S.M.A.R.T. Laboratory
Project Description

CdZnTe has shown potential as a room temperature semiconductor radiation detector, but the effect of severe ‘hole’ trapping inhibits the ability to efficiently collect the total charge. Conventional planar geometry radiation detectors require efficient charge collection of both electrons and holes for good energy resolution; hence hole trapping poses a serious problem for CdZnTe radiation spectrometers. Single charge carrier device designs alleviate much of the problems caused by hole trapping. Such ‘electron-only’ devices rely mostly on the transport of electrons to induce a signal and, by negating the deleterious effects of hole trapping, give improved energy resolution.


Frisch-grid-based designs have become a popular choice for semiconductor single carrier radiation detectors. The Frisch grid is a conductive screen structure originally fashioned for gas-filled ion chambers and is located near the anode.  Generally, a potential is applied to the device such that negative charges (electrons) drift through the grid and to the anode. A signal is induced on the anode by the charge motion between the grid and the anode, whereas the Frisch grid screens out the induced signal from slow moving positive ions drifting towards the cathode.  Placing the grid near the anode ensures that the origin of induced charge is from those electrons that drifted from the detector volume into the measurement region, thereby causing the signal to form mainly from electron motion.


Several methods of creating a Frisch grid effect without an embedded grid have been studied in semiconductor detectors, which include “co-planar” and “small-pixel-effect” devices. An alternative design, first demonstrated by Prof. McGregor, achieves single-carrier performance by placing side contacts as the Frisch grid. Unfortunately, all of the aforementioned designs suffer from problems, which include leakage current between the anode and grid, processing difficulties, or electric field distortions.

The non-contacting Frisch ring detector, described and patented by D.S. McGregor and R.A. Rojeski, eliminates grid-to-anode leakage current while still achieving single-carrier performance. The design utilizes a bar-shaped detector inserted into a conductive ring, which also allows for an insulator filling between the Frisch ring and the detector body. The conductive ring, when connected into the circuit between the anode and the cathode, confines the largest change in the weighting potential near the anode while effectively screening induction from charge motion in the region extending from the ring edge to the cathode.  The applied voltage need only drift electrons towards the anode, hence a variety of biasing schemes can be used, which includes connecting the ring directly to the cathode. The result of the simple, yet elegant, approach is outstanding gamma ray spectroscopic performance for a relatively inexpensive design. It is expected that the detectors will soon be available in the commercial market.


The high-energy resolution gamma ray detector consists of a bar-shaped detector placed inside a conductive ring. The CdZnTe device is completely separate from the non-contacting Frisch ring, and can be inserted into the ring after a teflon or other insulating coating has been applied. The simple device elimnates leakage current between the grid and the anode, a common problem with other single charge carrier detector designs. The simple device has yielded energy resolution better than 1.7% FWHM at 662 keV and can be tiled to produce a collimated array of gamma ray spectrometers.


Semiconductor Radiation Detectors with Frisch Collars and Collimators for Gamma Ray Spectroscopy, DOE NEER Grant, 2003-present. [report]


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