Radiant energy – Invisible radiation responsive nonelectric signalling – Optical change type
Reexamination Certificate
1998-12-18
2001-09-04
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiation responsive nonelectric signalling
Optical change type
Reexamination Certificate
active
06285031
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a radiation dosimetry method and associated devices for carrying out the method. More particularly, this invention relates to such a method and associated apparatus which compensates for variations in temperature and amounts of a radiation sensitive material in a dosimeter.
In facilities where radioactive materials are used, for example, in hospitals where cancer patients receive radiation treatments or in blood banks where blood products are irradiated, various methods are used to quantitatively determine the radiation dose. The methods practiced include the use of thermoluminescent dosimeters (TLD's), ionization-type radiation detectors, photographic film, and radiochromic materials. TLD's are inconvenient because they require a complicated and time-consuming read-out process. Ionization-type radiation detectors are awkward and unwieldy and require a complicated setup. Photographic film requires a time-consuming chemical processing procedure before read-out. Radiochromic materials are inconvenient in current practice because the calculation of the dose requires a complex sequence of steps, subject to operator error.
U.S. Pat. No. 5,637,876 describes a radiation dosimeter, exemplarily for use in determining a level of radiation to which a patient is subjected during radiation treatment, which comprises a substrate provided with a layer of radiation sensitive material. The radiation sensitive material has an optical density which varies systematically in accordance with the degree of radiation exposure. The dosimeter may take the form of a card or a flexible substrate which is positionable on the patient or other irradiation subject and which is also positionable in, or slidable through a slot in, a dose reader which includes a reflection or transmission densitometer.
The radiation sensitive material of a radiation dosimeter may be dispersions of crystalline pentacosadyinoic acid (PCDA). Subjecting monomeric PCDA crystals to ionizing radiation results in progressive polymerization, the degree of polymerization increasing with radiation dose. The amount of polymerization (and hence, the radiation dose) can be determined by measuring either the optical density or the spectral absorption of the exposed dosimeter. However, it has been found that these parameters also vary with both the temperature of the device when measured as well as the thickness of PCDA dispersion. Maximum accuracy of dose measurement must account for the temperature and thickness effects.
Temperature corrections are commonly applied to the output of sensors of whatever type; the method is straight forward. First the response or reaction of a sensor to a specific action is calibrated at a given (reference) temperature. Second, the change in response with respect to temperature is measured over a range of temperature and input action; this step characterizes the temperature dependence of the sensor. Then, during actual use of the sensor, assuming it is not maintained at the reference temperature, both the sensor's reaction (to the action its purpose it is to sense) as well as the sensor's temperature are measured. The measured reaction is corrected for temperature effect by using the data from step two. This yields a temperature corrected reaction, i.e., a calculated value of the reaction the sensor would be expected to have for the action at hand had the sensor been at its reference temperature. Finally, the amount of action the sensor is being subjected to is calculated from the corrected reaction by using the calibration data of step one. While giving good results the method suffers from the requirement to measure the sensor's temperature.
OBJECTS OF THE INVENTION
An object of the present invention is to provide a method for determining a radiation dose to which a dosimeter has been subjected, with compensation for temperature effects and/or thickness variations.
Another object of the present invention is to provide such a method wherein the temperature of the dosimeter need not be measured directly as a prerequisite to determining the radiation dose.
A further object of the present invention is to provide such a method which is efficient and easy to implement.
Yet another object of the present invention is to provide an associated apparatus for determining a radiation dose to which a dosimeter has been subjected, with compensation for temperature effects and/or thickness variations.
These and other objects of the present invention will be apparent from the drawings and descriptions herein.
BRIEF DESCRIPTION
Measurements of radiation exposed PCDA based dosimeter material show that its optical absorbance, while strongly a function of radiation, is not constant with temperature. Rather, it exhibits two peaks: a minor one at approximately 600 nanometers, a col at approximately 620 nm and a major peak at approximately 650 nm. In general, optical absorption increases with radiation dose. However, it has been observed that these spectra are temperature dependent, thus requiring temperature correction before dose can be determined from absorbance. Closer examination of sets of spectra for a PCDA dosimeter show that the spectra move in a regular manner in absorbance-wavelength space as temperature is varied. As opposed to characterizing the motion of the full spectrum (in absorbance—wavelength space as a function of temperature), characterizing the motion of some identifiable feature of the spectrum is much simpler. Readily discernible features are the two peaks and the col.
This invention is based on or incorporates the recognition that a predetermined point (e.g., peak or col) on a curve or surface in absorbance-wavelength space is associated with a unique temperature and a unique radiation dose. Determination of the absorbance and wavelength coordinates of the predetermined point thus enables one to determine the radiation dose to which the detector has been subjected.
A method, in accordance with the present invention, for monitoring exposure to radiation, with compensation for temperature variation of a sensor, utilizes a radiation dosimeter including a layer of radiation sensitive material on a substrate, the radiation sensitive material having an optical absorbance which varies in accordance with degree of radiation exposure and wavelength and which also varies in dependence on temperature. The method comprises, in accordance with a general embodiment of the present invention, exposing the layer of radiation sensitive material to a dose of radiation, optically measuring a spectral absorbance of the exposed layer of radiation sensitive material within a range of wavelengths, examining the measured spectral absorbance of the exposed layer of radiation sensitive material to determine an absorbance coordinate and a wavelength coordinate of a predetermined point on a spectral absorbance curve of the exposed layer of radiation sensitive material, and determining a radiation dose value associated with the absorbance coordinate and the wavelength coordinate. Generally, the radiation dose value is determined by consulting a table of absorbance and wavelength coordinates with associated dose values which have been previously measured for a batch of the radiation sensitive material, the batch having a uniform absorbance coefficient and a common concentration of the radiation sensitive material (e.g., PCDA).
In accordance with a particular feature of the present invention, the measuring of the spectral absorbance of the exposed layer of radiation sensitive material is accomplished by operating a spectrophotometer. The spectrophotometer detects the absorbance of the exposed radiation sensitive material at each wavelength within a predetermined range. Specific absorbance and wavelength values may be fed to a programmed circuit or other device, which automatically analyzes or examines the spectral data to determine the spectral absorbance curve and, more particularly, to determine the absorbance and wavelength coordinates of a predetermined poin
Lewis David F.
Listl Carl A.
Davis William J.
Goldberg Jules E.
Hannaher Constantine
ISP Investments Inc.
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