A. E. Bolotnikov

Effect of Te precipitates on the performance of CdZnTe detectors

Measurements using the National Synchrotron Light Source provided a detailed comparisons of the microscale detector response and infrared microscopy images for CdZnTe Frisch-ring x-ray and gamma detectors. Analysis of the data showed conclusively that local deteriorations of the electron charge collection and x-ray device response fully correlate with the presence of Te precipitates as seen in the IR images. Effects of the surface processing conditions on the detector performance were also clearly observed.

Single-charge-carrier-type sensing with an insulated Frisch ring CdZnTe semiconductor radiation detector

Performance optimization of an insulated Frisch ring design was investigated for a 3×3×6 mm CdZnTe planar semiconductor detector. The Frisch ring was composed of copper and was insulated from the detector surface with Teflon. Optimization variables included the Frisch ring length and the bias voltage. Optimized overall device performance was found using a 5 mm long Frisch ring extending from the cathode toward the anode, leaving a 1 mm separation between the Frisch ring and the anode. The best energy resolution observed was 1.7% full width at half maximum at 662 keV with the ring extending 4 mm from the cathode toward the anode.

Characterization of large cadmium zinc telluride crystals grown by traveling heater method

The focus of this paper is to evaluate thick, 20×20×10 and 10×10×10mm3, cadmium zinc telluride (CZT), Cd0.9Zn0.1Te, crystals grown using the traveling heater method (THM). The phenomenal spectral performance and small size and low concentration of Te inclusions/precipitates of these crystals indicate that the THM is suitable for the mass production of CZT radiation detectors that can be used in a variety of applications. Our result also proves that with careful material selection using IR and high-quality fabrication processes, the theoretical energy resolution limit can be achieved.

Characterization of Traveling Heater Method (THM) Grown

High-performance semi-insulating single crystals of n-type (CZT) were grown using the traveling heater method (THM). X-ray and -ray detector configurations fabricated from this material have a room-temperature mean energy resolution of 4.3% FWHM for a source (122 keV) and uniform pixel-to-pixel response on monolithic 20205 pixellated detectors. Energy resolution of 1% FWHM for (662keV) has been measured on virtual Frisch-grid 4411 devices useful for homeland security applications. Additional characterization techniques including mobility-lifetime measurements, infrared microscopy, X-ray topography, and OPTICAL Deep Level Transient Spectroscopy (ODLTS) have demonstrated the superior quality of this THM CZT.

{\hbox{Cd}}_{0.9}{\hbox{Zn}}_{0.1}{\hbox{Te}}

We studied the effects of small, <20 mum, Te inclusions on the energy resolution of CdZnTe gamma-ray detectors using a highly collimated X-ray beam and gamma-rays, and modeled them via a simplified geometrical approach. Previous reports demonstrated that Te inclusions of about a few microns in diameter degraded the charge-transport properties and uniformity of CdZnTe detectors. The goal of this work was to understand the extent to which randomly distributed Te-rich inclusions affect the energy resolution of CZT detectors, and to define new steps to overcome their deleterious effects. We used a phenomenological model, which depends on several adjustable parameters, to reproduce the experimentally measured effects of inclusions on energy resolution. We also were able to bound the materials-related problem and predict the enhancement in performance expected by reducing the size and number of Te inclusions within the crystals.

Crystals

The performance characteristics of Frisch-ring CdZnTe (CZT) detectors are described and compared with other types of CZT devices. The Frisch-ring detector is a bar-shaped CZT crystal with a geometrical aspect ratio of /spl sim/1:2. The side surfaces of the detector are coated with an insulating layer followed by a metal layer deposited directly upon the insulator. The simple design operates as a single-carrier device. Despite the simplicity of this approach, its performance depends on many factors that are still not fully understood. We describe results of testing several detectors fabricated from CZT material produced by different vendors and compare the results with numerical simulations of these devices.

Performance-limiting Defects in CdZnTe Detectors

The ideal operation of CdZnTe devices entails having a uniformly distributed internal electric field. Such uniformity especially is critical for thick long-drift-length detectors, such as large-volume CPG and 3-D multi-pixel devices. Using a high-spatial resolution X-ray mapping technique, we investigated the distribution of the electric field in real devices. Our measurements demonstrate that in thin detectors, < 5 mm, the electric field-lines tend to bend away from the side surfaces (i.e., a focusing effect). In thick detectors, > 1 cm, with a large aspect ratio (thickness-to-width ratio), we observed two effects: the electric field lines bending away from or towards the side surfaces, which we called, respectively, the focusing field-line distribution and the defocusing field-line distribution. In addition to these large-scale variations, the field-line distributions were locally perturbed by the presence of extended defects and residual strains existing inside the crystals. We present our data clearly demonstrating the non-uniformity of the internal electric field.

Performance characteristics of Frisch-ring CdZnTe detectors

In the past few years, significant progress has been achieved in the development of room-temperature semiconductor detectors, particularly those based on CdZnTe (CZT) crystals. Several types of electron-transport-only detectors have been developed: pixel, coplanar-grid, cross-strip, drift-strip, orthogonal coplanar strip, and virtual Frisch grid, many of which are now commercially available. Despite all these varieties in the detector designs, they have many common features and problems. This review summarizes the common detector design constraints and related factors limiting performance of CZT detectors: bulk and surface leakage currents, surface effects, properties of Schottky contacts and surface interfacial layers, charge sharing and loss in multielectrode devices, charge transport nonuniformities, and fluctuations in the pulse height for long-drift-length devices. We also describe unique capabilities at Brookhaven National Laboratory, Upton, NY, for CZT device characterization and recent progress utilizing these tools.

Internal Electric-Field-Lines Distribution in CdZnTe Detectors Measured Using X-Ray Mapping

One of the most challenging and potentially rewarding research applications of PET is imaging of the mouse brain. Although very high spatial resolution is required (< ~1 mm), there is a much wider variety of transgenic models in mouse compared to the rat. The solid state material CdZnTe (CZT) has long held promise for high resolution PET. Compared to scintillators, its limitations in time resolution and sensitivity can in some ways be compensated by its extremely high spatial and energy resolution, its compact geometry, and by sophisticated data processing techniques. Using such techniques, a time resolution of ~10 ns has been demonstrated for ~1 cm thick CZT pixel detectors, and this may be sufficient for mouse studies. The depth-of-interaction capability and high energy resolution can improve sensitivity by allowing detectors to be placed very close to the subject and by enabling both reconstruction of detector-scattered events and rejection of object-scattered events. A full-ring prototype scanner has been designed to demonstrate feasibility of the concept, consisting of 6 CZT pixel detectors in a novel geometry. The design of the detector, front-end electronics components, and data acquisition are presented, along with performance characterization of the custom-manufactured CZT detectors.

Factors limiting the performance of CdZnTe detectors

Pulse shape analysis is proved to be a powerful tool to characterize the performance of CdZnTe devices and understand their operating principles. It allows one to investigate the device configurations, electron transport properties, effects governing charge collection, electric-field distributions, signal charge formation, etc. This work describes an application of different techniques based on the pulse shape measurements to characterize pixel, coplanar-grid, and virtual Frisch-grid devices and understand the electronic properties of CZT material provided by different vendors. We report new results that may explain the performance limits of these devices.