Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1998-07-08
2001-04-17
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S385100, C250S389000
Reexamination Certificate
active
06218668
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to radiation detection, and more specifically to single polarity charge carrier sensing in ionization detectors.
Radiation detectors using simple planar electrodes and based on ionization measurements often suffer from poor collection of charge carriers of certain polarity types. For example, positive charge carriers (holes) may migrate through the detector medium at a much slower rate than negative charge carriers (electrons). As a result, such detectors produce signals that vary in amplitude depending on the location within the detector at which incident radiation interacts with the detector medium. Such detectors include semiconductor detectors, liquid ionization detectors, and gas ionization detectors.
In a simple planar electrode ionization detector, full-area electrodes are formed on two opposing faces of the detector medium. A bias voltage applied across the two electrodes provides an electric field to separate and collect the charge carriers that are created by the absorption of radiation in the detector medium. Induced charge signal on one of the electrodes due to the motion of carriers provides a measure of the energy of the radiation. Incomplete charge collection due to carrier trapping or slow carrier transport results in reduced signals, which vary in strength depending on the depth of radiation interaction. This degrades the energy resolution of the detector.
U.S. Pat. No. 5,530,249 describes a method and apparatus to improve the energy resolution of ionization-type radiation detectors suffering from incomplete charge collection. Two interlaced or interdigitated electrodes are used to sense the movement of charge carriers within the detector. The induced charge signals on these electrodes are subtracted to give a net signal that yields substantially improved energy resolution.
Thus, by reconfiguring the charge sensing electrode on a detector into a pair of interdigitated electrodes, the signal response can be modified such that the signal amplitude variation caused by poor carrier transport properties is greatly reduced. The coplanar interdigitated grid detector uses two interdigitated electrodes on the detector for charge sensing. The desired signal response is obtained by subtracting the induced signals on the two grid electrodes. By changing the relative gain of the two signals before subtraction, the detector response can be effectively tuned to match the charge transport properties of the material and thus optimize the spectral response.
While the two-electrode readout interdigitated grid detector is far superior to the full-area electrode detector, there are the problems of more complex and costly electronic circuitry, involving a two channel amplifier system with subtraction circuit, and greater electronic noise. Thus it would be desirable to have a detector which has the advantages of the interdigitated electrode structure, but with simpler electronics.
SUMMARY OF THE INVENTION
The invention is an ionization detector having a pair of coplanar interdigitated grid electrodes with single electrode readout. The detector signal is obtained from one of the pair of interdigitated electrodes whose relative areas are chosen to optimize performance. Only one electrode, the collecting electrode, is used to sense charge carriers. Only one channel of signal processing electronics is required and signal subtraction is not used.
According to the invention, one of the two opposing electrodes of an otherwise conventional detector is divided into two independent electrodes that are substantially interlaced. Each of the two electrodes may be a contiguous electrode or may consist of multiple electrode elements that are electrically connected together external to the detector. Each of the two electrodes may consist of interdigitated parallel strip electrode elements or elements of other shapes. As in the planar electrode detector, a bias voltage is applied across the detector to separate and collect the carriers. In addition, a smaller bias voltage is applied between the two interdigitated electrodes such that all carriers that drift in the direction of these two electrodes are collected at only one electrode. Only the induced signal on the collecting electrode is utilized for signal processing. By using different ratios of areas for the two interdigitated electrodes, different charge induction characteristics for the collecting electrode can be realized. Depending on the degree of charge trapping, there is an optimal charge induction characteristic that provides the best energy resolution. Therefore, by choosing an appropriate ratio of electrode areas, the detector response is optimized and the energy resolution of the detector is significantly improved.
Since the degree of charge trapping in a detector is affected by electric field as well as material properties, the design of the electrodes is optimized for the expected operating voltage of the detector. In practice, the electrodes can be designed to provide optimal response assuming a set of nominal charge trapping and detector operating parameters. Final optimization can then be achieved by adjusting the actual operating bias voltage of the detector.
The invention functions most effectively on detectors in which the collection efficiency of one polarity type of carriers is significantly worse than that of the opposite polarity type. This situation occurs in many types of detectors, such as compound semiconductor detectors (CdTe, CdZnTe, HgI
2
, etc.), gas ionization detectors and liquid ionization detectors, where the positive carriers (holes or ions) are much more poorly collected compared to the negative carriers (electrons). For these detectors, the optimal design for the electrodes is determined by the collection efficiency of electrons. For good electron collection, one would use a smaller collecting electrode.
The present detector is similar to the detector of U.S. Pat. No. 5,530,249 in that two interlaced electrodes are employed. They differ however in the way that optimal induced signals are obtained. In the previous detector, optimal response is obtained by subtracting the signal of one electrode from that of the other, whereas the present detector relies on selecting the relative areas of the two electrodes to give the desired signal response from only a single electrode. Only one electronic amplifier is required to process the signals in the present detector, compared to the previous case which requires two amplifiers and a signal subtraction circuit. Therefore, the electronics is much simplified and it is similar to that used in conventional full-area planar electrode detectors.
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Luke et al., “Electrode Design for Coplanar-Grid Detectors,” IEEE Transactions on Nuclear Science 44(3), Jun. 1997, pp. 713-720.*
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Luke et al., “Performance of CdZnTe Coplanar-Grid Gamma-Ray detectors,” IEEE Transactions on Nuclear Science 43(3), Jun. 1996, pp. 1481-1486.
Hannaher Constantine
Sartorio Henry P.
The Regents of the University of California
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