Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
2001-08-14
2004-08-24
Porta, David (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S370120
Reexamination Certificate
active
06781134
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates to a portable device for detecting radiation and, more particularly, to a handheld CZT radiation detector.
2. The Relevant Technology
Radioactive materials are unstable and emit radiation in the form of alpha, beta, gamma, or X-rays. Many different types of radiation detectors have been designed and manufactured to produce data corresponding to radioactive materials.
One type of radiation detector is a pulse mode detector, in which a separate electrical pulse is generated for each individual radiation quantum (e.g., a gamma ray) that interacts with a detector. A high-purity germanium detector, which is often cooled by liquid nitrogen, is one example of a pulse-mode detector. By way of example, a gamma ray interacts with a detector surface coupled to a cathode and an anode. A portion of the energy of the gamma ray may be deposited on the detector to produce a charge. From the point of interaction, freed electrons drift towards the anode and ions (or holes) drift towards the cathode. A signal relating to the produced charge is often captured and manipulated by charge-sensitive preamplifiers and shaping amplifiers, resulting in a voltage pulse. Before entering a shaping amplifier, the pulse may have a long tail because the energy produced by the gamma-ray interaction decreases gradually. The shaping amplifier cuts off that tail, enabling detection of more pulses within a fixed period of time. The peak amplitude of such a voltage pulse is proportional to the energy deposited on a detector by a gamma ray.
Analog-to-digital converters (ADCs) are frequently employed to generate a digital number indicating the height, or the amplitude, of each voltage pulse. Such digital pulse data may be gathered and analyzed to learn more about the corresponding radioactive material. For example, the digitized pulse data may be categorized into channels, each channel indicating a specific energy level range into which the amplitude of the pulse falls. Energy levels are often measured in kiloelectron volts (KeVs). Devices that analyze multiple channels of pulse data are called multi-channel analyzers. Pulse data is often displayed on a chart showing the number of pulses (or counts) that the detector receives at a specific energy level range.
These charts frequently show a series of consecutive energy level ranges and a number of counts received in each range. This data so configured is frequently referred to as pulse height data or a pulse height distribution. By analyzing pulse height data, experts in the field may make determinations regarding the corresponding radioactive material. Such determinations may be made by automated analysis algorithms, a visual inspection, or a combination of the two.
Pulse height data is particularly useful in determining the composition of a corresponding radioactive material. Different radioisotopes emit radiation at varying energy levels. For example, plutonium-239 emits gamma radiation of approximately 203, 330, 375, 414, and 451 KeVs, among other energy levels. By examining the energy levels and intensity of such peaks within the pulse height data, the source of the radiation may be identified.
One application of this technology relates to radioisotope detection and identification to prevent illegal transportation of nuclear materials. The U.S. Customs Service, the Federal Bureau of Investigation, the U.S. Secret Service, and the International Atomic Energy Agency (IAEA.) share this common interest. The United States is particularly concerned about shipments of fissionable nuclear materials such as uranium or plutonium. Likewise, environmentalists and consumer health advocates are similarly concerned about detecting and identifying radioactive materials.
A radioisotope detector's resolution affects its ability to accurately detect and identify radioisotopes. Ideally, pulses generated by a detector fall within discrete channels, creating tall and narrow peaks. However, because of poor resolution, peaks are frequently spread over a number of channels. Poor resolution can be the result of a number of factors, including noise produced by pulse-processing electronics and variation in a detector's parameters. If the resolution of a detector is poor, it may be impossible to identify discrete peaks characteristic of a particular radioactive material because the peaks will simply blend together and be indistinguishable, thus making it difficult or impossible to accurately identify a corresponding radioisotope.
The resolution of a detector is often measured employing a full-width-at-half-maximum (FWHM) terminology. FWHM may be expressed as a ratio of the width of the peak at half of the peak's maximum value over the peak's maximum value. This ratio is frequently given as a percentage, and small values correspond to narrow peaks and good energy resolution.
Conventionally, thallium (Nal(Tl)) scintillators and high-purity germanium detectors have been used for in-field radioisotope analysis. Scintillators, however, suffer from poor resolution, resulting in a low-confidence level in data produce thereby. The resolution of a scintillator is typically about only 7% FWHM at 662 KeV.
While high-purity germanium detectors provide excellent resolution, they suffer from a number of serious disadvantages. These detectors require in-field calibration to ensure the accuracy of the readings taken. To calibrate these devices, of course, requires transportation of a radioactive material from which to gauge the detector. This is extremely cumbersome, dangerous, and frequently requires governmental licensing. Furthermore, high-purity germanium detectors must be cooled to liquid-nitrogen temperatures and are extremely fragile. Extensive training is required to correctly operate this type of detector.
Another concern with conventional portable radiation detectors deals with automated analysis to determine the composition of a corresponding radioactive material. Conventional techniques, such as the Chi-square analysis, are inflexible and do not consider the variables present with in field analysis. In-field analysis often involves unknown distances between the detector and radioactive material, an unknown form of the radioactive material (gas, solid, or liquid), and unknown barriers between the radioactive material and detector. Conventional techniques are rigid and, as such, may provide unsatisfactory results given these variables. Moreover, the conventional techniques are computationally intensive, particularly for the limited resources of portable devices. Thus, employing such techniques may draw substantial resources (e.g., battery power) from the portable detector and may not produce timely or accurate results.
Consequently, it would be an advancement in the art to provide a portable radiation detector having automated radioisotope identification capabilities sufficiently flexible to adapt to the variables of in-field analysis. It would be a further advancement in the art to provide a portable radiation detector which operates at room temperatures, does not require in-field calibration, is not fragile, does not require training to use, and yet provides higher resolution.
BRIEF SUMMARY OF THE INVENTION
A handheld cadmium zinc telluride (CZT) radiation detector provides a portable radiation detector implementing a fuzzy-logic radioisotope identification procedure adapted to in-field analysis. This fuzzy-logic procedure is computationally less intensive and more flexible than conventional identification algorithms used in portable radiation detectors, thus providing timely and more accurate results to an end-user and extending the detector's battery life. Furthermore, in one embodiment, the handheld CZT radiation detector implements a coplanar grid CZT gamma ray sensor which provides higher resolution than conventional scintillators, but does not require in-field calibration or cooling to liquid-nitrogen temperatures like high-purity germanium detectors. To be more specific, the ha
Baird William
Butterfield Kenneth B.
Murray William S.
Madson & Metcalf
Porta David
Sung Christine
The Regents of the University of California
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