Title: Non-uniformity Effects in CdZnTe Coplanar-Grid Detectors

The spectral performance of coplanar grid detectors using currently available CdZnTe materials is examined theoretically and experimentally. Calculated spectral response based on the typical carrier mobility lifetime products of current CdZnTe materials show that energy resolution close to the charge-statistics limit can be achieved. Charge transport nonuniformity, which may limit the spectral performance of present detectors, is studied using alpha particle scanning.

CdZnTe is a relatively new semiconductor that is being developed for use as a nuclear radiation detector. Its highly atomic numbers provide good detection efficiency for gamma rays, while its wide bandgap allows operation at room temperature. The biggest drawback of this material is its poor charge transport characteristics, especially for the holes. The coplanar-grid charge-sensing technique has been developed over the past several years as a method to circumvent this problem so that detectors with good energy resolution can be realized. The technique is based on the use of two coplanar, interdigitated anodes (grids) to sense the collection ofmore » carriers in a detector. During detector operation, a voltage is applied between the two grids so that electrons are collected only to one of the grids. The signals induced on the collecting and noncollecting grids as a result of charge collection are subtracted to give a net output signal. By adjusting the relative gain of the two signals before subtraction, the output signal can be made insensitive to the effects of both hole trapping and electron trapping. The coplanar-grid technique has been successfully applied to CdZnTe detectors, and good energy resolution ({approximately}2% full-width at half-maximum for 662-keV gamma rays) combined with high efficiency has been obtained for detectors with volumes up to 2.2 cm{sup 3}. A single-electrode readout method has also been developed in which only the collecting anode signal is processed and signal subtraction is not used. In this case, the optimization of detector response is accomplished by adjusting the relative areas of the two anodes. An attractive feature of the coplanar-grid technique, whether employing signal subtraction or single-electrode readout, is that only simple readout electronics is needed. This enables the fabrication of small, low-power detector systems that are particularly well suited for field use. The authors are currently developing coplanar-grid CdZnTe gamma-ray detectors for use in environmental remediation and nuclear safe-guards applications.« less

Device simulations of (1) the laterally contacted-unipolar-nuclear detector (LUND), (2) the SpectrumPlus, (3) and the coplanar grid made of Cd{sub 0.9}Zn{sub 0.1}Te (CZT) were performed for {sup 137}Cs irradiation by 662.15 keV gamma-rays. Realistic and controlled simulations of the gamma-ray interactions with the CZT material were done using the MCNP4B2 Monte Carlo program, and the detector responses were simulated using the Sandia three-dimensional multi-electrode simulation program (SandTMSP). The simulations were done for the best and the worst expected carrier mobilities and lifetimes of currently commercially available CZT materials for radiation detector applications. For the simulated unipolar devices, the active devicemore » volumes were relatively large and the energy resolutions were fairly good, but these performance characteristics were found to be very sensitive to the materials properties. The internal electric fields, the weighting potentials, and the charge induced efficiency maps were calculated to give insights into the operation of these devices.« less

The coplanar-grid (CPG) and other electron only detection techniques have made possible the use of CdZnTe-based detectors for gamma-ray spectroscopy when high efficiency, good energy resolution, and near room temperature operation are required. Despite the demonstrated potential of the technologies, widespread use remains hampered in part by the limited availability of the highly uniform CdZnTe material required for high-resolution spectroscopy. However, it has been recently shown that mild cooling of CdZnTe CPG detectors can result in a significant improvement in the energy resolution of the detectors thereby allowing a wider range of material to be used for high-resolution applications. Inmore » this paper, we show that improved spectroscopic performance can consistently be achieved through a combination of detector cooling and increased detector bias. Energy resolutions of about 1 % FWHM at 662 keV for detector volumes up to 2.3 cm{sup 3} have been obtained at -20 C. With the electronic noise subtracted, this amounts to an intrinsic resolution of 0.76 %. We also show that further cooling of the detectors to -30 C leads to field polarization and a loss of spectroscopic performance.« less