Detection of radiation effects in polymer gel dosimeters using 129Xe NMR.

Polymer gel dosimeters consist of monomers, with or without cross-linking agents, dispersed in a gel. Upon exposure to ionizing radiation, polymerization proceeds within the gel matrix, thereby changing several measurable physical properties that can then be related quantitatively to absorbed dose. Several previous studies have examined how various nuclear magnetic resonance (NMR) properties, such as the relaxation rates of water protons, change with dose, and magnetic resonance imaging (MRI) has been used successfully to measure three-dimensional dose distributions in irradiated polymer gels. Here we report our first observations of the manner in which the chemical shift of xenon gas (129Xe) dissolved in a gel changes with absorbed dose, and we introduce the potential use of high resolution xenon NMR spectra for understanding better the dose response of gels. 129Xe possesses a large chemical shift range and xenon spectra are sensitive to subtle changes in the physical and chemical environments in which the gas is dissolved. For doses ranging from 0 Gy to 40 Gy we found that the mean chemical shift of 129Xe was linearly related to dose, and that the gel dosimeter could be described in terms of a two-component model undergoing fast exchange. We found no evidence of radiation damage to the gelatin matrix at doses between 0 Gy and 40 Gy. At 40 Gy, the fast-exchange model begins to break down, and distinct gelatin and poly(methacrylate) resonances are observed at higher doses. High resolution NMR measurements of xenon provide a novel method for probing radiation dose effects in irradiated polymer gels.

Studies of magnetization transfer and relaxation in irradiated polymer gels--interpretation of MRI-based dosimetry.

Magnetization transfer and NMR relaxation rates were measured for water protons in two types of polymer gels developed for radiation dosimetry with MRI in order to quantify the contributions of different relaxation processes to the radiation response in such gels. Measurements included the rate of magnetization transfer between proton pools and the ratio of the sizes of exchanging pools, R1 and R2. A model of relaxation in irradiated gels is presented to explain their properties. The model incorporates three proton pools: free water, macromolecular and interfacial. Two pools are insufficient to model the data. In these systems, radiation-induced polymerization appears to increase the size of a solid-like macromolecular proton pool but does not affect the rate constant of magnetization transfer per proton from macromolecular protons to the free water protons. The relation between R1 and the pool size ratio is consistent with free water exchanging with a macromolecular pool with an R1 of approximately 8 Hz. In addition, the rate of magnetization transfer is not limited by the rate of chemical exchange between the free water and the interfacial protons, and magnetization transfer most probably occurs via labile proton exchange rather than via bound water molecules.

Polymer gels for magnetic resonance imaging of radiation dose distributions at normal room atmosphere.

Polymer gels whose NMR and optical properties change when irradiated offer unique advantages for measuring radiation dose distributions. To date, all acrylic polymer gel dosimeters must be manufactured, stored and irradiated in hypoxic conditions which severely limits their use and stability. A new formulation of acrylic dosimeter gel has been developed that responds well in normal atmosphere and which we have named MAGIC (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper). To produce dosimeter gels, an aqueous solution of gelatin, open to the atmosphere, is mixed with methacrylic acid, copper(II) ions, ascorbic acid and hydroquinone. It is believed that the copper(II) and ascorbic acid form a complex with oxygen which (with radiolysis of water) serves as a free radical source for the initiation of the polymerization of methacrylic acid. At room air the water proton spin relaxation rate R2 in MAGIC gels is proportional to absorbed dose though the precise relationship depends on the composition of the gel and the initiating complex. For example, in the range 0-30 Gy the slope of the response of R2 versus dose at 20 MHz was 0.300, 0.519 and 0.681 s(-1) Gy(-1), respectively, when the concentration of MAA was 3, 6 and 9%. The slopes increased to 0.310, 0.567 and 0.868 s(-1) Gy(-1) at 85 MHz. An important determinant of the sensitivity to detect small dose changes is shown to be the slope-to-intercept ratio of the dose-response curve. These varied from 0.08 to 0.17, comparable to hypoxic gels described earlier. MAGIC gels can be manufactured and used much more easily than the previous formulations and can be imaged by magnetic resonance imaging or optical scanning, and thus they will likely be of considerable interest to radiation physicists.

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