EPR dosimetric properties of 2-methylalanine: EPR, ENDOR and FT-EPR investigations.

To find an EPR dosimeter material that is sensitive enough for clinical use, the substance 2-methylalanine (2MA) with the chemical structure (CH(3))(2)C(NH(3)(+))COO(-) was tested for its sensitivity to ionizing radiation, dose response, and radical stability over time. At equal and moderate settings of microwave power and modulation amplitude, 2MA was found to be 70% more sensitive than L-alpha-alanine, which is the most common EPR dosimeter material today. The dose response is linear, at least in the dose range of interest (0.5-00 Gy), and the time-dependent variations in signal intensity are very small and may be corrected for easily. The energy dependence of the stopping power and energy absorption was calculated and was found to be similar to that of alanine. The dependence of the signal intensity on microwave power and modulation amplitude was investigated, and the optimal settings were found to be 25 mW (Bruker ER 4102ST) and 12 gauss, respectively. Single crystals of 2MA were analyzed using ENDOR and ENDOR-induced EPR to identify the radiation-induced radicals that formed. Only one radical, in which the amino group is detached from the original molecule, was identified. This radical is obviously dominating and is apparently the only one relevant for dosimetry purposes. The complete set of coupling parameters for three hyperfine couplings is reported. The power saturation properties and spectral line width are ruled by the relaxation times T(1) and T(2). To determine the relaxation times of 2MA, pulsed EPR experiments were performed on single crystals. Two different values of T(1) were obtained, one in the range 1-3 micros, shown to be of importance for the dosimetry properties, and another that is strongly anisotropic with a value between 10 and 35 micros that does not seem to affect the saturation behavior. T(2) was estimated to be of the order of 200-300 ns.