Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
1998-05-08
2001-01-16
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370060
Reexamination Certificate
active
06175120
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is related to copending application entitled “Ionization Detector, Electrode Configuration and Single Polarity Charge Detection Method” filed Dec. 11, 1996 and having U.S. Ser. No. 08/763,675, now U.S. Pat. No. 5,777,338, provisional application entitled “Semiconductor Single Charge Carrier Device or Semiconductor Single Carrier Spectrometer” filed Jul. 10, 1997 and having Ser. No. 60/052,092, and application entitled “Method and Apparatus for Electron-Only Radiation Detectors From Semiconductors Materials” filed on the same day as this application.
1. Technical Field
This invention relates to ionization detectors and, in particular, to high-resolution ionization detectors and an array of such detectors.
2. Background Art
Ionization detectors such as semiconductor radiation detectors have been studied since 1945, in which tiny AgCl crystals cooled to low temperatures were used to observe radiation induced electrical pulses. Such devices were impractical due to the stringent cooling requirements and their diminutive size. Semiconductor radiation detectors were interesting devices and were studied for many years, but were deemed impractical when compared to the (then) superior performance of scintillation detectors.
The introduction of Li drifting in Ge and Si by E. M. Pell provided a method to produce reasonably large Ge and Si detectors with much higher energy resolution than scintillators could achieve. Hence, single element semiconductors radiation detectors became the main devices used for high energy resolution measurements of ionizing radiation.
The remaining inconvenience to the Ge(Li) and the Si(Li) detectors is the requirement of cryogenic operating temperatures (77K). Si(Li) detectors must be cooled in order to reduce the concentration of thermally excited charge carriers that add noise to the measurement, thereby reducing the energy resolution. The case for Ge(Li) devices is much more severe, in which the crystal must remain cooled to 77K to prevent redistribution of the Li in the device (through diffusion), as well as reduce thermal noise. Ge(Li) detectors are ruined if they are allowed to warm up to room temperature, and the devices can only be re-drifted once or twice before severe gamma ray energy resolution degradation appears.
Zone refinement of Ge eventually eliminated the need for Li drifting in Ge material to produce high-energy resolution gamma ray detectors. The high purity Ge (HPGe) detectors do not need to be constantly cooled, but must be cooled to 77K during operation to reduce thermal noise and to prevent harmfully high leakage currents from passing though the detector. Hence, the cryogenic requirement remains. Float zone refinement of Si also has worked to produce high quality material for charged particle detectors, but Si based X-ray detectors are still manufactured from Li drifted material.
Overall, the inconvenient cryogenic operating condition required for Ge and Si radiation detector operation has severely limited their use, and remotely operated devices are still manufactured mainly from scintillators. Also, the gamma ray absorption efficiency of Ge and Si are tremendously less than that of inexpensive and readily available scintillators such as NaI(Tl). The case is much worse when one considers that NaI(Tl) material can be obtained in sizes greater than 2 feet in diameter and several inches thick, whereas the largest commercially available HPGe detectors are only 5 inches in diameter and perhaps 5 inches thick. Additionally, the cost of such a large HPGe device could shock even the most dedicated gamma ray spectroscopist into preferring a NaI(Tl) detector when gamma ray efficiency is the primary concern.
One method to increase intrinsic gamma ray interaction efficiency in a semiconductor radiation detector is to utilize semiconductors composed of relatively high atomic number elements. Such a condition becomes practical only if the material of choice is available in reasonably large volume. A method to reduce the cryogenic cooling requirement is to use a semiconductor with a relatively large band gap (1.4 eV-2.5 eV) such that the concentration of thermally produced charge carriers is minimal at room temperature. Hence, a reasonably competitive semiconductor material for a room temperature operated gamma ray spectrometer would be composed of materials with high Z values and have a wide band gap. The only single element semiconductors are carbon (diamond—C), silicon (Si), germanium (Ge) and tin (Sn). Carbon does not satisfy the high Z requirement, and Si does not satisfy the band gap requirement or the high Z requirement. Ge barely meets the Z requirement with its atomic number of 32, but fails the band gap energy requirement, and Sn has overlapping conduction and valence bands that makes it perform as a conductor rather than a semiconductor. As a result, the only choices that remain for room temperature operated semiconductor radiation detectors are compound semiconductors.
The search for a semiconductor material to operate as a room temperature gamma ray spectrometer has been conducted for many decades and only recently have promising results been observed. W. J. Price's initial concerns were valid, and he remains correct in his assumptions regarding compound semiconductor radiation detectors to this very day. Imperfections and charge carrier “traps” impede detector performance and cause undesirable degradation in the observed energy resolution. To reduce trapping effects, compound semiconductor radiation detectors have been manufactured to be very small, on the order of a few tens of microns to a few millimeters in thickness. Such devices are generally considered interesting novelties, yet have only practical applications where tiny room temperature operated radiation detectors are required. The major difficulty with charge carrier extraction from a semiconductor radiation detector is the misbalance of hole charge carrier characteristics with that of electron charge carrier characteristics. Generally, holes suffer from trapping losses much more than electrons, and the resulting position dependent charge collection characteristics cause severe energy resolution degradation.
CdZnTe material only recently has become commercially available, and the material showed promise as a possible room temperature operated gamma radiation spectrometer. With a band gap near 1.6 eV and material atomic numbers of 48, 30 and 52, CdZnTe has the fundamental properties required for a room temperature operated semiconductor detector. Permutations of the single polarity electron sensing idea have been realized with CdZnTe material with interesting results. For instance, the co-planar grid concept allowed for the realization of fairly high-resolution CdZnTe detectors at room temperature with energy resolution of 3.7% FWHM at 662 keV for a 5 mm×5 mm×5 mm device (125 mm
3
) as described in the U.S. patent to Luke U.S. Pat. No. 5,530,249. These results placed much attention on the co-planar design, in which two series of serpentine patterned parallel anode electrodes were biased at different voltages on one surface and a large single cathode electrode was biased on the opposite surface. The claim that a Frisch grid could not be made on or in a semiconductor was announced as well by Luke.
Later results using the co-planar single polarity device sensing concept demonstrated 3.4% FWHM at 662 keV for another 5 mm×5 mm×5 mm device (125 mm
3
) as described in the above-noted application Ser. No. 08/763,675. Yet, due to imbalances in the induced charge “weighting distribution” of the co-planar devices and the remaining effect of electron trapping, higher energy resolution was not observed. With electronic correction techniques, a 1 cm
3
CdZnTe co-planar detector with 1.79% FWHM at 662 keV has been demonstrated.
A permutation of the co-planar idea is described in U.S. Pat. No. 5,677,539, in which a single dot electrode was put on one surface and surrounded by what is referred to as a “contro
McGregor Douglas S.
Rojeski Ronald A.
Brooks & Kushman P.C.
Hannaher Constantine
The Regents of the University of Michigan
LandOfFree
High-resolution ionization detector and array of such detectors does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with High-resolution ionization detector and array of such detectors, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High-resolution ionization detector and array of such detectors will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2482788