Nuclear magnetic relaxation characterization of irradiated Fricke solution.

The spin-lattice relaxation rate R1(= T1(-1) of irradiated Fricke solution was studied as a function of the absorbed dose D. The R1 increases linearly with dose up to D approximately 400 Gy after which the response saturates. A model describing the R1 of a solution of either ferrous (Fe2+) or ferric (Fe3+) ions is presented; it is based on the spin relaxation of protons on water molecules in the bulk and protons on water molecules in the coordination shells of the ions with fast exchange occurring between the two water environments. All inherent relaxation parameters of the different proton groups are determined empirically at NMR frequencies of 9 and 25 MHz. An extension of the model is made to describe the spin-lattice relaxation behavior of irradiated Fricke solution. Good agreement between model predictions and experimental results is observed. The model relates the spin-lattice relaxation rate of a Fricke dosimeter to the chemical yield of ferric ion, thus potentially creating an absolute NMR dosimetry technique. Various practical aspects of the NMR-Fricke system, such as the optimal initial ferrous concentration and the NMR frequency dependence of the sensitivity, are described.