Radiant energy – Invisible radiation responsive nonelectric signalling – Luminescent device
Reexamination Certificate
1999-05-11
2002-07-02
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiation responsive nonelectric signalling
Luminescent device
C250S484400, C250S484300
Reexamination Certificate
active
06414324
ABSTRACT:
TECHNICAL FIELD
This invention relates to the general subject of ultraviolet radiation detection and, more specifically, to methods and devices for detection and measurement of exposure to ultraviolet-B radiation, wherein the amount of exposure is measured using either thermoluminescence or optically stimulated luminescence from &agr;-Al
2
O
3
:C.
BACKGROUND OF INVENTION
Atmospheric ozone is generally considered to absorb all but approximately 1% of wavelengths below about 320 nm. Recent evidence of ozone depletion in the stratosphere, however, has generated interest in the biological impact on plants and animals resulting from increased exposure to wavelengths below 320 nm. This has created the need for ultraviolet (UV) radiation dosimetry for wavelengths in the UVB region of the electromagnetic spectrum, namely from 320 nm to 280 nm. Specifically, in order to search for possible links between enhanced UVB exposure and potential DNA damage in plants and/or animals the need for a small, portable, integrating UVB dosimeter has arisen. Additionally, the ideal device should also be unaffected by variations in ambient temperatures and by humidity and should be a passive—as opposed to an electronic or active—device, i.e., a device that does not require electrical power while it is operating.
One example of the current state-of-the-art in passive UVB dosimeters is one based on biological indicators such as
Bacillus subtilis,
pre-Vitamin D and bacteriophage 77. (See, for example, Quintern, L. E., Puskeppeleitm M., Rainer, P., Weber, S., El Naggar, S., Escweiler, U., and Horneck, G.
Continuous Dosimetry of the Biologically Harmful UV-Radiation in Antarctica with the Biofilm Technique,
in Photochem. Photobiol. B, 22, 59-66 (1994), the disclosure of which is incorporated herein by reference). These sorts of dosimeters are small in size, portable, do not need a power source, and have a linear response to increasing radiation.
Another approach to UV dosimetry measurement involves the use of thermoluminescence (TL). Thermoluminescence is the luminescence emitted from a suitable phosphor when the phosphor is heated following exposure to radiation. The intensity of the TL emitted is a measure of the dose of the absorbed radiation. For UV dosimetry, two approaches are generally used. The first approach is to expose the material directly to UV and then to heat the phosphor immediately after this exposure, yielding a TL signal which is related to the dose of absorbed UV radiation. The alternative approach is to pre-treat the sample by irradiating it with ionizing radiation (such as gamma or beta radiation) which places electronic charge into metastable charge centers (or “defects”) within the phosphor's crystal lattice. After the pre-treatment, the sample is exposed to UV radiation, which transfers the electronic charge into defect centers that can be directly stimulated by subsequently heating the sample. During heating, a phototransferred TL (or PTTL) signal is recorded, the intensity of which—for a given gamma or beta radiation dose—is proportional to the UV exposure. Whether the TL or PTTL approach is used, the sensitivity of the detector to the different wavelengths of UV depends critically upon the material chosen as the phosphor. Previous work by the instant inventors and others (see, for example, Colyott, L. E., Akselrod, M. S. and McKeever, S. W. S.,
Phototransferred Thermoluminescence &agr;
-
Al
2
O
3
:C
_ Radiat. Prot. Dosim 65, 263-266 (1996), the disclosure of which is incorporated herein by reference) has shown that &agr;-Al
2
O
3
:C offers many of the favorable properties that one would desire in a UV dosimeter. For example, this material is a sensitive TL radiation detector and it displays a PTTL sensitivity to wavelengths in the UVB range that, along with other desirable properties, make it potentially a versatile base upon which to construct a UVB dosimeter with the desired basic characteristics outlined above.
In addition to its favorable TL and PTTL properties, however, it has been demonstrated in the literature that this material is a sensitive optically stimulated luminescence (OSL) radiation detector (see, for example, Bøtter-Jensen, L. and McKeever, S. W. S.,
Optically Stimulated Luminescence Dosimetry Using Natural and Synthetic Materials,
Radiat. Prot. Dosim. 65, 273-280 (1996), the disclosure of which is incorporated herein by reference). In the OSL mode of operation, a sample previously irradiated with gamma or beta radiation will luminesce when illuminated with light in the visible range of wavelengths. The luminescence signal is termed OSL and several illumination methods are available, including continuous or steady-state illumination (cw-OSL), pulsed illumination (POSL), and linearly modulated illumination (LM-OSL). Among the many advantages of using an optical stimulation method rather than a thermal stimulation method are that the need for heating the sample is removed. Therefore, the devices that read the luminescence emission require less electrical power to operate; and, most importantly, by stimulating the luminescence emission at low temperature (specifically, ambient temperature) the problem of thermal quenching of the luminescence is avoided. Thermal quenching in &agr;-Al
2
O
3
:C is an effect in which the luminescence efficiency decreases as the temperature increases (see, Akselrod, M. S., Whitley, V., Agersnap Larsen, N., and McKeever, S. W. S.,
Thermal Quenching of Luminescence from Aluminum Oxide,
J. Appl. Phys. 84, 3364-3372 (1998), the disclosure of which is incorporated herein by reference). Thus, not only is &agr;-Al
2
O
3
:C known to be one of the most sensitive TL phosphors currently available, but it is even more sensitive, in terms of luminescence output per unit absorbed radiation dose, when used as an OSL phosphor.
Thus, it should be clear to those familiar with the UV dosimetry arts that there is, and has been for some time, a need to develop a small, portable, integrating dosimeter capable of sensitively measuring doses of absorbed UVB radiation. Additionally, the resulting dosimeter should be capable of measuring integrated UVB exposures of durations ranging from a few minutes to several days of total exposure with a near-linear response to the total UV exposure. Further, the dosimeter should be capable of measuring UVB exposure in either air or water. Still further, the UV radiation to which the dosimeter has been exposed should be determinable via either a TL technique or an OSL technique. Even further, the dosimeter should exploit the many advantages of using &agr;-Al
2
O
3
:C as a UV detecting material. Accordingly, it should be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a device that exhibits the various characteristics described above.
Before proceeding to a detailed description of the present 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 a first aspect of the instant invention, there is provided a dosimeter which measures absorbed ultraviolet-B radiation dose for light wavelengths centered at 307 nm and which is based on the phenomenon of the phototransferred luminescence properties (either PTTL, or PT cw-OSL) of &agr;-Al
2
O
3
:C. In the preferred embodiment &agr;-Al
2
O
3
:C detectors (either in the form of single crystals, thin powder layers on a suitable substrate, polycrystalline chips, or any other form of &agr;-Al
2
O
3
:C, including amorphous &agr;-Al
2
O
3
:C) are used as the UVB detector. The dosimeter formed therefrom can be used in air or water, and will have a near-linear response with a dynamic range of
Akselrod Mark S.
Colyott Leslie E.
McKeever Stephen W. S.
Fellers Snider Blankenship Bailey & Tippens, P.C.
Gagliardi Albert
Hannaher Constantine
The Board of Regents for Oklahoma State University
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