Coplanar Grid CdZnTe Research Articles

A Spherical Active Coded Aperture for $4\pi $ Gamma-Ray Imaging

Gamma-ray imaging facilitates the efficient detection, characterization, and localization of compact radioactive sources in cluttered environments. Fieldable detector systems employing active planar coded apertures have demonstrated broad energy sensitivity via both coded aperture and Compton imaging modalities. However, planar configurations suffer from a limited field of view, especially in the coded aperture mode. To improve upon this limitation, we introduce a novel design by rearranging the detectors into an active coded spherical configuration, resulting in a $4 {\pi }$ isotropic field of view for both coded aperture and Compton imaging. This paper focuses on the low-energy coded aperture modality and the optimization techniques used to determine the optimal number and configuration of 1-cm3 CdZnTe coplanar grid detectors on a 14-cm diameter sphere with 192 available detector locations.

Investigation of the Depth Reconstruction and Search for Local Performance Variations With a Large Coplanar-Quad-Grid CdZnTe Detector

CdZnTe coplanar-grid (CPG) detector is used in the COBRA experiment to search for neutrinoless double beta decay. In the next phase of the experiment, significantly larger detectors with a volume of $(20 \times 20 \times 15)~\text {mm}^{3}$ and a coplanar-quad-grid will be used. This paper presents the results of a spatially resolved investigation of the performance of one detector of this type, focusing on its usability in a double-beta experiment. With these data, two modifications of the interaction depth formula, which is of great importance for background reduction in COBRA, are investigated for the first time with such a detector, and the results are compared with a detector with a conventional CPG. Furthermore, we report the mobility-lifetime product for electrons for each sector of the detector.

Charge transport in CdZnTe coplanar grid detectors examined by laser induced transient currents

Laser-induced transient current technique was used for the visualization of charge transfer in the coplanar CdZnTe radiation detector including distinction to the collecting and noncollecting grids. Transient current waveforms measured at different intergrid biasing allowed us the identification of principal charge transit features and the final redistribution of collected charge between grids. We have demonstrated that while at the initial period of the charge passage through the detector bulk the current waveform shapes on the collection and non-collection electrodes are nearly the same, the intergrid biasing entails significant current deviation at the final part of the transients, where inverted current through the non-collecting grid induces markedly reduced/enhanced collected charge in the non-collecting/collecting electrode. An optimum ratio of cathode/intergrid biasing was found to be close to the ratio of detector/intergrid dimensions. Theoretical simulations demonstrating all charge-transfer characteristics were presented and proved the validity of the experimental data.

Measurements on the spectroscopic performance of CdZnTe coplanar grid detectors

A performance study was performed for CdZnTe coplanar grid (CPG) detectors when used as γ-ray spectrometers. The detectors have the crystal volumes of 1, 1.6875 and 2.25cm3, respectively. Time stability of each CdZnTe CPG detector was investigated in a long-term operation (time span of 0.25 to about 100h). The spectroscopic performances were measured at different bias voltages and at various photon energies ranging from 59.6keV (241Am) to 1332.5keV (60Co) for each detector, and evaluated by using the following parameters: energy resolution in FWHM, peak tailing in peak-to-valley (P/V) ratio and in FWHM/FW.25M ratio, and photofraction using the acquired γ-ray spectra. No polarization effect was observed in terms of count rate, energy resolution and peak centroid shift. The obtained results indicate that better time stability and excellent spectroscopic performances of the present CdZnTe CPG detectors are shown for a room temperature γ-ray spectroscopy.

Evaluation of CdZnTe as neutron detector around medical accelerators

The operation of electron linear accelerators (LINACs) and cyclotrons can produce a mixed gamma-neutron field composed of energetic neutrons coming directly from the source and scattered lower energy neutrons. The thermal neutron detection properties of a non-moderated coplanar-grid CdZnTe (CZT) gamma-ray detector close to an 18 MV electron LINAC and an 18 MeV proton cyclotron producing the radioisotope (18)F for positron emission tomography are investigated. The two accelerators are operated at conditions producing similar thermal neutron fluence rates of the order of 10(4) cm(-2) s(-1) at the measurement locations. The counting efficiency of the CZT detector using the prompt 558 keV photopeak following (113)Cd thermal neutron capture is evaluated and a good neutron detection performance is found at the two installations.

Hard x-ray response of a CdZnTe ring-drift detector

We present the results of an experimental study of a special type of CdZnTe detector of hard x and γ rays—a ring-drift detector. The device consists of a double ring electrode structure surrounding a central point anode with a guard plane surrounding the outer anode ring. The detector can be operated in two distinctively different modes of charge collection—pseudohemispherical and pseudodrift. We study the detector response profiles obtained by scanning the focused x-ray beam over the whole detector area, specifically the variations in count rate, peak position, and energy resolution for x rays from 10 to 100 keV. In addition, at 662 keV the energy resolution was shown to be 4.8 keV, more than a factor of 2 better than for CdZnTe coplanar grid detectors. To interpret the experimental data, we derive an analytical expression for the spatial distribution of the electric field inside the detector and neglecting carrier diffusion, and identify carrier collection patterns for both modes of operation within the drift model approximation. We show that this model provides a good understanding of measured profiles.

Performance Comparison of a Large Volume CZT Semiconductor Detector and a LaBr$_3$(Ce) Scintillator Detector

The development of portable nuclear instrumentation demands compact high sensitivity detectors operated at room temperature. The sensitivity of these detectors mainly depends on two parameters: absolute efficiency and energy resolution. In order to provide high efficiency, large volumes are needed. For semiconductor detectors able to operate at room temperature, the largest effective volumes with acceptable resolution are achieved with CdZnTe coplanar-grid and pixel detectors. On the other hand, new scintillation materials were recently developed with spectrometric capabilities only reachable some years ago with semiconductors. In this work we compare the performance of two state of the art detectors of different technologies with a relative large volume: a coplanar-grid CdZnTe detector with dimensions 15 mmtimes15 mmtimes10 mm and a LaBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (Ce) crystal with volume 18 mmtimes18 mmtimes30 mm. The CdZnTe crystal was made by Yinnel Tech (USA) and the detector was manufactured by BSI (Latvia). The LaBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (Ce) scintillator was grown and encapsulated by Saint-Gobain (France). The energy resolution for the CZT detector is 2.05% FWHM at 662 keV. The resolution for the scintillator at this energy was near 3%. The total efficiency was studied in a setup with calibrated point sources. The experimental spectra were compared with Monte-Carlo simulations performed with Geant4. The implications of the results of this comparison are discussed in the context of the practical use of these units

Hard X- and γ-ray measurements with a large volume coplanar grid CdZnTe detector

The recent introduction of coplanar grid techniques has led to resurgence in interest in developing large volume compound semiconductor detectors that have a reasonable γ-ray response but also good spectroscopic resolution. We report the results of a series of hard X- and γ-ray measurements on a large 15×15 mm2, 10 mm thick CdZnTe coplanar grid detector. The measurements were carried out at the HASYLAB and ESRF synchrotron radiation facilities using highly monochromatic pencil beams across the energy range 20–800 keV. Additional full-area measurements were carried out using radioactive sources. All measurements were carried out at room temperature. The measured energy resolution under full-area illumination was 8 and 12 keV FWHM at 59.95 and 662 keV, respectively. Under pencil beam illumination, the measured resolutions were essentially the same. The detector energy response function was found to be linear over the energy range 20–1400 keV with an average rms nonlinearity of 1.4%, consistent with statistics. The spatial uniformity of the detector was evaluated at 30, 60 and 180 keV by raster scanning a 20×20 μm2 monoenergetic X-ray beam across the active area. Apart from a few localized areas, the detector response was found to uniform at the few percent level, consistent with statistics. At 180 keV, the nonuniformity in the energy response due to the grids was estimated to be at the 0.7% level.