Coplanar Grid Research Articles

Synchrotron radiation response characterization of coplanar grid CZT detectors

Commercial 15/spl times/15/spl times/7.5 mm/sup 3/ coplanar grid CdZnTe detectors were studied on the micron-scale using a collimated high-energy X-ray beam provided by Brookhaven's National Synchrotron Light Source. This powerful tool enables simultaneous studies of detector response uniformity, electronic properties of the material, and effects related to the device's contact pattern and electric field distribution. The availability of a front-end Application Specific Integrated Circuit, developed at Brookhaven's Instrumentation Division, providing low noise amplification of grids and cathode signals, corresponding timing signal and adjustable relative gain, allowed to correlate performance mapping and fluctuations in collected charge. We observed the effect of the strip contacts comprising the coplanar grids on the energy resolution of the coplanar-grid device.

Investigation of the asymmetric characteristics and temperature effects of CdZnTe detectors

The results of our newest depth sensing coplanar grid CdZnTe detectors will be discussed. These detectors, which utilize the third-generation coplanar anode design, were fabricated by Baltic Scientific Instruments, Ltd., using crystals acquired from Yinnel Tech, Inc., having dimensions of 1.5 cm /spl times/1.5cm/spl times/ 0.9 cm and 1.5 cm /spl times/ 1.5cm /spl times/ 0.95 cm. Employing various compensation techniques, an energy resolution of about 1.7% full width at half maximum (FWHM) was achieved for 662-keV gamma rays. We observed that the spectral performance was highly dependent upon the biasing orientation of the detector. The /spl mu//sub e//spl tau//sub e/ product for one of the crystals was measured and it was found to be a high value of 1.13 /spl times/ 10/sup -2/ cm/sup 2//V. A study of the spectroscopic performance of CdZnTe for reduced temperature operation was implemented and we observed degraded performance for temperatures below 0/spl deg/C and a complete loss of spectral information at -30/spl deg/C.

Front-end ASIC for Co-Planar grid sensors

An Application Specific Integrated Circuit for Cadmium Zinc Telluride Co-Planar Grid sensors is presented. The ASIC provides low-noise amplification of grids and cathode signals, difference between grid signals with adjustable relative gain, shaped signals with baseline stabilization, and timing signals. In the current version the peaking time of the shaped pulses is 5 /spl mu/s and the gain can be switched between 36 mV/fC and 18 mV/fC covering an energy range up to 3 MeV. Designed in CMOS 0.25/spl mu/m technology it dissipates 25 mW from a single +2.5 V supply. A description of the ASIC and the results of its characterization with CdZnTe CPG sensors are presented. The system is analyzed in terms of resolution, and the impact of the noise correlation due to the inter-grid capacitance is discussed.

Characteristics of depth-sensing coplanar grid CdZnTe detectors

The latest depth-sensing coplanar grid CdZnTe detectors have been tested. Two of these have dimensions 1.5×1.5×1.0 cm 3 and one is a cylindrical detector with 1.5 cm diameter and 1.0 cm length, all of them using the third-generation coplanar anode design. Energy resolutions of 2.0% and 2.4% FWHM at 662 keV γ-ray energies were obtained. Detector performance has been observed experimentally as a function of depth of the γ-ray interaction, and as a function of radial position near the anode surface. The measured results show the improvement of the third-generation anode design. Material uniformity of CdZnTe crystals manufactured by eV Products have been directly observed and compared on two 1.5×1.5×1.0 cm 3 detectors.

Tetra-phenyl porphyrin based thin film transistors

Long channel p-type field effect transistors were realized by using 5,10,15,20-tetra-phenyl porphyrin. The organic was deposited by spray coating from a chloroform solution onto thermally oxidized silicon substrates where chromium source and drain contacts were realized. Coplanar grid structures with a high geometric ratio were defined by fine line lithography in order to increase the collected current. Gate bias up to −100 V and drain to source bias up to −30 V were used. Leakage paths through the dielectric, also at low voltage, were evidenced by monitoring the gate current. Source to drain currents were evaluated considering the Kirchhoff law keeping into account the leakage currents. On these basis, a complete characterization of the transistors was carried out. Threshold voltage equal to −7.5 V was estimated on a 40 μm long device. Hole field effect mobility equal to 0.007 cm 2/Vs was evaluated from zero-bias channel conductance. Besides, field dependent mobility as high as 0.012 cm 2/Vs and transconductance equal to 6.5 μA/V were estimated from the analysis of the achieved results. The operation mechanism of the transistor and current modulation at low and high field is presented and discussed.

Effect of differential bias on the transport of electrons in coplanar grid CdZnTe detectors

Segmented and pixilated electrode structures are used to compensate for poor hole transport in CdZnTe devices used for gamma-ray spectrometry and imaging. Efforts to model these structures have focused primarily on geometric effects; however, device performance also depends on the physical properties of the bulk and surface material, as well as the electrodes. In this paper, we describe experiments to characterize the electric field near the anode of a coplanar grid detector. The experiment is contrasted with a calculation that is based on an assumption commonly used to reduce the computational effort required to determine internal electric fields. Explanations for differences between the calculation and the experiment are proposed.

Stability and characteristics of large CZT coplanar electrode detectors

The single polarity charge sensing method based on coplanar grid electrodes has demonstrated promising results on CZT detectors having volumes of 1 cm/sup 3/. This work presents the performance of two coplanar grid CZT detectors having dimensions of 1.5 cm/spl times/1.5 cm/spl times/1.0 cm, among the largest CZT spectrometers that have been reported. About 2.3% and 2.0% FWHM energy resolutions have been obtained on the two detectors at 662 keV gamma-ray energy using basic analog techniques. Improvement can be achieved with depth sensing and digital pulse processing techniques. Additional investigations have studied detector characteristics critical to practical application of these devices. This includes the stability of detector response over a period of several months and the variation of detector performance in environments with high gamma radiation fluxes and at different counting rates.

Simultaneous ballistic deficit immunity and resilience to parallel noise sources: A new pulse shaping technique

A new and different time-variant pulse processing system has been developed based on a simple CR-RC filter and two analog switches. The new pulse processing technique combines both ballistic deficit immunity and resilience to parallel noise without a significant compromise to the low energy resolution, generally considered a mutually exclusive requirement. The filter is realized by combining two different pulse shaping techniques. One of the techniques creates a low rate of curvature at the pulse peak, which reduces ballistic deficit, while the second technique increases the tolerance to low-frequency noise by modifying the noise history. Several experimental measurements are presented, including tests on a coplanar grid CdZnTe detector. Improvements on both the resolution and line shape are shown for the 662 keV line of /sup 137/Cs.

Development of high efficiency, multi-element CdZnTe detectors for portable measurement applications

We describe the development of detector arrays and electronics for large-volume, hand-held CdZnTe detectors with the same counting efficiency as portable NaI(Tl) detectors presently used for nuclear material measurement applications. The pulse-height resolution of the multi-element detectors is at least three times better than NaI(Tl) over a wide energy range (from 100 keV to several MeV), enabling more accurate measurements of gamma-rays emitted by special nuclear material. Arrays of up to eight coplanar grid detectors can be combined to make detectors ranging in size from 4 to 14 cm3. Because the number of spectroscopy channels is small, low-power, hand-held detectors can be manufactured with conventional printed circuit board technology, thus keeping the cost of multi-element detectors to a minimum. The design and performance of an 8-element detector is presented.

Performance improvement of CdZnTe detectors using modified two-terminal electrode geometry

Recently, much has appeared in the literature concerning methods to improve the resolution and photopeak efficiency of CdZnTe radiation detectors operating at or around ambient temperature. These methods generally involve either the use of modified electrode structures (e.g. coplanar grids, three-terminal devices) or pulse processing techniques, both of which add complexity, and hence cost, to the production and operation of such devices. In this paper, we will report on results obtained with a simpler, modified two-terminal device. The detector structure combines a planar anode with an extended surface cathode, and relies on a standard, single-channel preamplifier/shaping amplifier system. The results obtained demonstrate that the charge collection efficiency of the device, as shown by the peak-to-valley ( P/ V) ratio, is significantly improved when compared to the standard planar geometry, especially at higher (>200 keV) photon energies.

A theoretical assessment of the relative performance of spherical phased arrays for ultrasound surgery

Computer modeling of spherical-section phased arrays for ultrasound surgery (tissue ablation) is described. The influence on performance of the number of circular elements (68 to 1024), their diameter (2.5 to 10 mm), frequency (1 to 2 MHz), and degree of sparseness in the array is investigated for elements distributed randomly or in square, annular, and hexagonal patterns on a spherical shell (radius of curvature, 120 mm). Criteria for evaluating the quality of the intensity distributions obtained when focusing the arrays both on and away from their center of curvature, and in both single focus and simultaneous multiple foci modes, are proposed. Of the arrays studied, the most favorable performance, for both modes, is predicted for 256 5-mm diameter, randomly distributed elements. For the single focus mode, this performed better than regular arrays of 255 to 1024 elements and, for the case of nine simultaneous foci produced on a coplanar 3x3 grid with 4-mm spacing, better than square, hexagonal, or annular distributed arrays with a comparable number of elements. Randomization improved performance by suppressing grating lobes significantly. For single focus mode, a several-fold decrease in the number of elements could be made without degrading the quality of the intensity distribution.

Effect of electron transport properties on unipolar CdZnTe radiation detectors: LUND, spectrumplus and coplanar grid

Device simulations of (1) the laterally contacted-unipolar-nuclear detector (LUND), (2) the SpectrumPlus, (3) and the coplanar grid made of Cd0.9Zn0.1Te (CZT) were performed for 137Cs irradiation by 662.15 keV gamma-rays. Realistic and controlled simulations of the gamma-ray interactions with the CZT material were done using the MCNP4B2 Monte Carlo program, and the detector responses were simulated using the Sandia three-dimensional multi-electrode simulation program (SandTMSP). The simulations were done for the best and the worst expected carrier mobilities and lifetimes of currently commercially available CZT materials for radiation detector applications. For the simulated unipolar devices, the active device volumes were relatively large and the energy resolutions were fairly good, but these performance characteristics were found to be very sensitive to the materials properties. The internal electric fields, the weighting potentials, and the charge induced efficiency maps were calculated to give insights into the operation of these devices.

Performance of CdZnTe detectors passivated with energetic oxygen atoms

Noise caused by surface-leakage current can degrade the performance of CdZnTe spectrometers, particularly devices with closely spaced contacts such as coplanar grid detectors. In order to reduce surface leakage, we are treating CdZnTe detector surfaces with energetic, neutral oxygen atoms. Energetic oxygen atoms react with the surface to form a resistive oxide layer. Because the reaction is effective at room temperature, deleterious heating of the substrate is avoided. In most cases, leakage current and noise are shown to decrease significantly after treatment. The effect of the treatment on the performance of coplanar grid detectors is presented.

Coplanar grid patterns and their effect on energy resolution of CdZnTe detectors

This paper describes diagnostic techniques using depth and radial position-sensing methods that have been applied to identify and remove the non-symmetric effect of coplanar grid electrodes. Our experimental results show that the non-symmetric effect can degrade significantly the energy resolution of single polarity charge sensing CdZnTe detectors and can be minimized by balancing the weighting potentials of coplanar anodes. The coplanar electrode design has been modified based on the knowledge gained from this study and improvements in detector performance have been achieved.

Performance of CdZnTe coplanar-grid gamma-ray detectors

The spectral performance of coplanar grid detectors using currently available CdZnTe materials is examined theoretically and experimentally. Calculated spectral response based on the typical carrier mobility-lifetime products of current CdZnTe materials shows that energy resolution close to the charge-statistics limit can be achieved. Charge transport nonuniformity, which may limit the spectral performance of present detectors, is studied using alpha particle scanning.

Development of an orthogonal-stripe CdZnTe gamma radiation imaging spectrometer

We report performance measurements of a sub-millimeter resolution CdZnTe strip detector developed as a prototype for astronomical instruments operating with good efficiency in the 30-300 keV photon energy range. The prototype is a 1.4 mm thick, 64/spl times/64 contact stripe CdZnTe array of 0.375 mm pitch in both dimensions. Pulse height spectra were recorded in orthogonal-stripe coincidence mode which demonstrate room-temperature energy resolution <10 keV (FWHM) for 122 keV photons with a peak-to-valley ratio >5:1. Good response is also demonstrated at higher energies using a coplanar grid readout configuration. Spatial resolution capabilities finer than the stripe pitch are demonstrated. We present the image of a /sup 133/Ba source viewed through a collimator slit produced by a 4/spl times/4 stripe detector segment. Charge signals from electron and hole collecting contacts are also discussed.

Reversibility of brain tissue loss in anorexia nervosa assessed with a computerized Talairach 3-D proportional grid.

We describe the results of our follow-up magnetic resonance imaging (MRI) study of underweight patients with anorexia nervosa, using rigorous methodology to control for head position across time. All subjects first underwent an initial scan and rescan to verify that our computerized three-dimensional co-planar grid method for volume measurement was reliable and accurate, regardless of head positioning. After a period of several months, subjects had a follow-up scan to assess for changes that may have occurred following significant weight gain. Ventricular and total brain volume measurements from the initial scans were compared with the scans from an age- and sex-matched normal control group to determine whether we could replicate previous findings of ventricular enlargement compared with controls and whether brain volume is reduced compared with controls. Anorexic subjects had significantly larger ventricles when compared with normal controls but did not differ significantly in total brain volume. Using a repeated measures analysis of variance, a priori contrasts compared the initial/rescan pair volumes with each other and the initial/rescan pair volumes with the follow-up volume. These analyses showed that ventricular and total brain volumes derived from the initial/rescan pair were nearly identical, but that at follow-up ventricular volume decreased significantly and total brain volume increased significantly after weight gain.

The silicon dioxide storage tube

The basic structure, principal mode of operation, and performance characteristics are described of a storage tube utilizing the coplanar grid effect. An array of 600 000 silicon dioxide storage elements/cm2has been fabricated, which provides continuous read-out of stored TV quality images for several minutes. Write-in can be accomplished in 1 frame and erasure in 1 to 4 frames.

Field effects in insulating thin film secondary emitters

A horizontal shift in the collector characteristic curves suggests the presence of an electric field of 10 6V/cm or greater within insulating thin film secondary emitters. This assumption is supported by the fact that changes in secondary yield occur corresponding to changes in internal field. Furthermore, effects have been observed which suggest that the structure of insulating films may be an important factor relating to their properties because of the coplanar grid effect. This work was supported by the Wright Air Development Division, Air Research and Development Command, Wright-Patterson Air Force Base, Ohio, under Contract No. AF33(616)-6239.