Coplanar Grid Research Articles

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.

Fiducial Volume of the Edelweiss Germanium Bolometers for Dark Matter Search Using Cosmogenic Activation

In this work, we use 10.36 and 8.98 keV X-rays originating from the decay of 68Ge and 65Zn which are homogeneously produced in a Ge crystal in order to evaluate the fiducial volume of a 400 g coplanar grid HP-Ge bolometers in operation in the Edelweiss-II experiment. Different energy estimators are tested on the experimental data and a fiducial fraction of (39.9±5)% is found. The results are compared to simulations taking into account the anisotropy of the electron transport in Ge.

Hot Carrier Velocities in Doped and in Ultra-pure Germanium Crystals at Millikelvin Temperatures

The velocity laws of electrons and holes in germanium single crystals at millikelvin temperatures are determined as a function of the electric field in the 〈001〉 orientation, based on time-of-flight measurements in cryogenic coplanar grid Ge detectors. Results obtained in two n-type crystals (|N a −N d |<1010 cm−3 and doped to 1011 cm−3) are compared with the experimental data from previous investigations, and shown to be consistent with Monte-Carlo simulations of carrier transport.

Performance of a Large Volume 20&lt;formula formulatype="inline"&gt; &lt;tex Notation="TeX"&gt;$\times$&lt;/tex&gt;&lt;/formula&gt;20&lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;$\times$&lt;/tex&gt; &lt;/formula&gt;15 mm&lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;$^{3}$&lt;/tex&gt;&lt;/formula&gt; CPG Detector

Large-volume CdZnTe (CZT) single crystals with electron lifetimes exceeding 10 μs recently became available commercially for making large effective area gamma-ray arrays. However, significant variations were observed in the performance of detectors made from such arrays. We evaluated the spectroscopic performance of such a large-volume, 20 × 20 × 15 mm3, coplanar-grid (CPG) CdZnTe detector, intended for use in a handheld radioisotope-identifier, which unexpectedly showed a poor spectral performance with an energy resolution of 3.5-4% FWHM measured with 662-keV gamma-rays. To understand the factors affecting its performance, we applied several characterization techniques, viz., white X-ray diffraction topography measurements, IR microscopy, and micron-spatial resolution X-ray mapping. They allowed us to identify major crystal defects in the device that limit its energy resolution and detection efficiency.

Synchrotron radiation studies of spectral response features caused by Te inclusions in a large volume coplanar grid CdZnTe detector

We report preliminary results from a synchrotron radiation study of Te inclusions in a large volume single crystal CdZnTe (CZT) coplanar-grid detector. The experiment was carried out by probing individual inclusions with highly collimated monochromatic X- and γ-ray beams. It was found that for shallow X-ray interaction depths, the effect of an inclusion on measured energy loss spectra is to introduce a ∼10% shift in the peak centroid energy towards lower channel numbers. The total efficiency is however not affected, showing that the net result of inclusions is a reduction in the Charge Collection Efficiency (CCE). For deeper interaction depths, the energy-loss spectra shows the emergence of two distinct peaks, both downshifted in channel number. We note that the observed spectral behaviour shows strong similarities with that reported in semiconductors, which exhibit polarization effects, suggesting that the underlying mechanism is common.

Internal Electric Field Estimation, Charge Transport and Detector Performance of As-Grown ${\rm Cd}_{0.9}{\rm Zn}_{0.1}{\rm Te:In}$ by THM

In this article we report the evaluation of thick (up to 10 mm) as-grown Cd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> Te(CZT):In detectors fabricated from ingots grown by traveling heater method (THM). The crystals were investigated by mapping the electron mobility (μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> ), (μτ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> and the internal electric field of as-grown CZT samples. The detector performance was also evaluated in co-planar grid geometry. The excellent (μτ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> value and the detector performance of 10 mm thick as-grown detectors is the indication that the THM technique can be employed successfully to fabricate the detectors directly from the as-grown boule at substantially lower cost.

Development of the gamma-ray analysis digital filter multi-channel analyzer (GMCA)

Abstract Gamma-ray spectroscopic systems with high efficiency and energy resolution have become important in many fields. Conventional systems with analog shaping are being replaced by digital systems that can provide higher throughput, better energy resolution and better stability. Therefore, we present the development of our new versatile real time Multi-Channel Analyzer with digital signal processing, the GMCA. The device has two independent channels and is compatible to various detectors like Coplanar Grid (Cd,Zn)Te, HPGe, LaBr 3 , NaI and CsI. Different sensors for temperature, pressure and humidity provide additional information. In this work we will show energy resolution measurements and compare spectra measured with different detectors connected to our system and to commercial systems, both analog and digital.

True coincidence-summing corrections for the coincident γ-rays measured with coplanar grid CdZnTe detectors

In this study, true coincidence-summing (TCS) correction factors have been measured for the sources (22)Na, (60)Co, (133)Ba and (152)Eu by use of three large volume coplanar grid CdZnTe (acronym: CZT) detectors. In case of a close-in detection geometry, two different TCS calculation algorithms were used to compute the required TCS correction factors. Both of the algorithms are based on the measured total-to-peak (TTP) ratio and full-energy peak (FEP) efficiency values that were obtained using almost "single" energy and coincidence-free nuclides. The results for TCS correction factors obtained by two different algorithms were agreeable to each other. The obtained TCS factors were ranged from about 7% to 30.5% in a 2250 mm(3) CZT detector when a close counting geometry was used. For other two detectors with a volume of 1000 and 1687.5mm(3), the resulted TCS correction factors were relatively smaller and varied between about 0.1% and 20% at the close counting geometry condition. Therefore, the results indicate that there is a need for the estimation of TCS corrections in CZT detectors, especially when their crystal volumes are greater than 1cm(3) and these detectors are used in the case of a close-in detection geometry.

Detecting multi-hit events in a CdZnTe coplanar grid detector using pulse shape analysis: A method for improving background rejection in the COBRA [formula omitted] experiment

A number of experiments are underway to search for a rare form of radioactivity, neutrinoless double beta decay, as a measurement of the half-life would enable the neutrino mass to be determined. The COBRA collaboration [1,2] (Zuber, 2001; Dawson, 2009) employs CdZnTe detectors in such a search. This paper describes techniques using pulse shape analysis for identifying two-centre events in a coplanar grid CdZnTe detector. This enables Compton scatter events to be identified and so suppressing the background present within the COBRA detectors.

A new high-background-rejection dark matter Ge cryogenic detector

A new design of a cryogenic germanium detector for dark matter search is presented, taking advantage of the coplanar grid technique of event localisation for improved background discrimination. Experiments performed with prototype devices in the EDELWEISS II setup at the Modane underground facility demonstrate the remarkably high efficiency of these devices for the rejection of low-energy β, approaching 105. This opens the road to investigate the range beyond 10−8 pb in the WIMP–nucleon collision cross-sections, as proposed in the EURECA project of a one-ton cryogenic detector mass.

An investigation on cooling of CZT co-planar grid detectors

The effect of moderate cooling on CdZnTe semiconductor detectors has been studied for the COBRA experiment. Improvements in energy resolution and low energy threshold were observed and quantified as a function of temperature. Leakage currents are found to contribute typically ∼ 5 keV to the widths of photopeaks.

Energy resolution improvement in room-temperature CZT detectors

We present methods to improve the energy resolution of single channel, room-temperature Cadmium-Zinc-Telluride (CZT) detectors. A new preamplifier design enables the acquisition of the actual transient current from the crystals and straightforward data analysis methods yield unprecedented energy resolution for our test-detectors. These consist of an eV-CAPture Plus crystal as standard and 1 cm cube Frisch collar crystals created in-house from low-grade coplanar grid detectors. Energy resolutions of 1.9% for our collar detectors and 0.8% for the eV crystal at 662 keV were obtained. The latter compares favourably to the best existing energy resolution results from pixel detectors.

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.

Simulation and Design of Orthogonal Capacitive Strip CdZnTe Detectors

Orthogonal strip detectors are single-carrier devices that allow reduction of the amount of read-out channels while obtaining a good spatial resolution. They are especially interesting for low count rate imaging applications like gamma-ray medical imaging. Nevertheless, applying a bias voltage between X and Y strips is necessary in order to localize correctly the interactions. Unfortunately, this bias voltage induces a leakage current, thus generating an unwanted shot noise. We propose in this paper a new setup using capacitive contacts, which suppresses leakage currents between collecting and non-collecting strips. In this paper, we first briefly present the state-of-the-art relative to coplanar grid and strip detectors, their advantages and their drawbacks. In a second part, we will focus on the theoretical background of the capacitive contact. In a third part, a simulation study of the electrode geometry is reported. Thanks to the spectral approach, we have computed the 3D applied and weighting electric fields, bulk resistances and capacitances for various geometries of the design. Afterwards, we have used these data to simulate pulse waveforms and study the associated electronics. Finally the fabrication of the devices is presented and our first experimental results are reported. As a conclusion, we will discuss the interests and the limitations of this kind of devices.

Theory of the dynamic response of a coplanar grid semiconductor detector

The authors have developed a theoretical model for the response of a coplanar grid semiconductor detector to hard x- and γ-ray radiation. Carrier drift trajectories were obtained by solving the coupled dynamical equations for carriers driven by electrostatic fields of the coplanar grid configuration. The pulse spectra calculated by summing the individual contributions for all carriers are compared to experimental results for a large volume optimized cadmium zinc telluride coplanar grid detector and good agreement is obtained.

Temperature Study of CdZnTe Coplanar-Grid Detectors

The coplanar-grid (CPG) and other electron-only detection techniques have made possible the use of CdZnTe-based detectors for gamma-ray spectroscopy when high efficiency, good energy resolution, and near room temperature operation are required. Despite the demonstrated potential of the technologies, widespread use remains hampered in part by the limited availability of the highly uniform CdZnTe material required for high-resolution spectroscopy. However, it has been recently shown that mild cooling of CdZnTe CPG detectors can result in a significant improvement in the energy resolution of the detectors thereby allowing a wider range of material to be used for high-resolution applications. In this paper, we show that improved spectroscopic performance can consistently be achieved through a combination of detector cooling and increased detector bias. Energy resolutions of about 1% FWHM at 662 keV for detector volumes up to 2.3 cm <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^3$</tex> have been obtained at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$-20^circ$</tex> C. With the electronic noise subtracted, this amounts to an intrinsic resolution of 0.8%. We also show that further cooling of the detectors to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$-30^circ$</tex> C leads to field polarization and a loss of spectroscopic performance.

Characterization of an ASIC for CPG sensors with grid-only depth of interaction sensing

A new version of the front-end application specific integrated circuit (ASIC) for co-planar-grid (CPG) sensors is presented. Compared to the first version, the ASIC is optimized for signal/noise performance, modified in peaking time, and it implements timing signals to measure the depth of interaction using the grids signals only. Experimental results using state-of the-art CPG sensors and different approaches for reducing the error due to electron trapping are reported. A new technique that makes use of the sum and difference of the grids signals is also presented.

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.

Single carrier sensing techniques in compound semiconductor detectors

For conventional radiation detectors fabricated from compound semiconductors, the wide disparity between the transport properties of the electron and holes means that detector performances are limited by the carrier with the poorest mobility–lifetime product (μτ). Finite drift lengths introduce an energy dependent depth term into the charge collection process, which effectively limit maximum detector thicknesses to a few mm for spectroscopy applications. Since mobility is a fundamental material property, the only practical way of improving μτ products is to increase carrier lifetimes, which in turn depends greatly on detector material quality and stoichiometry. Until the particular traps/defect(s) can be identified and corrected, single carrier pulse processing (e.g., rise time compensation) or sensing techniques (e.g., co-planar grids) offer the only practical means of obviating the problem. We review carrier collection in compound semiconductor radiation detectors and examine various approaches to single carrier correction and collection techniques.

Carrier dynamics and resolution of co-planar grid radiation detectors

Co-planar grid detectors utilizing single carrier sensing techniques, offer a practical solution for developing compound semiconductor gamma-ray detectors with high resolving power. We present an analytical model for describing the response of co-planar grid detector. The response of a co-planar grid detector is shown to be affected by intrinsic lateral inhomogeneity, which is inherent to its design. We evaluate the effect of both lateral and bulk inhomogeneities and show that the resolution of a co-planar grid detector is dominated by inhomogeneous broadening of the signal and not by Fano or electronic noise.