample of the hard x-ray contents of our galaxy as well as of the extragalactic sky, (2) alarms and precise positions for transient phenomena on all timescales from seconds to days, and (3) continuous spectral/temporal studies of these objects over a vast range of timescales from seconds to days, to months, to years. DESI will extend LUCY's coverage even deeper by viewing a small portion of the hard x-ray sky (0.01 sr) for background limited studies of individual objects considerably deeper than present or future hard x-ray instruments.
The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) is a proposed joint ESA/NASA/Russia gamma-ray astronomy mission which will provide both imaging and spectroscopy. It is currently at the final stages of an ESA phase-A study which it is hoped will lead to it being adopted during 1993 as the second 'medium-class' mission within ESA's Horizon 2000 plan. Launched in less than 10 years time it will be the successor to the current generation of gamma-ray spacecraft, NASA's Compton Observatory (GRO) and the Soviet- French Granat/Sigma mission. The baseline is to have two main instruments covering the photon energy range 50 keV to 10 MeV, one concentrating on high-resolution spectroscopy, the other emphasizing imaging. In addition there will be two monitors--an X-ray monitor which will extend the photon energy range continuously covered down to a few keV, and an Optical Transient Camera which will search for optical emission from gamma-ray bursts.
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