Radiant energy – Invisible radiation responsive nonelectric signalling – Optical change type
Reexamination Certificate
1998-10-07
2001-06-19
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiation responsive nonelectric signalling
Optical change type
C250S484500, C376S247000
Reexamination Certificate
active
06249004
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a composite material and its use as a high dose dosimeter typically used in the field of ionizing radiation processing and sterilization. More specifically the invention is the incorporation of an inorganic dosimeter material powder into a polymer or a liquid with subsequent formation of composite material dosimeters.
In this patent, the term dosimeter is used to indicate a device that incorporates dosimeter material that absorbs ionizing radiation and is subsequently interrogated for indication of ionizing radiation dose using the methods of either optical absorption and/or optical reflectivity.
BACKGROUND OF THE INVENTION
The present industrial standard high dose dosimeter is a plastic composite dosimeter comprising a mixture of a polymer and an organic dye of various compositions that has the property of absorbing optical photons (UV or visible) in proportion to the amount of ionizing radiation dose. The two primary commercial vendors are Far West (Goleta, Calif.) and Harwell (United Kingdom). While suitable to their purpose, these two manufacturer's products suffer from dose rate dependency, dependency of irradiation temperature during exposure, post-irradiation temperature effects, pre-concurrent-, and post irradiation humidity effects, light-induced response changes, and handling problems that make the dose determination a difficult process. The currently commercially available high dose dosimeters are analyzed by placing the polymer dosimeters within a spectrophotometer that measures optical absorption. One or more wavelengths are used to determine the ionizing radiation dose.
In addition, W. L. McLaughlin et. al. in their paper ELECTRON AND GAMMA-RAY DOSIMETRY USING RADIATION-INDUCED COLOR CENTERS IN LiF,
Radiat. Phys. Chem.
V 14. Pp. 467-480, 1979 describe a method for using single crystals of lithium fluoride as dosimeters. In this method, ionizing radiation creates F-centers in the lithium fluoride crystals. The amount of ionizing radiation received by large single crystals of lithium fluoride has been shown to be related to the absorption of light at a wavelength corresponding to the F-center absorption. In the case of lithium fluoride, the F-center absorption wavelength is in the ultraviolet range with a &lgr;
max
of about 247 nm. While this method has minimal irradiation temperature and dose rate dependence, it relies on a single crystal. Despite efforts to make uniform single crystals, the single crystals reported by McLaughlin were not uniform in their optical absorption response to ionizing radiation and therefore would have to be individually calibrated. Further, alkali halide crystals such as LiF are soft and easily scratched. Once scratched, the crystal is useless for optical measurements. Finally, polished single crystals are far too costly at $10 to $100 each to be used for routine high dose dosimetry.
In addition, A. Waibel et. al. in their paper A METHOD OF INDIVIDUAL CALIBRATION OF LiF OPTICAL ABSORPTION DOSEMETERS,
Radiation Protection Dosimetry
V 47 Pp. 581-583, 1993 describe a method for using single crystals of lithium fluoride as dosimeters at high dose and high temperature. This work validated the use of F-center absorption in single lithium fluoride crystals as a method of dosimetry. Waibel also validated the large (20%) coefficient of variation between the response of different crystals of LiF to the same ionizing radiation dose.
In addition, optically stimulated luminescent dosimetry as taught by Miller in U.S. Pat. Nos. 5,272,348; 5,567,948 and 5,569,927 utilizes M-center re-emission of a photon of longer wavelength than the interrogating photon. This dosimetry is subject to dose rate dependence and temperature effects at high doses which lessens it utility as a routine dosimeter in commercial practice for high dose dosimetry at doses greater than 10 kiloGray.
SUMMARY OF THE INVENTION
The present invention is based upon the discovery that certain plastic/dosimeter and liquid/dosimeter material combinations provide sufficient sensitivity that rapid readings (less than 10 sec) are possible and it is not necessary to remove the dosimeter material from the carrier liquid or plastic for purposes of interrogation. A composite material containing a dosimeter material powder mixed into a carrier can be molded, extruded, or otherwise formed into dosimeters of various shapes is described. Such dosimeter material powder is an inorganic material that exhibits F-center absorption of optical radiation between 200 and 700 nm when exposed to ionizing radiation. The carrier is non-opaque to the optical radiation absorbed by the ionizing radiation induced F-center in the dosimeter material powder. The carrier in the composite dosimeter can be a polymer or a liquid. When a polymer is used, the dosimeter can be made into any convenient shape. When a liquid is used as the carrier, the dosimeter can take the shape of any container. In the present invention, the absorption of light in a composite material dosimeter is used as the basis for the dosimetry.
In addition to traditional methods of transmission-absorption spectrophotometry, the dosimeter can be interrogated with a reflectometer that measures the optical reflection intensity. The greater the ionizing radiation-induced absorption, the less optical light is reflected back to the detector. The reflection method can be used as a quick screening tool to verify if a package has been irradiated or as a less precise (for example, 10% to 25% uncertainty) method of measuring dose. A possible application for such a screening tool is ionizing radiation indication labels for packages being processed with ionizing radiation. In a specific example of the present invention a LiF powder composite polymer dosimeter is quantified by measuring the amount of ultraviolet light absorption in a range of 200-300 nm with the peak of the absorption being measured at 240 nm using an uncalibrated spectrophotometer.
An advantage of the present invention is that multiple particles of the dosimeter material powder provide an integrated response with much less variation than a single crystal. Another advantage of the present invention is the discovery that optical reflection and/or absorption measurements are substantially independent of dose rate and temperature of irradiation.
The basis for this absorption technique is the fact that ionizing radiation exposure produces radiation-induced defects in crystalline solids (i.e. alkali-halides). These ionizing radiation-induced defects cause a characteristic optical absorption that depends upon the type of crystal being irradiated. In the case of LiF, an F-center is created that absorbs ultraviolet light in proportion to the about of ionizing radiation received. LiF and other crystals display monotonically increasing optical absorption with increasing ionizing radiation exposure. Virtually any crystalline solid will undergo some sort of optical absorption when irradiated with ionizing radiation. The temperature stability and light stability and the wavelength of the peak optical absorption are the major differences between the different crystals. At present, LiF powder is the preferred material for its stability, sensitivity, and economics. Other crystalline materials, such as NaCl, may also prove useful after further testing.
As used herein, the term optically transmissive includes a broad range of transmissivities from optically clear for the wavelengths of interest to optically translucent for the wavelengths of interest. Another term considered synonymous is non-opaque.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.
REFERENCES:
patent: 498011
Hannaher Constantine
Israel Andrew
Sunna Systems Corp.
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