Electron spin resonance (ESR) spectroscopy in combination with certain irradiated solids is a known method for measuring radiation doses. In the present study, the combination of ESR and granulated sugars exposed to low doses of X-rays (0–2000 mGy) was first examined to confirm the validity of this method. This method was then used to measure the accumulated radiation doses in the teeth of field mice living around the nuclear power plant in Fukushima.

Four hundred mg of glycine powder containing 5.0×10¹³ spins of 1,1-diphenyl-2- picrylhydrazyl (DPPH) was used as a standard to estimate the amounts of free radicals in both the irradiated sugars and the teeth of field mice. After the granulated sugars (400 mg for each) were subjected to X-ray doses of 0–2000 mGy, ESR signals were obtained by tracing an 18.4-mT magnetic field for 3.6 s and repeating this process 100 times using a JEOL JFS-RE1X ESR spectrometer; the ESR conditions were 1 mW of microwave power with 9.08-GHz field modulation of 0.4-mT amplitude. After the background ESR signal of non-irradiated sugar was subtracted from each ESR signal, the resulting signal was doubly integrated using the WINRAD computer software (Radical Research Company, ver. 1.20) to convert the signal into intensity. The amounts of free radicals in X-ray irradiated sugars were calculated by comparing the doubly integrated values of the ESR signals of irradiated sugars with the corresponding values of the DPPH standard.

A linear relationship between the amount of free radicals and irradiation dose was obtained for doses in the range of 0–2000 mGy. The applicability of ESR spectroscopy as a dosimeter was certified in the dose range of 33 mGy or greater. On the basis of the slope of the straight line, the efficiency of free-radical production in granulated sugars per 1 Gy irradiation was estimated to be 2.1×10¹⁴/g/Gy. This method was then used to measure radiation doses accumulated in the teeth of field mice living around the nuclear power plant in Fukushima. Nineteen field mice were collected between November 15 and 17, 2013. ESR signals of the teeth (40 mg for each) were obtained by repeating the ESR measurements 20 times under the same conditions previously described. The teeth of field mice collected in Hokkaido were used as non-irradiated control samples. Because the control samples gave large background ESR signals, a statistically significant difference between the teeth of mice collected in Fukushima and those of mice collected in Hokkaido was not clear.