N. R. Hilton

Device Simulation of a Unipolar Gamma-Ray Detector

The pulse height spectra from a new kind of unipolar gamma-ray detectors were predicted using a new three-dimensional simulation program developed at Sandia National Laboratories. The detectors were fabricated at Sandia and RMD Inc., and tested at Sandia. They were fabricated from Cd{sub 1{minus}x}Zn{sub x}Te crystals and they were electron-transport-only devices. For the simulation, a successive overrelaxation method was used to determine the three-dimensional internal electric field within a detector, and to find the weighting potentials for the anode and the cathode. Uniform irradiation and ionization from a {sup 137}Cs source was assumed, and the charge transport and the signal induction within the detector were numerically computed using the appropriate materials and design parameters. The simulation gave excellent agreement with experimental pulse height spectra, and it demonstrated the power of such a simulation to correlate the materials parameters and the device design to the actual detector performance.

Semiconductor material requirements for orthogonal strip detectors

We discuss the physical and electrical properties of semiconducting crystals necessary for use in orthogonal strip detectors. We also compute what constraints the properties of existing CdZnTe crystals place on the design of orthogonal strip detectors. First we consider the constraints imposed by uniform material that has limited charge carrier transport properties and resistivity. Next, we consider what effects spatially varying electrical properties (non-uniformities) have on the performance of detectors. Finally, we discuss the properties of CZT crystals available today that we have measured in our laboratory, and what ramifications these measured properties have on the design and construction of orthogonal strip detectors.

Low-cost cadmium zinc telluride radiation detectors based on electron-transport-only designs

The goal of this project was to utilize a novel device design to build a compact, high resolution, room temperature operated semiconductor gamma ray sensor. This sensor was constructed from a cadmium zinc telluride (CZT) crystal. It was able to both detect total radiation intensity and perform spectroscopy on the detected radiation. CZT detectors produced today have excellent electron charge carrier collection, but suffer from poor hole collection. For conventional gamma-ray spectrometers, both the electrons and holes must be collected with high efficiency to preserve energy resolution. The requirement to collect the hole carriers, which have relatively low lifetimes, limits the efficiency and performance of existing experimental devices. By implementing novel device designs such that the devices rely only on the electron signal for energy information, the sensitivity of the sensors for detecting radiation can be increased substantially. In this report the authors describe a project to develop a new type of electron-only CZT detector. They report on their successful efforts to design, implement and test these new radiation detectors. In addition to the design and construction of the sensors the authors also report, in considerable detail, on the electrical characteristics of the CZT crystals used to make their detectors.

Multi-parameter high-resolution spatial maps of a CdZnTe radiation detector array

Resistivity results from a 48/spl times/48 pixelated CdZnTe (CZT) radiation detector array are presented alongside X-ray topography and detector mapping with a collimated gamma-ray beam. By using a variety of measurements performed on the same sample and registering each data set relative to the others, the spatial dependence of relationships between them was examined. The local correlations between resistivity and one measure of detector performance were strongly influenced by the positions of grain boundaries and other gross crystal defects in the sample. These measurements highlight the need for material studies of spatially heterogeneous CZT to record position information along with the parameters under study.

Gamma-ray imaging and spectroscopy system using room-temperature semiconductor detector elements

We report on the design, construction, and testing of a gamma-ray imaging system with spectroscopic capabilities. The imaging system consists of an orthogonal strip detector made from either HgI2 or CdZnTe crystals. The detectors utilize an 8×8 orthogonal strip configuration with 64 effective pixels. Both HgI2 or CdZnTe detectors are 1 cm2 devices with a strip pitch of approximately 1.2 mm (producing pixels of 1.2 mm × 1.2 mm). The readout electronics consist of parallel channels of preamplifier, shaping amplifier, discriminators, and peak sensing ADC. The preamplifiers are configured in hybrid technology, and the rest of the electronics are implemented in NIM and CAMAC with control via a Power Macintosh computer. The software used to readout the instrument is capable of performing intensity measurements as well as spectroscopy on all 64 pixels of the device. We report on the performance of the system imaging gamma-rays in the 20–500 keV energy range and using a pin-hole collimator to form the image.

Orthogonal strip gamma-ray imaging system for use with HgI2 and Cd1-xZnxTe detectors

We have designed and constructed an orthogonal strip imaging system for use with room temperature semiconductor strip detectors. The system has been tested with both HgI2 and Cd1-xZnxTe (CZT) detector elements. Our first system consists of complete readout electronics and software for spectroscopy and imaging with 8 by 8 orthogonal strip detectors. The readout electronics consist of 16 channels of hybrid charge sensitive preamplifiers, and 16 channels of parallel discriminators, shaping amplifiers, and a 16 channel ADC implemented in CAMAC and NIM. The software used to readout the instrument is capable of performing intensity measurements as well as spectroscopy on all 64 pixels of the device. In this paper we describe measurements to determine the factors limiting the performance of this system.