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A 3 cm3 multichannel gamma spectrometer for DOE applications is under development by Digirad Corporation. The device is based on a position sensitive detector packaged in a compact multi-chip module (MCM) with integrated readout circuitry. The modular, multichannel design will enable identification and quantitative analysis of radionuclides in extended sources, or sources containing low levels of activity. The MCM approach has the advantages that the modules are designed for imaging applications, and the sensitivity can be arbitrarily increased by increasing the number of pixels, i.e. adding modules to the instrument. For a high sensitivity probe, the outputs for each pixel can be corrected for gain and offset variations, and summed digitally. Single pixel results obtained with discrete low noise readout indicate energy resolution of 3 keV can be approached with currently available CdZnTe. The energy resolution demonstrated to date with MCMs for 511 keV gamma rays is 10 keV.

In order to gain insight on a proposed CZT imaging device with orthogonal coplanar anodes currently under development, measurements have been made on an available commercial device with pixels and control electrodes. Charge signals induced on both the pixel and control electrodes by alphas incident at two positions on the cathode side are measured and compared with model predictions. Signals produced by c-rays are also discussed. Measurements and simulations are found to be in reasonably good agreement. It is concluded that the proposed anode geometry might exhibit excellent energy resolution and good position resolution in two dimensions as well as some position information along the depth dimension. ( 1999 Elsevier Science B.V. All rights reserved.

We report the development of a CdZnTe gamma ray imager whose advantages over the conventional Anger camera include: 1) improved contrast and spatial resolution resulting from superior energy resolution and unambiguous position determination and 2) the small size and weight and high reliability inherent in an all-solid-state construction. It provides high energy resolution and peak efficiency by incorporating Digirad's new SpectrumPlus/sup TM/ detector technology. The new imager employs a modular design, in which each 1"/spl times/1" module incorporates a monolithic 64-element CdZnTe detector array and ASIC-based circuitry that provides signal conditioning for every channel, identification of valid events and addressing functions. Imagers with a wide range of sizes and shapes can be assembled by timing modules together on a specially designed signal routing board.

Room-temperature semiconductor gamma-ray detectors have exhibited charge loss due to incomplete collection of hole charge, resulting in spectra with significant low-energy tailing, and good energy resolution has typically been achieved only with very small detectors. Researchers at Digirad Corporation have developed an innovative method for eliminating the effects of hole trapping in radiation detectors made from compound semiconductors, such as CdTe or CdZnTe. The technique involves placing one or more additional electrodes on or near the surface of the detector, biased for optimum charge collection at the signal electrode. The additional electrode(s) focuses signal charge to the signal-collecting electrode and shields that electrode from charge trapped in the detector volume. This results in excellent photopeak efficiency because essentially all of the absorbed photons are counted in the single spectral peak. Working devices have been manufactured in a variety of configurations including imaging arrays. The configuration of the new device and present understanding of the basic principles of operation are described.

A new electrode configuration comprising a cathode, a small anode and a “control electrode” resulted in energy resolutions as low as 0.72% and photopeak efficiencies near unity at 511 keV with CdZnTe room temperature nuclear detectors. The control electrode transiently accepts a large fraction of charge that would otherwise be induced on the anode by mobile charges in transit and, in effect, enhances the near-field effect. Control electrode configurations are adaptable to both single-element detectors and multi-element arrays, and are compatible with existing semiconductor detector manufacturing technology.

We report on the construction and initial testing of a 48 × 48 CdZnTe array with 125 μm pixel spacing and multiplexer readout. Large portions of the array function well but there was a loss of pixels near one corner of the array due to non-interconnecting indium bumps. This problem is readily correctable. Excellent single-pixel spectra were obtained with a 99mTc source using an adjustment technique that accounts for energy deposited in neighboring pixels. A point-spread function (PSF) taken at 140 keV yielded a spatial resolution of 230 μm, much better than required for nuclear medicine applications. No problems were found that are not readily correctable or of much less significance for CdZnTe arrays having larger pixel spacing. We are now constructing 64 × 64 CdZnTe arrays with 380 μm pixel spacing for use in an ultra-high resolution brain SPECT imaging system.

We report performance measurements of a sub-millimeter resolution CdZnTe strip detector developed as a prototype for astronomical instruments operating with good efficiency in the 30-300 keV photon energy range. The prototype is a 1.4 mm thick, 64/spl times/64 contact stripe CdZnTe array of 0.375 mm pitch in both dimensions. Pulse height spectra were recorded in orthogonal-stripe coincidence mode which demonstrate room-temperature energy resolution 5:1. Good response is also demonstrated at higher energies using a coplanar grid readout configuration. Spatial resolution capabilities finer than the stripe pitch are demonstrated. We present the image of a /sup 133/Ba source viewed through a collimator slit produced by a 4/spl times/4 stripe detector segment. Charge signals from electron and hole collecting contacts are also discussed.

This paper reports the first high spatial resolution room-temperature semiconductor gamma detector arrays. Results of wafer-scale fabrication of 48 by 48 element CdZnTe (CZT) arrays with 125 μm element pitch are presented. Performance characteristics are presented, including pulse height spectra using charge-sensitive preamplifier readout from witness test pixels, which show excellent energy resolution. High bulk and surface resistivities have been verified for these arrays, thus good energy resolution with multiplexer readout is expected.

Results of recent efforts in the growth of Cd1−xZnxTe crystals by a high pressure Bridgman (HPB) method and their use in gamma- and X-ray detector applications are presented. Evidence for crystals of relatively low defect content include etch pit densities of ≤ 104cm−2, double crystal rocking curve linewidths of 10–15″ and sharp, bright emission lines with excitonic features in low temperature photoluminescence measurements. Resistivities in excess of 1011 ohm cm are achieved without impurity doping. The resulting low leakage currents lead to good energy resolution

The results of an effort to improve the performance at CdTe detectors by addressing starting element purity and crystallinity are described. Structural perfection was improved by alloying with ZnTe to form crystals of Cd/sub 1-x/Zn/sub x/Te. Crystals were grown by a high-pressure Bridgman method. Evidence for significant enhancements of the mu tau products resulting from these efforts is presented. Features of Cd/sub 0.8/Zn/sub 0.2/Te detectors include: energy resolutions at 122 keV >

Experimental parameters of Cd,,Z%Te detectors relevant to nuclear medical imaging are presented and discussed. An image of a thyroid phantom was acquired with an early-stage prototype Cd,JqTe detector array imager using conditions to emulate observations in a human patient and a nuclear medicine imaging parallel-hole collimator. The phantom was filled with a Tc-99m solution. Results were compared with an image obtained from a modern commercial Anger camera under the same conditions. The Cd,-J%Te imager was found to exhibit resolution and contrast at least equal to those of the Anger camera.

We report performance measurements of a sub-millimeter resolution CdZnTe strip detector developed as a prototype for astronomical instruments. The prototype is a 1.4 mm thick, 64 × 64 stripe CdZnTe array with 0.375 mm pitch in both dimensions. Pulse height spectra in orthogonal stripe coincidence mode were recorded at several energies. The room-temperature energy resolutions is 5:1. We also demonstrate spatial resolution capabilities finer than the stripe pitch.

Replacing cryogenically cooled germanium (Ge) with room-temperature operable cadmium-zinc-telluride (CZT) semiconducting detectors as the first detector of an electronically collimated SPECT system would have certain practical advantages. To determine the feasibility of this approach, the authors have performed a theoretical study comparing the resolution and sensitivity of a Ge based system to that of a CZT based system. Also, a preliminary experimental study using a 5 mm/spl times/5 mm/spl times/1.5 mm thick CZT detector as the first detector and an uncollimated scintillation camera as the second detector was conducted to measure the point spread function. The results indicate that the efficiency of a CZT based system would be a factor of 2.5 lower than a comparable Ge based system at 140 keV and the spatial resolution would also be approximately a factor of two worse. However, at higher energies, the difference between CZT and Ge would decrease, and at 662 keV, for example, the CZT and the Ge systems could have nearly equal sensitivity and resolution. >

We report results of a study undertaken to experimentally determine the achievable spatial resolution of monolithic cadmium zinc telluride (CZT) arrays. It was demonstrated that CZT arrays can produce direct electronic X-ray images with fine spatial resolution and high detection efficiency. For low X-ray energies useful for imaging soft tissue such as the breast, 50 micron resolution with virtually 100% detection efficiency were demonstrated, while for higher energy X-rays where fluorescence dominates the signal spreading, 100 micron resolution was demonstrated. Due to electrostatic effects of small anodes, improved charge deficit tailing and pulse rise time results are observed for strips as well as pixels.

Researchers at Digirad Corporation have developed an innovative method for eliminating the effects of hole trapping in radiation detectors made from compound semiconductors such as CdTe or CdZnTe. The technique involves no additional electronics. Working devices have been manufactured in a variety of configurations including imaging arrays. This paper presents results from some simple structures.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Many future space missions will use cadmium zinc telluride (CdZnTe) gamma-ray detectors because their operation at room temperature makes compact, lightweight detector systems possible. Even though instruments for space using CdZnTe detectors have already been built, the effect of the high- energy particle space environment on these detectors has not been measured. To determine the effect of energetic charged particles on these detectors, we have bombarded several CdZnTe detectors with 199 MeV protons at the Indiana University Cyclotron Facility. Planar detectors of area 1 cm^2 and thickness 2-3 mm from both eV products and Digirad were irradiated, along with a 2 multiplied by 2 array of proprietary design from Digirad. Using standard gamma-ray sources, the response of the detectors was measured before and after bombardment in steps up to fluences of 5 multiplied by 10^9 p cm^(-2). Significant effects from the proton irradiation were observed in the gamma-ray spectra. In particular, the peak positions of the lines in the spectrum were shifted downward proportional to the fluence. The explanation is almost certainly the production of electron traps by the high energy proton interactions, resulting in a decrease of the mobility-lifetime (µτ) ) product of the electrons. Calculations were made to model the effect of a decrease in electron trapping length on the spectrum.

We report progress in ongoing measurements of the performance of a sub-millimeter pitch CdZnTe strip detector developed as a prototype for astronomical instruments. Strip detectors can be used to provide two-dimensional position resolution with fewer electronic channels than pixellated arrays. Arrays of this type are under development for the position-sensitive image plane detector for a coded-aperture telescope operating in the hard x-ray range of 20-200 keV. The prototype is a 1.5 mm thick, 64 by 64 orthogonal stripe CdZnTe detector of 0.375 mm pitch in both dimensions, approximately one square inch of sensitive area. In addition to energy and spatial resolution capabilities, as reported last year, we demonstrate the imaging capabilities and discuss uniformity of response across an 8 by 8 stripe, 64 'pixel', segment of detector. A technique for determination of the depth of photon interaction is discussed and initial results related to depth determination are presented. Issues related to the design and development of readout electronics, the packaging and production of strip detectors and the production of compact strip detector modules, including detector and readout electronics, are also discussed.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report the first performance measurements of a sub-millimeter CdZnTe strip detector developed as a prototype for space-borne astronomical instruments. Strip detector arrays can be used to provide two-dimensional position resolution with fewer electronic channels than pixellated arrays. Arrays of this type and other candidate technologies are under investigation for the position-sensitive backplane detector for a coded-aperture telescope operating in the range of 30 - 300 keV. The prototype is a 1.4 mm thick, 64 multiplied by 64 stripe CdZnTe array of 0.375 mm pitch in both dimensions, approximately one square inch of sensitive area. Pulse height spectra in both single and orthogonal stripe coincidence mode were recorded at several energies. The results are compared to slab- and pixel-geometry detector spectra. The room-temperature energy resolution is less than 10 keV (FWHM) for 122 keV photons with a peak-to-valley ratio greater than 5:1. The response to photons with energies up to 662 keV appears to be considerably improved relative to that of previously reported slab and pixel detectors. We also show that strip detectors can yield spatial and energy resolutions similar to those of pixellated arrays with the same dimensions. Electrostatic effects on the pulse heights, read-out circuit complexity, and issues related to design of space borne instruments are also discussed.

Preliminary investigations were carried out to evaluate the resolution of Cd1-xZnxTe detectors at temperatures achievable with commercially available, low power Peltier refrigerators. Detectors were in the form of cubes, 2 mm on a side. They were tested using a preamplifier with optical feedback. The input FET was cooled along with the detectors. Resolutions of 409 eV and 326 eV were observed at the 5.9 keV line of Fe-55, at temperatures of -10 degree(s)C and -20 degree(s)C, respectively. Results indicate that a straightforward extension of this work will lead to resolutions well below 200 eV.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A prototype portable gamma ray camera using 32 X 32 channels was developed. An experimental 3 X 3 sub-array of 5 mm X 5 mm CZT detectors was fabricated for use in system checkout and to investigate the applicability of CZT imaging arrays to nuclear medical imaging. Experiments were carried out to make a direct comparison of the imaging capabilities of the CZT sub-array with a state-of-the-art Anger camera. In a linespread study using a Tc-99m source embedded in a tissue equivalent absorber, contrasts of 9.5 for the CZT array and 3.4 for the Anger camera were observed. In a dynamic imaging experiment, the CZT imager appeared to have comparable resolution to and be somewhat more regular than the Anger camera.

A discussion of recent results in the preparation of Cd 1-x Zn x Te crystals by a high pressure Bridgman (HPB) method and use of the crystals for gamma- and x-ray detectors is presented. Resistivities in excess of 10 11 ohm-cm are achieved in Cd 1-x Zn x Te without impurity doping. The consequential low detector leakage currents lead to excellent energy resolution, 8.4% (FWHM) at 30 keV, for example. Useful energy spectroscopy can be performed at temperatures up to 100 degree(s)C. The dependence of leakage current on temperature from -40 degree(s)C to 100 degree(s)C implies a Fermi level at mid-gap for x equals 0.2. HPB grown Cd 1-x Zn x Te crystals exhibit relatively low defect content, as evidenced by etch-pit-densities 4 cm -2 , double-crystal-rocking-curve linewidths of 10 - 15 arc-seconds and sharp, bright emission lines, with excitonic features, in low temperature photoluminescence measurements. Results of flash x ray experiments indicate high current sensitivity, low leakage current, and good temporal response. Preliminary results of Cd 1-x Zn x Te imaging detector array studies are discussed.