Radiation Dosimetry Using Alanine and Electron Paramagnetic Resonance (EPR) Spectroscopy: A New Look at an Old Topic

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The detection and quantification by electron paramagnetic resonance (EPR) spectroscopy of stable radicals formed in alanine by exposure to γ-radiation is used as a secondary standard for radiation dosimetry measurements, even though the EPR signal is actually derived from >1 radical with different spectral properties. For high radiation doses, microwave power saturation and spectral linewidths are both dependent on the received dose, and result in non-linear calibration curves. Furthermore, using a high-sensitivity microwave cavity, the power at which EPR signal saturation commences is ~0.3–0.4 mW for samples with irradiation doses ≤10 kGy; these values are an order of magnitude lower than those normally used in alanine dosimetry. In addition, the central peak of the first derivative spectrum, the height of which is commonly used in dosimetry measurements, is the most susceptible to microwave power saturation. Therefore, for high-level dosimetry we now recommend that analyses be performed under non-saturating conditions, and that the spectral acquisition parameters should be determined with a standard irradiated to ≤10 kGy to eliminate any intensity problems associated with variable saturation characteristics. At low radiation doses, variations in spectral saturation characteristics are negligible, and partially saturating conditions along with modulation amplitudes much higher than those normally used can reliably produce improved signal-to-noise ratios and allow extension of the methodology to practical working limits of ~0.1–0.2 Gy.