M. Petryk

Drift Time Variations in CdZnTe Detectors Measured With Alpha Particles and Gamma Rays: Their Correlation With Detector Response

Homogeneity of properties related to material crystallinity is a critical parameter for achieving high-performance CdZnTe (CZT) radiation detectors. Unfortunately, this requirement is not always satisfied in todays commercial CZT material due to high concentrations of extended defects, in particular subgrain boundaries, which are believed to be part of the causes hampering the energy resolution and efficiency of CZT detectors. In the past, the effects of subgrain boundaries have been studied in Si, Ge and other semiconductors. It was demonstrated that subgrain boundaries tend to accumulate secondary phases and impurities causing inhomogeneous distributions of trapping centers. It was also demonstrated that subgrain boundaries result in local perturbations of the electric field, which affect the carrier transport and other properties of semiconductor devices. The subgrain boundaries in CZT material likely behave in a similar way, which makes them responsible for variations in the electron drift time and carrier trapping in CZT detectors. In this work, we employed the transient current technique to measure variations in the electron drift time and related the variations to the device performances and subgrain boundaries, whose presence in the crystals were confirmed with white beam X-ray diffraction topography and infrared transmission microscopy.

Comparison of Analog and Digital Readout Electronics on the Processing of Charge-Sharing Events in Pixelated CdZnTe Detectors

We present our results from testing 15×15×10 mm3 CdZnTe pixelated detectors using a readout system based on the H3D ASIC. Data obtained with an uncollimated 137Cs source helped reveal details of the operational principle of such devices, and how the pulse-processing electronics may influence their performance. The responses of individual pixels were compared by using two different pulse processing approaches based on the analog shaping (H3D ASIC) and fitting of the waveforms as read out from hybrid preamplifiers.

Design Considerations and Testing of Virtual Frisch-Grid CdZnTe Detector Arrays Using the H3D ASIC

We discussed the design implementation and results from testing 2 × 2-, 3 × 3-, and 2 × 4-arrays of 6×6 ×15 mm3 CdZnTe virtual Frisch-grid detectors. In these measurements we employed a data acquisition system based on the H3D ASIC developed by BNLs Instrumentation Division in collaboration with the University of Michigan for 3D position-sensitive detectors. We used CZT crystals with a range of performance attributes to evaluate practical array configurations and detector-assembling procedures. The detector ratings were assigned based on the pulse-height spectra and correlated with data from X-ray diffraction topography measurements and X-ray response mapping obtained at BNLs National Synchrotron Light Source. The results helped us to better understand the performance limits of these detectors, and to identify future improvements in the arrays design and requirements for the new readout ASIC.

Arrays of Position-Sensitive Virtual Frisch-Grid CdZnTe Detectors: Results From a

Position-sensitive virtual Frisch-grid (VFG) CdZnTe (CZT) detectors offer a unique capability for correcting the response nonuniformities caused by crystal defects. This allowed us to achieve high energy resolution, while using typical-grade commercial CZT crystals with relaxed requirements to their quality, thus reducing the overall cost of detectors. Another advantage of the VFG detectors is that they can be integrated into arrays and used in small compact hand-held instruments or large-area gamma cameras that will enhance detection capability for many practical applications, including nonproliferation, medical imaging, and gamma-ray astronomy. Here, we present the results from testing small array prototypes coupled with front-end application-specified integrated circuit. Each detector in the array is furnished with 5-mm-wide charge-sensing pads placed near the anode. The pads signals are converted into XY coordinates, which combined with the cathode signals (for Z coordinates) provide 3-D position information of all interaction points. The basic array consists of a number of detectors grouped into

4\times 4

2\times 2

Array Prototype

subarrays, each having a common cathode made by connecting together the cathodes of the individual detectors. These features can significantly improve the performance of detectors while using typical-grade low-cost CZT crystals to reduce the overall cost of the proposed instrument.

New Virtual Frisch-Grid CdZnTe Detector Design With Sub-Millimeter Spatial Resolution

We evaluated the performance of a position-sensitive virtual Frisch-grid (VFG) CdZnTe detector, 6 mm ×6 mm ×15 mm, via sensing strips on its side surfaces. Once the signals were collected from the anode, and from four or eight strips attached to the detectors sides, we assessed the anodes energy spectra and derived histograms from the side electrodes to evaluate the feasibility of achieving sub-millimeter spatial resolution in the X-Y plane. Using a highly collimated 30-keV X-ray beam at the National Synchrotron Light Source, and applying corrections to the raw data, we determined the photon-interaction points by conventional Anger logic and via a more sophisticated statistics-based positioning (SBP) algorithm. With the VFG detectors current configuration, we achieved a resolution below 1 mm, even for low-energy X-rays.

Use of virtual Frisch-grid CdZnTe detectors to attain sub-millimeter spatial resolution

The goal of our study was twofold: To determine the distribution of signals in position-sensitive CdZnTe (CZT)-based virtual Frisch-grid detectors (VFGDs) with side-sensing pads, and to evaluate the feasibility of accurately measuring the X- and Y-coordinates where a photon interaction occurs within a single VFGD module. Accordingly, we collected signals from an anode, and from four or eight sensing pads attached to four sides of a CZT crystal. We assessed the anodes energy spectra and derived histograms from the side electrodes so to evaluate the feasibility of employing VFGDs as imaging devices. Using a highly collimated 30-keV X-ray beam at the National Synchrotron Light Source (NSLS), and applying some corrections to the raw signal data, we found that the signals acquired from one side of the detector were well separated from those measured at the opposite side. We also determined the photon interaction points by conventional Anger logic and via a more sophisticated statistics-based positioning (SBP) algorithm. With the current VFGD configuration, preliminary results showed that our positioning methods could increase the resolution above the intrinsic resolution of the VFGD (6 mm). Using SBP, we achieved a resolution below 1 millimeter for low-energy X- and gamma-rays.

Characterization of CdZnTe crystals and radiation detectors

Data obtained with BNLs National Synchrotron Light Source (NSLS) has helped to elucidate, in detail, the roles of non-uniformity and extended defects on the performance of CZT detectors, as well as the root cause of device polarization during exposure to a high flux of incident X-rays. Measurements of carrier traps will be reported, including their nature and relationships to different growth methods (conventional Bridgman, high-pressure Bridgman, traveling heater, and floating zone methods). Most findings will be correlated with the performance of spectrometer-grade CZT Xray and gamma detectors, and new directions to resolve the material deficiencies will be offered.