James E. Berry

Improved resolution for 3-D position sensitive CdZnTe spectrometers

Two three-dimensional position sensitive CdZnTe gamma-ray spectrometers based on VAS2/TAT2 ASICs were constructed and fully tested. Each 1.5/spl times/1.5/spl times/1 cm/sup 3/ CdZnTe detector employs 11/spl times/11 pixelated anodes. Each pixel location provides the interactions lateral coordinates, while the cathode/anode signal ratio and the electron drift time are both used to obtain the interaction depth. Data-acquisition and processing software were developed to perform calibration and real-time spectroscopy. An energy resolution of 1.11% full-width at half-maximum (FWHM) at 662 keV was obtained for single-pixel events from the 120 working pixels of one detector, and an energy resolution of 1.14% FWHM was obtained on the other detector. Significant progress in improving energy resolution for multiple pixel events has been realized. The energy resolutions at 662 keV for two-pixel events and three-pixel events were 1.57% FWHM and 2.13% FWHM, respectively.

3-D position sensitive CdZnTe spectrometer performance using third generation VAS/TAT readout electronics

Three-dimensional (3-D) position-sensitive CdZnTe (CZT) gamma-ray spectrometers employing new VAS3.1/TAT3 ASIC readouts were developed and tested. Each spectrometer is a 1.5 /spl times/ 1.5 /spl times/ 1 cm/sup 3/ CdZnTe crystal with 11 /spl times/ 11 pixellated anodes wire-bonded to the readout electronics using an intermediate ceramic substrate with plated-through-via. The signals from the anode pixels and the cathode were all read out using these ASICs. The pixel position provides the lateral coordinates of interactions, while the cathode to anode signal ratio and electron drift times are used to obtain interaction depths. Using the 3-D position information, the variation in weighting potential, electron trapping and material nonuniformity can be accounted for to the scale of the position resolution, /spl sim/1.27 /spl times/ 1.27 /spl times/ 0.2 mm. The new VAS3.1/TAT3 ASIC has less gain and baseline drift, lower cross-talk noise, more uniform thresholds, better linearity and better timing resolution than our previous VAS2/TAT2 system. For example, the 32 keV K X-ray from a /sup 137/Cs source was observed for the first time. Two 3-D position sensitive CZT spectrometers were tested and both achieved better than 1% FWHM energy resolution (at 662 keV, room temperature operation, with an uncollimated source) for single-pixel events. The experimental results for these two 3-D position sensitive CZT spectrometer systems are presented and discussed.

A data acquisition and processing system for 3-D position sensitive CZT gamma-ray spectrometers

A PC-based data acquisition and processing system has been developed for two 3-D position-sensitive CZT /spl gamma/ ray detectors. For each 3-D CZT detector, an integrated readout chip (VA1) is used to read out the signals from the 11/spl times/11 anode pixels. Two independent preamplifiers are used to read out the signals from the cathode and the grid electrodes that are used as trigger signals for low and high energy incident /spl gamma/ rays, respectively. A PC-based DAQ board samples the amplitudes of the signals from the anode pixels and the cathode. The software collects and processes the energy spectra from the sampling data. The typical equivalent electronic noise of this system is measured to be 6 keV and 7 keV for the two 3-D CZT detectors, and the dynamic range of the system is up to 1 MeV. Spectra from 60, 122 and 662 keV /spl gamma/ rays have been successfully measured using the two 3-D CZT detectors.

Experimental results from an Imarad 8×8 pixellated CZT detector

Abstract This paper reports experimental results obtained from an 8×8 pixellated CZT detector provided by Imarad Imaging Systems. Three arbitrarily selected pixels were independently tested using γ rays of different energies. The signals from an anode pixel and the cathode were read out simultaneously. From the observations of the pulse waveforms from both the anode pixel and the cathode, the electron-injection effect leading to electron–hole combination was not observed. The best energy resolution at 122 keV from the three pixels was 7.8, 6.6 and 9.4 keV, respectively, while resolution at 662 keV was 29.3, 11.8 and 26.5 keV, respectively. The electronic noise from the three pixels was similar (5 keV), so the variation in the best energy resolution indicates material nonuniformity in the detector. For the material underneath the three selected pixels, the mobility-lifetime product for electrons was measured to be 3.4, 2.4 and 1.8 ( ×10 −3 cm 2 / V ), respectively, and the resistivity was measured to be 8, 10 and 8.3 ( ×10 9 Ω cm ), respectively. The variation in these results also shows the material nonuniformity across the detector. In the test with 662 keV γ rays, the pulse-height ratio of the signals from the cathode and pixel can be used to infer the interaction depth of single-pixel events. Results and analyses are presented in this paper.

A data acquisition system for flat-panel imaging arrays

An electronic data acquisition system for pixelated, two-dimensional, amorphous silicon x-ray imaging arrays has been developed. The system was designed in a modular fashion with digital control provided by field-programmable logic devices. This approach allows sections of the design to be upgraded with little impact an other aspects of the system. Good analog noise performance was obtained by matching the preamplifier design to the characteristics of the array outputs. The design of this system is presented and its performance quantified. >

A programmable, low noise, multichannel ASIC for readout of pixelated amorphous silicon arrays

Pixelated amorphous silicon arrays used for detecting X-rays have a number of special requirements for the readout electronics. Because the pixel detector is a high density array, custom integrated circuits are very desirable for reading out the column signals and addressing the rows of pixels to be read out. In practice, separate chips are used for readout and addressing. This paper discusses a custom integrated circuit for processing the analog column signals. The chip has 32 channels of low noise integrators followed by sample and hold circuits which perform a correlated double sample. The chip has several programmable features including gain, bandwidth, and readout configuration.

An asynchronous, pipelined, electronic acquisition system for Active Matrix Flat-Panel Imagers (AMFPIs)

Abstract The development of a full-custom electronic acquisition system designed for readout of large-area active matrix flat-panel imaging arrays is reported. The arrays, which comprise two-dimensional matrices of pixels utilizing amorphous silicon thin-film transistors, are themselves under development for a wide variety of X-ray imaging applications. The acquisition system was specifically designed to facilitate detailed, quantitative investigations of the properties of these novel imaging arrays and contains significant enhancements compared to a previously developed acquisition system. These enhancements include pipelined preamplifier circuits to allow faster readout speed, expanded addressing capabilities allowing a maximum of 4096 array data lines, and on-board summing of image frames. The values of many acquisition system parameters, including timings and voltages, may be specified and downloaded from a host computer. Once acquisition is enabled, the system operates asynchronously of its host computer. The system allows image capture in both radiographic mode (corresponding to the capture of individual X-ray images), and fluoroscopic mode (corresponding to the capture of a continual series of X-ray images). A detailed description of the system architecture and the underlying motivations for the design is reported in this paper.

A CAMAC based data acquisition system for flat-panel image array readout

A readout system has been developed to facilitate the digitization and subsequent display of image data from two-dimensional, pixellated, flat-panel, amorphous silicon imaging arrays. These arrays have been designed specifically for medical X-ray imaging applications. The readout system is based around hardware and software developed for various experiments at CERN and the Fermi National Accelerator Laboratory. Additional analog signal processing and digital control electronics were constructed specifically for this application at Michigan. The authors report on the form of the resulting data acquisition system, discuss aspects of its performance, and consider the compromises which were involved in its design. Although there are some features of the resulting system which are not optimized for the particular application (e.g., analog noise considerations and data transfer bandwidth), it has been possible to perform numerous investigations of the signal and noise characteristics of these imaging devices. In addition, the first diagnostic and megavoltage images of anatomical detail have been generated, thereby verifying the concept of this new detector technology. >

Hand‐Held Gamma‐Ray Imaging Sensors Using Room‐Temperature 3‐Dimensional Position‐Sensitive Semiconductor Spectrometers

This paper demonstrates the capability of compact gamma‐ray imaging devices using 3‐dimensional position sensitive CdZnTe semiconductor gamma‐ray spectrometers, developed at the University of Michigan. A prototype imager was constructed and tested using two 1 cm cube 3‐dimensional position sensitive CdZnTe detectors. Energy resolutions of 1.5% FWHM for single pixel events at 662 keV gamma‐ray energy were obtained on both detectors, and an angular resolution of about 5° FWHM was demonstrated. The capabilities of proposed devices, which can cover a wider energy range up to 2.6 MeV, are discussed.

Depth reconstruction validation in pixelated semiconductor detectors

The depth of interaction in pixelated CdZnTe and HgI2 detectors is determined using either the cathode-to-anode signal ratio (CAR) or drift time information. The CAR can be used if a γ-ray interacts only once in the detector. The drift time must be used if the γ-ray interacts over multiple pixels. The accuracy of these reconstruction techniques is evaluated using a collimated fan beam of γ-rays that can scan across the entire depth of the detector, from the pixelated anode to the planar cathode. Signals are read-out using the VAS UM/TAT4 ASIC. First, the beam position associated with the anode and cathode of a detector is determined. To measure the anode position, the polarity of the applied bias must be reversed to ensure that electrons, rather than holes, drift through the bulk. Once the beam is properly calibrated to determine the actual cathode and anode locations, many reconstruction properties are studied. The discrepancy between the reconstructed depth and the true collimator position is found to be small for all detectors tested, with a minor reconstruction bias for interactions near the anode side. The thickness of the inactive layer near the anode has been measured to be less than 300μm for two 15mm thick CdZnTe detectors. The experiments have also been used to simultaneously evaluate the time-amplitude-walk calibrations and the reconstruction of multiple-pixel events based on the drift time.