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.

Continued improvements in the manufacturing of Cd1-xZnxTe (CZT) material have resulted in a practical thermoelectrically cooled X-ray and gamma-ray detector of very high energy resolution. A high resolution spectroscopy system was used to measure the Fano factor in CZT at temperatures down to −40°C. The best resolution of the 5.9 keV 55Fe peak was measured to be 188 eV FWHM, while the best resolution of the 5.9 keV 241Am peak was measured to be 482 eV FWHM. The minimum measured Fano factor was 0.082, with several measurements yielding a value of 0.089±0.005. With a resolution of 4.2 keV FWHM for the 662 keV peak of 137Cs, these detectors demonstrate excellent performance in detecting X-rays and gamma rays.

Semiconductor multiple-electrode detectors have been developed for the purpose of reducing effects of hole trapping in room-temperature radiation detectors.1,2 Some reported geometries maintain a nearly-uniform electric field inside the detector, but others generate an electric field that is very non-uniform and highly-concentrated at the anode. This paper reports the results of mapping such a detector (having a non-uniform electric field) with a finely collimated gamma-ray beam to determine the detector response as a function of position.

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.

Results of a program to improve the performance of Cd1-xZnxTe detectors by adjusting growth parameters to achieve low-strain, high purity low defect crystals, investigating surface effect phenomena and contacting methods, and establishing reproducible manufacturing methods are reviewed and discussed. Processing and fabrication methods were developed which are applicable throughout the composition range. Energy spectra for room temperature Cd1-xZnxTe detectors exhibit resolutions (FWHM) from 2.16 keV at 14 keV to 6.9 keV at 122 keV. An energy resolution of 910 eV at 5.9 keV was achieved at −25 C. Stable ohmic contacts and gamma ray detection for ZnTe are reported for the first time. Applications of Cd1-xZnxTe to nuclear medicine and X-ray fluorescence are discussed. New gamma ray imagers using Cd1-xZnxTe detector arrays are described, and imaging data for a 32 × 32 monolithic array of 1 mm2 elements on a 42mm × 42mm substrate are presented.

Cd1−xZnxTe room temperature semiconductor detectors offer improved measurement capability, stability and lifetime over currently employed devices such as CdTe and HgI2 detectors. These improved features along with the compactness, durability and other properties inherent in solid-state devices often make it the correct decision to use Cd1−xZnxTe in place of detectors such as NaI(Tl) scintillators and proportional counters, and provide the basis for new approaches to instrumentation, including the use of imaging arrays and current-mode operation.

Charge-carrier mobilities have been measured for the first time in Cd0.8Zn0.2Te single crystals using time-of-flight measurements of charge carriers produced by short (10 ns) light pulses from a frequency-doubled Nd:YAG laser (λ=532 nm). The electron mobility displayed a T−1.1 dependence on the absolute temperature T in the range 200–320 K, with a room-temperature mobility of 1350 cm2/V s. The hole mobility displayed a T−2.0 dependence in the same temperature range, with a room-temperature mobility of 120 cm2/V s. Cd0.8Zn0.2Te appears to be a very favorable material for a room-temperature electronic nuclear radiation detector.

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.

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 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.

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.

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, <6% at 59.5 keV for example. The dependence of leakage current on temperature from 233 K to 373 K implies a Fermi level at mid-gap for x=0.2. The results of flash X-ray experiments indicate that the high current sensitivity, low leakage current and good temporal response of Cd1−xZnxTe detectors make them attractive candidates for applications involving short pulses at high dose rates.