Optimization of microwave power in ESR dosimetry of tooth enamel in Japanese macaques.

To apply electron spin resonance (ESR) dosimetry to wild Japanese macaques captured in the ex-evacuation area during the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, the improvement of the detection limit is crucial. In this study, we optimized the microwave power in ESR measurements to suppress the noise of native radicals and enhanced the signals of radiation-induced carbonate radicals. Tooth enamels of a Japanese macaque captured in a control area were prepared and irradiated with gamma rays from 60Co source. The ESR spectra of the enamel samples with different absorbed doses ranging from 0 to 1800 mGy were measured with varying microwave power. The ESR spectra were analyzed by an in-house multi-component decomposition code using a simulated annealing method. Intensities of both components originating from carbonate and native radicals saturated and decayed as the microwave power increased. The intensity ratio of carbonate radicals to native radicals, i.e., signal to noise ratio, increased monotonically at microwave powers below 30 mW. We also examined the linearity of the intensity of carbonate radicals against the absorbed doses and recommended a microwave power range of 5-25 mW. In this study, we showed that optimizing the microwave power is an effective way to improve the quantitation accuracy of carbonate radicals in samples with low absorbed doses. The improved measurement conditions will expand the applicable range of ESR dosimetry for research on the effects of radiation on wild animals related to the FDNPP accident.