Apparatus and method for OSL-based, remote radiation monitoring and spectrometry

[0001] This invention was made with Government support under Contract DE-AC0676RLO1830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.

[0002] The present invention generally relates to the field of zero- or low-power, remote radiation detection, and more particularly, to OSL-based radiation sensors and spectrometers and a method for extracting spectroscopic information from said spectrometer.

[0003] Commercially-available devices for unattended gamma-ray detection and spectrometry commonly have relatively high power requirements and contain expensive components. The high power consumption and cost renders most of the commercially-available, gamma-ray sensors and spectrometers unsuitable for long-term, unattended radiation detection and spectrometry. For example, typical sensors or spectrometers utilizing scintillators or room-temperature semiconductors as detectors must constantly draw power to record the energy of each incident event since the pulse processing electronics in most of these detectors must be “always on.” Furthermore, the components in these sensors can be prohibitively expensive for one-time use and/or large-scale deployment in most, if not all, applications.

[0004] While OSL materials have been used in environmental radiation dosimetry for years, and have recently gained widespread use in personnel dosimetry, in the aforementioned dosimetry modes, OSL cannot provide spectroscopy information nor would it serve as an effective, long-term, remote radiation sensor. Thus, a need for zero-power to ultra-low-power, OSL-based sensors and spectrometers for long-term, unattended radiation monitoring exists.

[0005] In view of the foregoing and other problems, disadvantages, and drawbacks of traditional compact, radiation detection technologies, the present invention has been devised. The invention encompasses an apparatus for and a method of gamma-ray and neutron detection and spectroscopy. One embodiment of the apparatus encompasses a compact sensor comprising an OSL pixel, a light source and a luminescence detector, both proximally located to the OSL pixel. A power source electrically connects to the light source and the luminescence detector, while a wireless communication device operably connects to the light source, the luminescence detector, and the power source. Another version of the apparatus encompasses a compact spectrometer comprising a plurality of OSL pixels and at least one radiation filter surrounding at least a portion of the OSL pixels for modulating a flux of incident radiation. The compact spectrometer can acquire spectroscopic information regarding the incident radiation flux.

[0006] The invention also encompasses a method for acquiring spectroscopic information about the incident radiation, which can comprise gamma-rays and/or neutrons. For a compact spectrometer that has been exposed to an incident flux of radiation, the method comprises the steps of optically stimulating the plurality of OSL pixels, thereby resulting in a readout luminescence of deposited energy values, and then detecting the readout luminescence to obtain a readout signal. An incident-energy spectrum can then be reconstructed from the readout signal.

[0007] It is an object of the present invention to provide compact, zero- to ultra-low-power sensors and spectrometers capable of long-term, unattended radiation monitoring.

[0008] It is another object to provide effective and economical sensors and spectrometers for large-scale deployment in a variety of operational environments.

[0009] It is yet another object to develop sensors and spectrometers for instrumented containers.

[0010] A further object is to provide the capability to use pulsed light sources for interrogation of the OSL pixels in the sensors.

[0011] Another object of the present invention is to further expand the applicability of OSL materials, especially in applications where zero or ultra-low power consumption is advantageous or required.

[0018] Many of the shortcomings of commercially-available devices could be overcome using a radiation detection and/or spectrometry device based on optically stimulated luminescence (OSL) materials. OSL materials constitute an integrating detector technology and are capable of recording total deposited energy during a measurement period. An enabling mechanism can be the creation of trapped charge carriers in the material by the ionizing radiation. Depending on the particular OSL material, these trapped charge carriers can have lifetimes measurable on geologic time scales. OSL materials and dosimetry are discussed in detail in “Optically Stimulated Luminescence Dosimetry” written by Lars Botter-Jensen, S. W. S. McKeever, and A. G. Wintle (Elsevier, 2003), which detailed discussions are incorporated herein by reference.

[0019] The present invention is directed to long-term and/or unattended radiation monitoring using OSL-based sensors and spectrometers. One version of the invention encompasses a compact sensor comprising an OSL pixel, a light source, a luminescence detector, a power source, and a wireless communication device. Referring to FIG. 1, the light source 101 interrogates the OSL pixel 102 by optically exciting the OSL material at a first wavelength while the luminescence detector 103 converts light emitted at a second wavelength into a readout signal. Accordingly, both the light source 101 and the luminescence detector 103 can be proximally located to the OSL pixel and connected electrically to the power source 104. As used herein, proximally located can refer to the position of the light source and the luminescence detector relative to the OSL pixel, such that the OSL pixel can be exposed to light emitted from the light source and the detector can be exposed to light emitted by the OSL pixel. The light source 101, luminescence detector 103, and OSL pixel 104 need not be coaxial, but should not be positioned such that exposure of the OSL pixel or the detector to light from the light source or the OSL pixel, respectively, is prevented. Furthermore, optical elements 105 can be used to tune and direct the light delivered to both the OSL pixel 102 and the luminescence detector 103. Such elements can be placed in an optical path between the excitation light source and the OSL pixel as well as between the OSL pixel and the luminescence detector, and can include among other things, lenses, filters, prisms, gratings, mirrors, and apertures. As will be appreciated by one skilled in the art, the distance between the light source and the OSL pixel and between the OSL pixel and the detector can affect the intensity of the exposure. Larger distances may require longer exposure times and/or more sensitive OSL materials and detectors. The wireless communication device 106 imparts remote-readout capability and power-consumption control as described below. The wireless communication device can operably connect to the light source 101, the luminescence detector 103 and the power source 104. An enclosure can contain the above-mentioned components.

[0020] The OSL pixel can comprise a number of OSL materials including, but not limited to Al₂O₃, doped Al₂O₃, Pb-loaded Al₂O₃, BaFBr, and doped BaFBr, as well as other storage phosphors. The OSL materials can also be mixed with high-Z materials to make composites such as the Pb-loaded Al₂O₃. Examples of doped Al₂O₃ and BaFBr include Al₂O₃:C and BaFBr:Eu, respectively. More broadly, the OSL pixel can comprise OSL materials having decay constants less than approximately 100 msec. As used herein, “decay constant” refers to the time for the luminescence intensity to decrease to a value of (1/e) I_p, where I_p is the peak intensity. The pixels can comprise a variety of shapes, for example, planar, cylindrical, spherical, and combinations thereof. Cylindrical and spherical pixels can be advantageous because the attenuation length can have a relatively reduced angular dependence.

[0021] The compact sensor can further comprise a radiation filter surrounding the OSL material. The radiation filter can serve to isolate particular energy ranges of interest for detection and can comprise an attenuating material such as metals, alloys, ceramics, glasses, polymers, and combinations thereof. For neutron detection, the sensor can further comprise a neutron converting material that surrounds the OSL pixel. The neutron converting material can comprise ¹⁰B, ⁶Li, Gd, Sm, or combinations thereof.

[0022] In one embodiment of the invention, referring to FIG. 2a, the OSL pixel 203 is cylindrical and the radiation filter comprises an open-ended radiation filter “sheath” 201. Incident radiation 202 can impinge the pixel 203 normal to the longitudinal axis of the cylinder. Readout of the OSL pixel would involve illuminating the end of the pixel with the excitation light source 204 that is essentially parallel to the longitudinal axis of the pixel. The luminescence detector 205 would then detect the resultant OSL emission from the opposite end. In another embodiment, the OSL pixel and radiation filter are spherical, as shown in FIG. 2b.

[0023] Ionizing radiation that impinges an OSL pixel can generate trapped charge carriers in the OSL material. In some OSL materials, the trapped charge carriers can have lifetimes measurable on geologic time scales. Thus, appropriately-selected OSL materials can exhibit a “memory” of incident radiation events. This memory enables intermittent readout while the sensor is in the field. Stimulation by the light source depopulates the trapped charge carriers, which can result in the emission of a characteristic luminescence photon. Light sources can include, but are not limited to flash lamps, light emitting diodes (LED), arc lamps, and lasers and can be continuous-wave or pulsed. The appropriate wavelength for depopulating the trapped charges can depend on the particular OSL material and can be determined by measuring the OSL emission spectra and then finding the optimal excitation wavelengths that maximize the OSL. A preferred technique can comprise measuring the OSL emission peak while scanning across a wide band of excitation wavelengths until the optimal wavelengths are found. Alternatively, the open literature can be used to determine the best excitation and emission wavelengths for known OSL materials.

[0024] The use of pulsed light can be preferred because it can minimize the generation of heat often associated with continuous-wave sources, thereby eliminating the need for heat sinks and minimizing competing thermoluminescent emissions from the OSL material. Pulsed light sources can also allow for less stringent optical filtering as well as self-calibration. If the OSL output is measured between pulses after the excitation light has died away, then less optical filtering is required compared to measuring the OSL signal simultaneously with the excitation light. A self-calibration procedure can be performed internally by comparing the detected light signal with one or more of the excitation pulses to ensure that the signal resulting from the combination of the light source and the photodetector is within acceptable tolerances. If the light source becomes too intense or too weak, or if the photodetector becomes faulty, the resulting signal will change over time and this self-calibration procedure can reveal the problem.

[0025] The ability to average multiple luminescence counts resulting from a plurality of excitation pulses can provide a criterion for rejecting scintillation events. Specifically, a probability distribution on the mean luminescence response for each excitation pulse can be averaged over multiple pulses to discriminate against scintillation events. Thus, readout measurements can comprise a luminescence response from a single stimulation pulse or an averaged luminescence response from a plurality of stimulation pulses.

[0026] In one example of the compact sensor, referring to FIG. 3, pulses of green light at approximately 530 nm±30 nm, stimulated the OSL pixel, which subsequently resulted in the emission of blue light at approximately 420 nm±30 nm. The plot shows the luminescence responses from OSL pixels that had previously received a variety of gamma-ray exposures ranging from approximately 50 microR to 1600 microR, though the present invention is not limited to these particular values. The pulse duration 301 was approximately 5 msec and the elapsed time between pulses 302 was approximately 280 msec. The OSL material had a decay constant of roughly 30 msec.

[0027] In preferred embodiments, the compact sensor would utilize OSL pixels having very short decay constants such as BaFBr-based materials, which can have decay constants of approximately 800 nanoseconds. OSL materials with very short decay constants can reduce the need for high-sensitivity detectors by optimizing the signal to noise ratio of the detected luminescence. Regardless of which OSL material is used, the invention can employ pulse separation times ranging from a value approximately greater than, or equal to, the decay constant to a value approximately less than, or equal to, 10 times the decay constant.

[0028] While the luminescence detector can comprise several types of photodetectors such as photomultiplier-tube (PMT) photosensors and charge-coupled devices (CCD), photodiodes can be preferred based on current costs and availability. A non-limiting example of a compact, photodiode-based radiation sensor can include a single yellow LED, a BaFBr-based OSL pixel, and a Si photodiode. After exposure to ionizing radiation and the creation of trapped charges, the OSL pixel can be excited by the yellow light at approximately 590 nm and will subsequently emit blue luminescence photons at approximately 390 nm±40 nm, which can be detected by the Si photodiode.

[0029] The compact sensor encompassed by the present invention can further comprise a wireless communication device operably connected to the light source, the luminescence detector, and the power source. The wireless communication device can communicate from the field instances of ionizing radiation detected by the sensor. Communication modes can include radio frequency (RF), microwave, infrared (IR), and more. One version of the wireless communication device can comprise processors, or logic components, operably connected to an RF antenna. The processors, or logic components, manage the data and the communication signals while the RF antenna receives and transmits sensor commands and radiation data.

[0030] In addition to communicating sensor radiation data, the wireless communication device can receive commands associated with regulation of the sensor's power consumption. For example, the compact sensor can have at least two power consumption modes. A first mode would draw only enough power to maintain a communication link so that the sensor can receive particular commands such as a wireless “activation” signal. In the first mode, the light source and the luminescence detector would be deactivated and would not draw power. However, incidents of ionizing radiation would still create trapped charges, which would be retained in the OSL material until depopulated by optical stimulation.

[0031] Upon receiving the activation signal, the sensor would “wake up” and enter a second mode in which the light source, the luminescence detector and the wireless communication device would be fully active. In this active state, the sensor would perform readouts of the OSL pixel and transmit the associated data, which would include the amount of radiation exposure received by the sensor. A subsequent wireless “hibernation” signal would then initiate a return to the first mode to preserve power.

[0032] Additional power consumption modes may exist for purposes of transmitting sensor location, performing state-of-health checks, and “erasing” the trapped charges in the OSL pixel. Alternatively, mode switching can be initiated based on predetermined values of elapsed time. For example, the sensor may perform intermittent readouts every 24 hours for five minutes each time. Device wake up methods and apparatus have been described in detail in U.S. patent application Ser. No. 10/263,940, titled “Radio Frequency Identification Devices, Backscatter Communication Device Wake-up Methods, Communication Device Wake-up Methods, and a Radio Frequency Identification Device Wake-up Method” by inventors Richard M. Pratt and Mike Hughes, which detailed descriptions are incorporated herein by reference. Attention is also directed to the following commonly assigned applications, which details regarding wireless communication devices and methods are incorporated herein by reference:

[0033] U.S. patent application Ser. No. 10/263,826 entitled “Radio Frequency Identification Device Communication Systems, Wireless Communication Devices, Backscatter Communication Methods and Radio Frequency Identification Device Communication Methods” by inventors Mike A. Hughes and Richard M. Pratt; U.S. patent application Ser. No. 10/263/809, entitled “Method of Simultaneously Reading Multiple Radio Frequency Tags, RF Tag, and RF Reader”, by inventors Emre Ertin, Richard M. Pratt, Mike A. Hughes, Kevin L. Priddy, and Wayne M. Lechelt; U.S. patent application Ser. No. 10/263,873, entitled “RFID System and Method Including Tag ID Compression”, by inventors Richard M. Pratt and Mike A. Hughes; U.S. patent application Ser. No. 10/264,078, entitled “System and Method to Identify Multiple RFID Tags”, by inventors Mike A. Hughes and Richard M. Pratt; U.S. patent application Ser. No. 10/263,997, entitled “Wireless Communication Systems, Radio Frequency Identification Devices, Methods of Enhancing a Communications Range of a Radio Identification Frequency Device, and Wireless Communication Methods”, by inventors Richard Pratt and Steven B. Thompson; U.S. patent application Ser. No. 10/263,670, entitled “Wireless Communications Devices, Methods of Processing a Wireless Communication Signal, Wireless Communication Synchronization Methods and a Radio Frequency Identification Device Communication Method”, by inventors Richard M. Pratt and Steven B. Thompson; U.S. patent application Ser. No. 10/263,656, entitled “Wireless Communications Systems, Radio Frequency Identification Devices, Wireless Communications Methods, and Radio Frequency Identification Device Communications Methods”, by inventors Richard Pratt and Steven B. Thompson; U.S. patent application Ser. No. 10/263,635, entitled “A Challenged-Based Tag Authentication Model”, by inventors Mike A. Hughes and Richard M. Pratt; U.S. patent application Ser. No. 09/589,001, filed Jun. 6, 2000, entitled “Remote Communication System and Method”, by inventors R. W. Gilbert, G. A. Anderson, K. D. Steele, and C. L. Carrerider; U.S. patent application Ser. No. 09/802,408; filed Mar. 9, 2001, entitled “Multi-Level RF Identification System”; by inventors R. W. Gilbert, G. A. Anderson, and K. D. Steele; U.S. patent application Ser. No. 09/833,465, filed Apr. 11, 2001, entitled “System and Method for Controlling Remote Device”, by inventors C. L. Carrender, R. W. Gilbert, J. W. Scott, and D. Clark; U.S. patent application Ser. No. 09/588,997, filed Jun. 6, 2000, entitled “Phase Modulation in RF Tag”, by inventors R. W. Gilbert and C. L. Carrender; U.S. patent application Ser. No. 09/589,000, filed Jun. 6, 2000, entitled “Multi-Frequency Communication System and Method”, by inventors R. W. Gilbert and C. L. Carrender; U.S. patent application Ser. No. 09/588,998; filed Jun. 6, 2000, entitled “Distance/Ranging by Determination of RF Phase Delta”, by inventor C. L. Carrender; U.S. patent application Ser. No. 09/797,539, filed Feb. 28, 2001, entitled “Antenna Matching Circuit”, by inventor C. L. Carrender; U.S. patent application Ser. No. 09/833,391, filed Apr. 11, 2001, entitled “Frequency Hopping RFID Reader”, by inventor C. L. Carrender.

[0034] The power source supplies the electrical power required by the compact sensor, including the light source and the luminescence detector during active readout as well as the wireless communication device. Examples of suitable power sources can include, among others, batteries, fuel cells, solar cells, and combinations thereof.

[0035] The present invention can also encompass a compact spectrometer comprising a plurality of OSL pixels and at least one radiation filter surrounding a portion of the OSL pixels. Each of the various OSL materials and radiation filters can serve to detect and modulate a particular energy range of the incident radiation flux, respectively. Collectively, the array of pixels not only detects the presence of ionizing radiation, but it can also collect spectroscopic information, which can be extracted from the relative luminescence signal of each pixel to produce an incident energy spectrum.

[0036] The plurality of OSL pixels can form an array in which the pixels comprise at least one OSL material. While many OSL materials may provide acceptable performance, those having decay constants less than or equal to approximately 100 msec are preferred. The OSL material can comprise Al₂O₃, Pb-loaded Al₂O₃, BaFBr, etc., and combinations thereof. The invention also encompasses doped Al₂O₃ and BaFBr materials including, but not limited to Al₂O₃:C and BaFBr:Eu. The pixels can also comprise a variety of shapes, for example, planar, cylindrical, spherical, or combinations thereof.

[0037] The radiation filters can comprise attenuating materials of differing compositions, shapes, and thicknesses. The materials can include, but are not limited to metals, alloys, ceramics, glasses, polymers, minerals, and combinations thereof. Specific metals include, but are not limited to Pb, W, Al, and combinations thereof. The shape of the radiation filter can be planar, cylindrical, spherical, or combinations thereof and can vary in thickness for different pixels in the OSL array. For neutron detection, the spectrometer can further comprise a neutron converting material that surrounds the OSL pixel. The neutron converting material can comprise ¹⁰B, ⁶Li, Gd, Sm, or combinations thereof.

[0038] FIG. 4a depicts a simple, passive spectrometer having an OSL array 401 and a radiation filter 402 and illustrates the modulation of an incident radiation flux 403. The planar OSL pixels are partially encompassed by a “wedge-shaped” filter. For ionizing radiation that is normal to the surface of the OSL array, the radiation filter attenuates the energy of the incident radiation according to the thickness of the graded filter. Pixels in the portion of the array covered by relatively thin sections of the radiation filter will experience a greater exposure to the radiation flux compared to pixels covered by thicker portions of the filter. An example of such filters includes, but is not limited to a Pb filter having a thickness ranging from approximately 1 mm to 10 mm across at least one dimension of the filter. The OSL pixels can include, but are not limited to Al₂O₃ or Pb-loaded Al₂O₃ having a density of approximately 1.9 g/cm³ or 2.4 g/cm³, respectively, and having either a constant pixel thickness or an increasing thickness from approximately 1 to 20 mm across 10 pixels. In a preferred embodiment, referring to FIG. 4b, the pixels and filters composing the array are cylindrically 404 or spherically 405 shaped to minimize the angular dependence of the attenuation length through the filters.

[0039] The compact spectrometer encompassed by present invention can be passive, requiring no power, or it can be semi-active, requiring minimal power until activated. In the ultra-low power consumption, or semi-active, embodiment, the compact spectrometer can comprise at least the same components and can function in essentially the same manner as the compact sensor described above with the added capability of acquiring spectroscopic information. In the passive embodiment, OSL-pixel readout occurs after data collection, for instance, in a laboratory facility. Thus, the passive spectrometer comprising a plurality of OSL pixels and at least one radiation filter can be deployed for radiation monitoring, retrieved after a period of time and analyzed in a laboratory, where a trained technician extracts an incident energy spectrum according to the method described below.

[0040] While applicable to general, long-term radiation monitoring, the passive spectrometer is particularly suited for detection and spectroscopy in high-radiation applications, including those that might damage conventional electronics. Examples of such applications include, but are not limited to waste tanks, power plants, radiation hot cells, and down-hole monitoring.

[0041] Relative to the passive spectrometer, the compact, semi-active spectrometer can further comprise at least one light source, at least one luminescence detector, a wireless communication device, and a power source. As described earlier for the compact sensor, readout of the spectrometer pixels involves illuminating the OSL material and detecting the subsequent optical luminescence. Readout of the array in the compact spectrometer can use one or more light sources and detectors; each electrically connected to the power source and operably connected to the wireless transmission device. Optical elements such as lenses, mirrors, filters, prisms, gratings, apertures and masks can be used to tune and direct the excitation light or the luminescence to portions of the array or the detector, respectively.

[0042] The semi-active spectrometer can further comprise a processor, an analog-to-digital converter, and low-noise pulse processing components each operably connected to the power source, the light sources, the detectors, and the wireless communication device. These additional components can serve to process and manage the hardware components, the data, and the power consumption in a similar manner the one described for the compact sensor. The processor can further serve to reconstruct the incident-energy spectrum according to the method described below, which the present invention also encompasses.

[0043] The method provides for the extraction of spectroscopic information from a compact spectrometer comprising a plurality of OSL pixels and at least one radiation filter surrounding at least a portion of the OSL pixels. The steps comprise: 1) providing a compact spectrometer of the present invention that has been exposed to an incident radiation flux, 2) optically stimulating the OSL pixels, 3) detecting the resultant luminescence, and 4) reconstructing an incident-energy spectrum from the detected luminescence.

[0044] Reconstruction of the spectrum can involve solving for s, the incident energy vector, in the following equation:
s=H⁻¹*d
where d is a vector of the mean energy deposited on each OSL pixel and H is a system matrix describing the probability of an incident gamma-ray or neutron being absorbed by the OSL material without colliding and interacting with the attenuation filter. The system matrix can be created by simulating or measuring the modulation of the incident radiation flux by the filter materials. Non-limiting examples of techniques for solving the equation above include direct, ordinary least squares, non-negative least squares, and maximum likelihood. Reconstruction of the spectrum may also comprise analytically solving for the inversion of the system matrix.

[0045] While a number of embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the invention in its broader aspects. The appended claims, therefore, are intended to cover all such changes and modifications as they fall within the true spirit and scope of the invention.