Coplanar Grid Research Articles

Efficient Charge Collection in Coplanar-Grid Radiation Detectors

We have modeled laser-induced transient current waveforms in radiation coplanar grid detectors. Poisson's equation has been solved by finite element method and currents induced by photo-generated charge were obtained using Shockley-Ramo theorem. The spectral response on a radiation flux has been modeled by Monte-Carlo simulations. We show 10$\times$ improved spectral resolution of coplanar grid detector using differential signal sensing. We model the current waveform dependence on doping, depletion width, diffusion and detector shielding and their mutual dependence is discussed in terms of detector optimization. The numerical simulations are successfully compared to experimental data and further model simplifications are proposed. The space charge below electrodes and a non-homogeneous electric field on a coplanar grid anode are found to be the dominant contributions to laser-induced transient current waveforms.

Development of a position-sensitive CZT detector with coplanar grid electrode

The finite–element analysis was done to optimize position-sensing coplanar grid electrode geometry. CdZnTe crystals manufactured by PAM-XIAMEN with customized electrode geometry have been tested. After digitizing pre-amplifier signals, trapezoidal digital filter was adopted to enhance the signal to noise ratio. Energy resolution of 2.7% FWHM at 662 keV γ-ray energy was obtained. The measured results show the dramatic improvement with position-sensing technique.

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.

Chemical species tomographic imaging of the vapour fuel distribution in a compression-ignition engine

This article reports the first application of chemical species tomography to visualise the in-cylinder fuel vapour concentration distribution during the mixing process in a compression-ignition engine. The engine was operated in motored conditions using nitrogen aspiration and fired conditions using a gasoline-like blend of 50% iso-dodecane and 50% n-dodecane. The tomography system comprises 31 laser beams arranged in a co-planar grid located below the injector. A novel, robust data referencing scheme was employed to condition the acquired data for image reconstruction using the iterative Landweber algorithm. Tomographic images were acquired during the compression stroke at a rate of 13 frames per crank angle degree within the same engine cycle at 1200 r min−1. The temperature-dependent fuel evaporation rate and mixing evolution were observed at different injection timings and intake pressure and temperature conditions. An initial cross-validation of the tomographic images was performed with planar laser-induced fluorescence images, showing good agreement in feature localisation and identification. This is the first time chemical species tomography using near-infrared spectroscopic absorption has been validated under engine conditions, and the first application of chemical species tomography to a compression-ignition engine.

Conception and characterization of a virtual coplanar grid for a 11×11 pixelated CZT detector

Due to the low mobility of holes in CZT, commercially available detectors with a relatively large volume typically use a pixelated anode structure. They are mostly used in imaging applications and often require a dense electronic readout scheme. These large volume detectors are also interesting for high-sensitivity applications and a CZT-based blood gamma counter was developed from a 20×20×15 mm3 crystal available commercially and having a 11×11 pixelated readout scheme. A method is proposed here to reduce the number of channels required to use the crystal in a high-sensitivity counting application, dedicated to pharmacokinetic modelling in PET and SPECT. Inspired by a classic coplanar anode, an implementation of a virtual coplanar grid was done by connecting the 121 pixels of the detector to form intercalated bands. The layout, the front-end electronics and the characterization of the detector in this 2-channel anode geometry is presented. The coefficients required to compensate for electron trapping in CZT were determined experimentally to improve the performance. The resulting virtual coplanar detector has an intrinsic efficiency of 34% and an energy resolution of 8% at 662keV. The detector's response was linear between 80keV and 1372keV. This suggests that large CZT crystals offer an excellent alternative to scintillation detectors for some applications, especially those where high-sensitivity and compactness are required.

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.

Long-term stability of underground operated CZT detectors based on the analysis of intrinsic 113Cd β−-decay

The COBRA collaboration operates a demonstrator setup at the underground facility Laboratori Nazionali del Gran Sasso (LNGS, located in Italy) to prove the technological capabilities of this concept for the search for neutrinoless double beta-decay. The setup consists of 64 (1×1×1)cm3 Cadmium-Zinc-Telluride (CZT) detectors in Coplanar-Grid (CPG) configuration. One purpose of this demonstrator is to test if reliable long-term operation of CZT-CPG detectors in such a setup is possible. The demonstrator has been operated under ultra low-background conditions for more than three years and collected data corresponding to a total exposure of 218kgdays. The presented study focuses on the long-term stability of CZT detectors by analyzing the intrinsic, fourfold forbidden non-unique 113Cd single beta-decay. It can be shown that CZT detectors can be operated stably for long periods of time and that the 113Cd single beta-decay can be used as an internal monitor of the detector performance during the runtime of the experiment.

Voltage-Assisted Calorimetric Detection of Gamma Interactions in a Prototype Cryogenic Ge Detector of the EDELWEISS Collaboration for Dark Matter Search

As a part of an R&D program to improve the sensitivity of its detectors to low-mass ( $$<$$ 10 GeV) weakly interacting massive particles, the Edelweiss dark matter collaboration is developing cryogenic ionization-and-heat coplanar grid germanium detectors, operated in a high-bias mode where advantage is taken of the voltage-assisted amplification of the ionization signals for enhanced sensitivity to low-energy ( $$<$$ a few keV) interactions. First results of $$\upgamma $$ calibration experiments are presented for a 200 g prototype detector, capable of sustaining collection voltages up to 180 V with a corresponding gain of 60 in the heat measurement channel for electron recoil interactions. Event populations are analyzed based on ionization and heat data and on computer modeling of the detector signals, and a tentative interpretation of the results for the heat resolution is presented, involving athermal ballistic phonon losses in the device with consequent fluctuations in the thermometer response to the energy deposit of a particle.

Surface morphology and electrical properties of polycrystalline mercuric iodide films based on coplanar grid electrode

Polycrystalline mercuric iodide (HgI2) films have been grown on ITO-coated glass substrates using hot-wall physical vapor deposition (HWPVD) with the different source temperatures. The influence of source temperature on the structure and morphology of the polycrystalline HgI2 films is investigated. It is found that the source temperature plays an important role in the improvement of the structure and morphology. The uniformly (001)-oriented polycrystalline HgI2 films can be obtained at the source temperature of 85 °C. The electrical properties of high quality polycrystalline HgI2 films based on coplanar-grid electrode are investigated at room temperature under irradiation conditions by 5.9 keV X-ray irradiation from 55Fe source. For (001)-oriented polycrystalline HgI2 films, the dark current is in the order of 10−8 A and photo current is in the order of 10−7 A with the applied voltage of 40 V. However, a stable signal of capacity in the order of 10−12 F is also obtained under low and high frequency respectively.

Investigation of a CPG-array CdZnTe γ-ray imaging detector with single collecting electrodes readout.

The single-electrode readout method has been applied to a coplanar grid (CPG) array CdZnTe detector in order to halve the number of preamplifiers previously needed and to facilitate imaging applications of CPG detectors. A method of predetermining the width of the optimum collecting electrodes has also been proposed, using the calculated optimum relative gain factor G. Meanwhile, a detailed process for calculating the charge induction efficiency (CIE) is presented. To simplify the calculation process, the computational formula of the CIE was deduced through the integration of the weighting potential. For performance evaluation, a 2 × 2 CPG-array CdZnTe detector was elaborately designed and tested with (137)Cs at 662 keV. Experimental results showed the capability of using the CPG-array CdZnTe detector with single collecting electrode readout for γ-ray imaging applications, with the same complexity of associated readout electronics as that of the pixelated CdZnTe detectors.

Simulation and characterization of polycrystalline mercuric iodide film detector based on coplanar grid electrode

Polycrystalline mercuric iodide films are being developed as a new detector technology for X-ray imaging at room temperature. Polycrystalline mercuric iodide (HgI2) films have been grown on ITO-coated glass substrates using hot-wall physical vapor deposition (HWPVD) with the different source temperatures. The electrical properties of high quality polycrystalline HgI2 detector based on coplanar-grid electrode are investigated at room temperature. The HgI2 device of high quality polycrystalline films has low leakage current before X-ray radiation and stable capacity under low frequency. Different coplanar-grid electrode patterns are designed by varying the widths of grid and gap. The finite elements analysis is used to simulate the potential distribution of the coplanar-grid HgI2 device. The electrode design of pattern A (the width of grid is 150 μm and the width of gap is 500 μm) is the best. The electrical characteristic of coplanar-grid device using mask A verifies the feasible simulation of electrode design for pattern A.

Status and Perspectives of the COBRA Experiment

COBRA is a neutrinoless double beta decay (0νββ) experiment using an array of Cadmium-Zinc-Telluride semiconductor detectors, the isotope of interest being 116Cd with a Q-value of 2814 keV. To investigate the experimental challenges of operating CdZnTe detectors in low background mode and to identify potential background components, a demonstrator setup is operated at the Gran Sasso underground laboratory (LNGS) in Italy, while additional studies are proceeding in surface laboratories. The experiment consists of monolithic, calorimetric detectors of coplanar grid design (CPG de- tectors). These detectors have a size of 1×1×1cm3 and are arranged in four layers of 4×4 detectors. An overview of the current status and of future perspectives is given. Results of pulse shape analyses are presented as well as background estimates using the data collected so far.

A novel CZT detector using strengthened electric field line anode

In this paper, we report on the design, simulation and testing of a novel CZT detector with an electrode named the Strengthened Electric Field Line Anode (SEFLA). The Strengthened Electric Field (SEF) technique and Single Polarity Charge Sensing (SPCS) technique are implemented. It could achieve the same performance as Coplanar Grid, Pixel Array CZT detectors but requires only a simple readout system. Geant4, Ansoft Maxwell and a self-developed Induced Current Calculator (ICC) package are used to develop an understanding of how the energy spectrum is formed, and the parameters of the detector are optimized. A prototype is fabricated. Experimental results demonstrate the effectiveness of this design. The test shows that the SEFLA detector achieves a FWHM of 6.0% @59.5 keV and 1.6% @662 keV, which matches well with the simulations.

Design and study of a coplanar grid array CdZnTe detector for improved spatial resolution

Coplanar grid (CPG) CdZnTe detectors have been used as gamma-ray spectrometers for years. Comparing with pixelated CdZnTe detectors, CPG CdZnTe detectors have either no or poor spatial resolution, which directly limits its use in imaging applications. To address the issue, a 2×2 CPG array CdZnTe detector with dimensions of 7×7×5mm3 was fabricated. Each of the CPG pairs in the detector was moderately shrunk in size and precisely designed to improve the spatial resolution while maintaining good energy resolution, considering the charge loss at the surface between the strips of each CPG pairs. Preliminary measurements were demonstrated at an energy resolution of 2.7–3.9% for the four CPG pairs using 662keV gamma rays and with a spatial resolution of 3.3mm, which is the best spatial resolution ever achieved for CPG CdZnTe detectors. The results reveal that the CPG CdZnTe detector can also be applied to imaging applications at a substantially higher spatial resolution.

Pulse-shape discrimination of surface events in CdZnTe detectors for the COBRA experiment

Events near the cathode and anode surfaces of a coplanar grid CdZnTe detector are identifiable by means of the interaction depth information encoded in the signal amplitudes. However, the amplitudes cannot be used to identify events near the lateral surfaces. In this paper a method is described to identify lateral surface events by means of their pulse shapes. Such identification allows for discrimination of surface alpha particle interactions from more penetrating forms of radiation, which is particularly important for rare event searches. The effectiveness of the presented technique in suppressing backgrounds due to alpha contamination in the search for neutrinoless double beta decay with the COBRA experiment is demonstrated.

Current Reduction of CdZnTe via Band Gap Engineering

CdZnTe-based gamma detectors require a reduction of electronic noise contributed by apparent device and surface leakage current, especially for advanced readout schemes such as the Co-planar Grid or Pixelated Grid. In this work, we describe a combination of surface treatments and amorphous semiconductor layers that result in a reduction of both apparent device and surface leakage current compared to metal contacts on Br:MeOH etched devices. Characterization by scanning electron microscopy (SEM), spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), current-voltage (IV), and pulse height spectra is performed.

Status and perspectives of COBRA

The COBRA experiment is a search for neutrinoless double-beta decay using CdZnTe semiconductor detectors. Currently COBRA is collecting data from 32 coplanar grid (CPG) detectors installed in the Gran Sasso National Laboratory. This serves as a prototype for a proposed design of over 100 kg target mass in order to achieve the sensitivity necessary to probe new parameter space for 0νββ. Pixelated detector allowing tracking and thus particle identification are also explored as a unique option in double beta searches.

Analytical model for event reconstruction in coplanar grid CdZnTe detectors

Coplanar-grid (CPG) particle detectors were designed for materials such as CdZnTe (CZT) in which charge carriers of only one sign have acceptable transport properties. The presence of two independent anode signals allows for a reconstruction of deposited energy based on the difference between the two signals, and a reconstruction of the interaction depth based on the ratio of the amplitudes of the sum and difference of the signals. Energy resolution is greatly improved by modifying the difference signal with an empirically determined weighting factor to correct for the effects of electron trapping. This paper introduces a modified interaction depth reconstruction formula which corrects for electron trapping utilizing the same weighting factor used for energy reconstruction. The improvement of this depth reconstruction over simpler formulas is demonstrated. Further corrections due to the contribution of hole transport to the signals are discussed.

Current Status and Future Perspectives of the COBRA Experiment

The aim of the COBRA experiment is to prove the existence of neutrinoless double-beta-decay (0νββ-decay) and to measure its half-life. For this purpose a detector array made of cadmium-zinc-telluride (CdZnTe) semiconductor detectors is operated at the Gran Sasso Underground Laboratory (LNGS) in Italy. This setup is used to investigate the experimental issues of operating CdZnTe detectors in low-background mode and to identify potential background components, whilst additional studies are proceeding in surface laboratories. The experiment currently consists of monolithic, calorimetric detectors of coplanar grid design (CPG detectors). These detectors are 1 × 1 × 1 cm3and are arranged in 4 × 4 detector layers. Ultimately four layers will be installed by the end of 2013, of which two are currently operating. To date 82.3 kg·days of data have been collected. In the region of interest for116Cd around 2.8 MeV, the median energy resolution is 1.5% FWHM, and a background level near 1 counts/keV/kg/y has been reached. This paper gives an overview of the current status of the experiment and future perspectives.

Digital Spectroscopic System Based on Large Volume Stacked Coplanar Grid (Cd,Zn)Te Detectors

The Freiburg Materials Research Center (FMF) is developing a spectroscopic system with high efficiency and high energy resolution based on (Cd,Zn)Te detectors with coplanar grid (CPG) configuration. Our system features an improved multi-channel analyzer with digital signal processing and a large volume detector stack. Thermoelectric cooling is used to improve energy resolution and maintain detector temperature at a constant value. System performance under laboratory and outdoor conditions is investigated in this paper.