Radiant energy – Source with recording detector – Using a stimulable phosphor
Reexamination Certificate
1999-06-09
2001-11-13
Kim, Robert H. (Department: 2882)
Radiant energy
Source with recording detector
Using a stimulable phosphor
C250S497100
Reexamination Certificate
active
06316782
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to luminescence techniques for imaging radiation fields and, more specifically, to the use of experimental and mathematical methods to distinguish between static and dynamic irradiation and other, related, abnormal radiation exposure conditions. The instant invention would be most fully appreciated by those in the personnel and environmental radiation dosimetry fields, and to fields related thereto.
BACKGROUND
Current radiation dosimetry methods using photographic film take advantage of the imaging capabilities of the film to map a “radiation image”. This is done by inserting radiation absorbing filters between the source of the radiation and the film. When the film is subsequently developed an image of the filters is obtained. This capability is used in the radiation dosimetry community to detect abnormal exposure conditions. The main abnormal exposure condition of interest is the intentional, or accidental, exposure of the film to a radiation source when the radiation film “badge” is not being worn by a person. This might occur, by way of example, if someone placed his or her radiation film badge close to a radiation source in order to expose the badge—but not the person—to the radiation. Such “static” exposures of the badge will produce a sharp image of the filters on the developed film. This is to be distinguished from an exposure in which a person wears the film badge on his or her clothing over an extended period. The images of the radiation absorbing filters in this “dynamic” case are expected to be blurred. By subjectively distinguishing between “sharp” and “blurred” a judgment can be made regarding the likelihood of “static” exposure and, specifically, to distinguish between “static” exposure and the more usual “dynamic” exposure.
Other potential abnormal exposure conditions include the unintentional shielding of the film by external objects (coins, paper-clips, etc.) because the badge was worn incorrectly (say) in a person's pocket, rather than being worn correctly on the outside of a person's clothing. Such exposures are normally detected by observing an image of the object on the developed film. Still other abnormal exposure conditions include contamination of the film badge by radioactive contaminants, or physical damage of the film or the radiation filter. Such conditions typically exhibit “hot spots” or “cold spots” on the developed film.
Others have investigated the imaging of radiation exposure of large area phosphors using optically stimulated luminescence, see, for example, Luckey (U.S. Pat. No. 3,859,527), Ikedo et al. (U.S. Pat. No. 3,957,637), Kotera et al. (U.S. Pat. No. 4,258,264), Kato et al. (U.S. Pat. No. 4,315,318) and Gasiot et al. (U.S. Pat. No. 4,517,463). These references are cited more fully in the attached Bibliography, references (1) to (5) of which are specifically incorporated herein by reference. However, the field of application of each of these patents is medical imaging and the goal in each case is to capture an image of an irradiated object, such as a human body, on a luminescent plate. The latent image on the plate is typically read by scanning with a suitable laser beam. In particular, Luckey (U.S. Pat. No. 3,859,527) records the radiation image by exposing the phosphor to radiation through the object to be imaged, then reading the recorded information by stimulating the phosphor by scanning with a laser beam. The image is read by recording the output of a photodetector as a function of the position of the scanning laser beam on the surface of the phosphor.
Note that the foregoing inventors have focused their efforts on imaging an object, rather than on the problem under consideration herein; namely, the detection of non-standard irradiation of a phosphor sample. Furthermore, the imaging methods published previously do not use the pulsed, synchronized detection techniques described hereinafter.
Heretofore, as is well known in the radiation dosimetry arts, there has been a need for an invention that can provide rapid and reliable detection of abnormal dosimeter exposure conditions over a wide dynamic range of radiation doses (~1 mGy to ~100 Gy) without encountering significant background interference or stimulation light leakage. Additionally, the invention should preferably use a pulsed and synchronized luminescence detection scheme. Further, the invention should provide a complete method and system for abnormal exposure detection including the use of a luminescent thin powder layer, the use of a periodic radiation absorbing filter, the pulsed stimulation and synchronized luminescence detection scheme, and a method of analyzing and interpreting the recorded images. Finally, the system should provide some means of mathematically characterizing an image as containing either a normal or abnormal exposure.
Accordingly, it should now be recognized, as was recognized by the instant inventors, that there exists, and has existed for some time, a very real need for a method and apparatus that would address and solve the above-described problems. Before proceeding to a description of the instant invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.
SUMMARY OF THE INVENTION
According to one aspect of the instant invention, there is provided a method and apparatus for distinguishing between static and dynamic dosimeter exposure conditions, wherein Pulsed Optically Stimulated Luminescence (POSL) is applied to a luminescent material preferably containing aluminum oxide, carbon, and other essential elements, to assess the character of absorbed radiation doses. By way of general background, the technology associated with POSL is more fully described in co-pending patent application Ser. Nos. 08/710,780 and 08/879,385. In the instant invention, a luminescence detector, consisting of a thin layer of luminescent powder deposited on a suitable substrate, or sandwiched between thin films of (say) plastic, is placed under a radiation-absorbing filter, the filter having a spatially periodic structure with areas of higher radiation absorption coefficient and areas of lower, or zero, radiation absorption coefficient. Alternatively, two filters might be used—one positioned on either side of the luminescent powder—with each having spatially periodic absorption characteristics. A two-filter configuration would make the response of the dosimeter indifferent to its orientation—it would not have a “front” side or a “back” side. The detector and filter(s) are then typically made part of a badge or other device for measuring radiation exposure and sent to the field.
Upon its return from the field, the badge and its associated detector are preferably tested for exposure to radiation as follows. The luminescent layer is placed in the path of a beam of stimulating light in such a way as to uniformly, or nearly so, illuminate the whole of the area of interest of the luminescent layer. The stimulating light is preferably pulsed at a predetermined frequency, and the luminescence emitted from the luminescent layer is detected by an imaging system, such as a camera using a charge coupled device (CCD). The light detection is synchronized to occur between the pulsed laser stimulations and the image recorded by the camera is a representation of the pattern of luminescence emission from the luminescent layer, which in turn is a representation of the pattern of radiation absorption a s defined by the structure of the radiation absorbing filter.
The distinction between “static” and “dynamic” irradiation is then made by observation and/or analysis of the luminescent image so-recorded. In one preferred embodiment, a spatial frequency spectral analysis
Akselrod Mark S.
McKeever Stephen W. S.
Fellers Snider Blankenship Bailey & Tippens, P.C.
Kiknadze Irakli
Kim Robert H.
The Board of Regents for Oklahoma State University
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