Zhong He

Review of the Shockley-Ramo theorem and its application in semiconductor gamma-ray detectors

Abstract The Shockley–Ramo theorem is reviewed based on the conservation of energy. This review shows how the energy is transferred from the bias supplies to the moving charge within a device. In addition, the discussion extends the original theorem to include cases in which a constant magnetic field is present, as well as when the device medium is heterogeneous. The rapid development of single polarity charge sensing techniques implemented in recent years on semiconductor γ-ray detectors are summarized, and a fundamental interpretation of these techniques based on the Shockley–Ramo theorem is presented.

1-D POSITION SENSITIVE SINGLE CARRIER SEMICONDUCTOR DETECTORS

Abstract A single polarity charge sensing method has been studied using coplanar electrodes on 5 mm cubes of CdZnTe γ-ray detectors. This method can ameliorate the hole trapping problem of room-temperature semiconductor detectors. Our experimental results confirm that the energy resolution is dramatically improved compared with that obtained using the conventional readout method, but is still about an order of magnitude worse than the theoretical limit. A method to obtain the γ-ray interaction depth between the cathode and the anode is presented here. This technique could be used to correct for the electron trapping as a function of distance from the coplanar electrodes. Experimental results showed that a position resolution of about 0.9 mm FWHM at 122 keV can be obtained. These results will be of interest in the design of higher performance room-temperature semiconductor γ-ray detectors.

POSITION-SENSITIVE SINGLE CARRIER CDZNTE DETECTORS

Abstract Single polarity charge sensing on room temperature semiconductor gamma-ray detectors can be achieved by using the coplanar electrode read-out technique. This mehod can eliminate the hole-trapping problem of the wide band gap semiconductors which are currently available. Our previous results on 5 mm cube CZT detectors confirmed [6] that the energy resolution can be dramatically improved compared with that obtained using the conventional read-out method. This paper explores the application of this technique to CdZnTe detectors of larger volume, namely 1 cm 3 . In our previous work, we suggested a method to obtain γ-ray interaction depth and further progress is reported here. This technique can be used to correct for the electron trapping as a function of distance from the anode. The intrinsic position resolution has been analyzed and energy resolutions of less than 2% FWHM at 662 keV were obtained on both detectors tested. Finally, the factors which inhibit attaining the statistical energy resolution limit of CdZnTe detectors have been explored. These results will be of interest in the design of higher performance, portable and imaging-related, room-temperature semiconductor γ-ray detectors.

Evaluation of a Compton scattering camera using 3-D position sensitive CdZnTe detectors

Abstract A CZT Compton Camera (CCC) is being built using two three-dimensional (3-D) position-sensitive CZT detectors. Expected system performance was analyzed by analytical and Monte Carlo approaches. Based on the measurement of detector energy and position resolution, the expected angular resolution is ∼3° and ∼2° for a ±30° FOV for 511 keV and 1 MeV γ -rays, respectively. The intrinsic efficiency for a point source 10 cm from the first detector surface ranges from 1.5×10 −4 to 8.8×10 −6 for 500 keV–3 MeV.

Single charge carrier type sensing with a parallel strip pseudo-Frisch-grid CdZnTe semiconductor radiation detector

Wide band gap compound semiconducting materials offer great promise for use as room temperature operated radiation detectors. The most common semiconductor radiation detector design incorporates the use of a semiconducting block of material with metal contacts applied at opposite ends of the block. A voltage is applied across the block, which produces an internal electric field capable of drifting free charge carriers to the detector contacts. Gamma ray interactions occurring in the device excite electron-hole pairs which are separated by the applied electric field across the device bulk. Electrons are drifted towards the anode, and holes are drifted towards the cathode. The induced charge produced by the moving free charge carriers can be measured by an external circuit. Shockley 1 and Ramo 2 derived the dependence of the induced current and induced charge produced by point charges moving in an electric field, which was later shown to apply to semiconductor detectors as well. 3‐5 The Shockley‐Ramo theorem states that the induced charge that appears at the terminals of a planar device from moving point charges is proportional to the distance displaced by the moving point charges, regardless of the presence of space charge. Hence, the change in induced charge Q* can be represented by

COPLANAR GRID PATTERNS AND THEIR EFFECT ON ENERGY RESOLUTION OF CDZNTE DETECTORS

Abstract 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.

Improved resolution for 3-D position sensitive CdZnTe spectrometers

Two three-dimensional position sensitive CdZnTe gamma-ray spectrometers based on VAS2/TAT2 ASICs were constructed and fully tested. Each 1.5/spl times/1.5/spl times/1 cm/sup 3/ CdZnTe detector employs 11/spl times/11 pixelated anodes. Each pixel location provides the interactions lateral coordinates, while the cathode/anode signal ratio and the electron drift time are both used to obtain the interaction depth. Data-acquisition and processing software were developed to perform calibration and real-time spectroscopy. An energy resolution of 1.11% full-width at half-maximum (FWHM) at 662 keV was obtained for single-pixel events from the 120 working pixels of one detector, and an energy resolution of 1.14% FWHM was obtained on the other detector. Significant progress in improving energy resolution for multiple pixel events has been realized. The energy resolutions at 662 keV for two-pixel events and three-pixel events were 1.57% FWHM and 2.13% FWHM, respectively.

A Prototype Three-Dimensional Position Sensitive CdZnTe Detector Array

A new CdZnTe gamma-ray spectrometer system that employs two layers of modular detector arrays Is being developed under the collaboration between the University of Michigan and the Pacific Northwest National Labaratory (PNNL). Each layer can accommodate up to three by three 3-dimensional position sensitive CdZnTe gamma-ray spectrometers. This array system is based on the newly developed VAS_UM/TAT4 ASIC readout electronics. Each of the nine detector modules consists of a pixellated CdZnTe detector and a VAS_UM/TAT4 ASIC front-end board. Each 1.5times1.5times1.0 cm3 CdZnTe detector employs an array of 11 by 11 pixellated anodes and a planar cathode. The energy depositions and 3-dimensional positions of individual interactions of each incident gamma ray can be obtained from pulse amplitude, location of each pixel anode and the drift time of electrons. Ten detectors were tested individually and half of them achieved resolution of <1.0% FWHM at 662 keV for single-pixel events (~30% of all 662 keV full energy deposition events). Two of them were tested in a simple array to verify that the upgrade to an array system does not sacrifice the performance of individual detectors. Experimental results of individual detectors and a two-detector array system are presented, and possible causes for several worse performing detectors are discussed.

3-D position sensitive CdZnTe spectrometer performance using third generation VAS/TAT readout electronics

Three-dimensional (3-D) position-sensitive CdZnTe (CZT) gamma-ray spectrometers employing new VAS3.1/TAT3 ASIC readouts were developed and tested. Each spectrometer is a 1.5 /spl times/ 1.5 /spl times/ 1 cm/sup 3/ CdZnTe crystal with 11 /spl times/ 11 pixellated anodes wire-bonded to the readout electronics using an intermediate ceramic substrate with plated-through-via. The signals from the anode pixels and the cathode were all read out using these ASICs. The pixel position provides the lateral coordinates of interactions, while the cathode to anode signal ratio and electron drift times are used to obtain interaction depths. Using the 3-D position information, the variation in weighting potential, electron trapping and material nonuniformity can be accounted for to the scale of the position resolution, /spl sim/1.27 /spl times/ 1.27 /spl times/ 0.2 mm. The new VAS3.1/TAT3 ASIC has less gain and baseline drift, lower cross-talk noise, more uniform thresholds, better linearity and better timing resolution than our previous VAS2/TAT2 system. For example, the 32 keV K X-ray from a /sup 137/Cs source was observed for the first time. Two 3-D position sensitive CZT spectrometers were tested and both achieved better than 1% FWHM energy resolution (at 662 keV, room temperature operation, with an uncollimated source) for single-pixel events. The experimental results for these two 3-D position sensitive CZT spectrometer systems are presented and discussed.

CdZnTe semiconductor parallel strip Frisch grid radiation detectors

CdZnTe wide band gap compound semiconducting material offers promise as a room temperature operated gamma ray spectrometer. Position-dependent free charge carrier losses during transport can prevent efficient charge carrier extraction from semiconductor detectors and severely reduce energy resolution. Hole trapping losses in CdZnTe radiation detectors are far worse than electron trapping losses and resolution degradation in CdZnTe detectors results primarily from severe hole trapping during transport. Coplanar radiation detectors improve energy resolution by sensing the induced charge primarily from the motion of electrons. Demonstrated is an alternative approach to single free charge carrier sensing, in which a parallel strip Frisch grid is fabricated on either side of a parallelepiped block. The detectors are three terminal devices, but require only one preamplifier for the output signal. The prototype devices demonstrate a considerable increase in energy resolution when operated in the true Frisch grid mode rather than the planar mode, with a demonstrated room temperature energy resolution for 662 keV gamma rays of 5.91% at FWHM for a 10 mm/spl times/2 mm/spl times/10 mm device. Presently, high surface leakage currents prevent large voltages from being applied to the devices, which ultimately reduces their maximum achievable energy resolution. Further improvements are expected with the realization of reduced surface leakage currents.