We report progress in the study of thick single-sided charge-sharing cadmium zinc telluride (CZT) strip detector
modules designed to perform spectroscopy and 3-D imaging of gamma-rays. We report laboratory measurements
including spectroscopy, efficiency and 3-D imaging capability of prototype detectors (15 × 15 × 7.5mm3) with
11×11 unit cells. We also report on Monte Carlo simulations (GEANT4 v7.1) to investigate the effect of multihits
on detector performance in both spectroscopy and imaging. We compare simulation results with data obtained
from laboratory measurements and discuss the implications for future strip detector designs.
We report progress in the study of a thick CZT strip detector module designed to perform gamma-ray spectroscopy and 3-D imaging. We report preliminary performance measurements of 7.5 mm thick single-sided charge-sharing strip detector prototype devices. This design features both row and column contacts on the anode surface. This electron-only approach addresses problems associated with poor hole transport in CZT that limit the thickness and energy range of double-sided strip detectors. This work includes laboratory and simulation studies aimed at developing compact, efficient, detector modules for 0.05 to 1 MeV gamma measurements while minimizing the number and complexity of the electronic readout channels. This is particularly important in space-based coded aperture and Compton telescope instruments that require large area, large volume detector arrays. Such arrays will be required for the NASA Black Hole Finder Probe (BHFP)and Advanced Compton Telescope (ACT). This new design requires an anode pattern with contacts whose dimensions and spacing are roughly the size of the ionization charge cloud. The first prototype devices have 125 μm anode contacts on 225 μm pitch. Our results demonstrate the principle of operation but suggest that even finer anode contact feature sizes will be necessary to achieve the desired performance.
We report progress in the study of thick CZT strip detectors for 3-d imaging and spectroscopy and discuss two approaches to device design. We present the spectroscopic, imaging, detection efficiency and response uniformity performance of prototype devices. Unlike double-sided strip detectors, these devices feature both row and column contacts implemented on the anode surface. This electron-only approach circumvents problems associated with poor hole transport in CZT that normally limit the thickness and energy range of double-sided strip detectors. These devices can achieve similar performance to pixel detectors. The work includes laboratory and simulation studies aimed at developing compact, efficient, detector modules for 0.05 to 1 MeV gamma radiation measurements. The low channel count strip detector approach can significantly reduce the complexity and power requirements of the readout electronics. This is particularly important in space-based coded aperture or Compton telescope instruments requiring large area, large volume detector arrays. Such arrays will be required for NASA's Black Hole Finder Probe (BHFP) and Advanced Compton Telescope (ACT).
We report progress in our study of cadmium zinc telluride (CZT) strip detectors featuring orthogonal coplanar anode contacts. We specifically report on the performance, characterization and stability of 5 and 10 mm thick prototype CZT detectors fabricated using material from several manufacturers. Our ongoing work includes laboratory and simulation studies aimed at optimizing and developing compact, efficient, high performance detector modules for 0.05 to 1 MeV gamma radiation measurements with space-based instrumentation. The coplanar anode strip configuration retains many of the performance advantages of pixel detectors yet requires far fewer electronic channels to perform both 3-d imaging and spectroscopy. Minimizing the channel count is important for large balloon or space instruments including coded aperture telescopes (such as MARGIE or EXIST) and Compton imaging telescopes (such as TIGRE or ACT). We also present plans for developing compact, space qualified imaging modules designed for integration into closely packed large area detector arrays. We discuss issues associated with detector module and array electronics design and development.
KEYWORDS: Sensors, Electrodes, Polarization, Superposition, Semiconductors, Electric field sensors, Dielectric polarization, Signal detection, Dielectrics, Space operations
Ramo's theorem provides a convenient way of simulating the currents induced on the electrodes of a radiation detector by the movement of charge carriers. The concept of weighting potential or of weighting field is the key ingredient in the calculation of the induced currents. The effect of the space charge present in the detector on the weighting potentials is discussed. Three types of space charge are considered and are shown to have different effects on the induced currents. These are 1) the polarization space charge, 2) a fixed volume space charge and 3) the charge stored in the depletion layer of a semiconductor detector. A new definition of the weighting potential is proposed that is valid for all situations.
KEYWORDS: Sensors, Prototyping, Photons, Signal processing, Signal detection, Spatial resolution, Image resolution, Data acquisition, 3D image processing, Gold
We report on recent three-dimensional imaging performance and detection efficiency measurements obtained with 5 mm thick prototype CdZnTe detectors fabricated with orthogonal coplanar anode strips. In previous work, we have shown that detectors fabricated using this design achieve both very good energy resolution and sub-millimeter spatial resolution with fewer electronic channels than are required for pixel detectors. As electron-only devices, like pixel detectors, coplanar anode strip detectors can be fabricated in the thickness required to be effective imagers for photons with energies in excess of 500 keV. Unlike conventional double-sided strip detectors, the coplanar anode strip detectors require segmented contacts and signal processing electronics on only one surface. The signals can be processed to measure the total energy deposit and the photon interaction location in three dimensions. The measurements reported here provide a quantitative assessment of the detection capabilities of orthogonal coplanar anode strip detectors.
KEYWORDS: Sensors, Signal processing, Signal detection, Analog electronics, Prototyping, Device simulation, Field effect transistors, Gamma radiation, Spatial resolution, Data acquisition
We present the requirements, design, and performance of an analog circuit for processing the non-collecting anode strip signals from a cadmium zinc telluride (CZT) strip detector with orthogonal coplanar anodes. Detector signal simulations and measurements with a prototype are used to define the range of signal characteristics as a function of location of the gamma interaction in the detector. The signals from the non- collecting anode strip electrodes are used to define two of the three spatial coordinates including the depth of interaction, the z dimension. Analog signal processing options are discussed. A circuit to process the signals from the non- collecting anode strips and extract from them the depth of interaction is described. The circuit employs a time-over- threshold (TOT) measurement. The performance of the detector prototype with a preliminary version of this circuit is presented, and future development work is outlined.
We report on the simulation, construction and performance of prototype CZT imaging detectors employing orthogonal coplanar anodes. These detectors employ a novel electrode geometry with non-collecting anode strips in 1D and collecting anode pixels, interconnected in rows, in the orthogonal dimensions. These detectors retain the spectroscopic and detection efficiency advantages of single carried charge sensing devices as well as the principal advantage of conventional strip detectors with orthogonal anode and cathode strips, i.e. an N X N array of imagin pixels are realized with only 2N electronic channels. Charge signals induced on the various electrodes of a prototype detector with 8 X 8 unit cells are in good agreement with the simulations. The position resolution is about 1 mm in the direction perpendicular to the pixel lines while it is of the order of 100 micrometers in the other direction. Energy resolutions of 0.9 percent at 662 keV, 2.6 percent at 122 keV and 5.7 percent at 60 keV have been obtained at room temperature.
We report new performance measurements and computer simulations of a sub-millimeter pitch CdZnTe strip detector under study as a prototype imaging spectrometer for astronomical x-ray and gamma-ray observations. The prototype is 1.5 mm thick with 375 micron strip pitch in both the x and y dimensions. Previously reported work included demonstrations of half-pitch spatial resolution (approximately 190 microns) and good energy resolution and spectral uniformity. Strip detector efficiency measurements have also been presented. A model that includes the photon interaction, carrier transport and the electronics was developed that qualitatively reproduced the measurements. The new studies include measurements of the CdZnTe transport properties for this prototype in an effort to resolve quantitative discrepancies between the measurements and the simulations. Measurements of charge signals produced by laser pulses and (alpha) -rays are used to determine these transport properties. These are then used in the model to predict gamma-ray efficiencies that are compared with the data. The imaging performance of the detector is studied by scanned laser and gamma beam spot measurements. The results support the model's prediction of nearly linear sharing of the charge for interactions occurring in the region between electrodes. The potential for strip detectors with spatial resolution much finer than the strip pitch is demonstrated. A new design scheme for strip detectors is shortly discussed.
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