Abstract
Magnetization transfer and NMR relaxation rates were measured for waterprotons in two types of polymer gels developed for radiation dosimetry withMRI in order to quantify the contributions of different relaxationprocesses to the radiation response in such gels. Measurements included therate of magnetization transfer between proton pools and the ratio of thesizes of exchanging pools, R1 and R2. A model of relaxation inirradiated gels is presented to explain their properties. The modelincorporates three proton pools: free water, macromolecular andinterfacial. Two pools are insufficient to model the data. In thesesystems, radiation-induced polymerization appears to increase the size of asolid-like macromolecular proton pool but does not affect the rate constantof magnetization transfer per proton from macromolecular protons to thefree water protons. The relation between R1 and the pool size ratio isconsistent with free water exchanging with a macromolecular pool with anR1 of ~8 Hz. In addition, the rate of magnetization transfer isnot limited by the rate of chemical exchange between the free water and theinterfacial protons, and magnetization transfer most probably occurs vialabile proton exchange rather than via bound water molecules.
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