An international intercomparison on “self-calibrated” alanine EPR dosimeters

The results, obtained by six independent electron paramagnetic resonance (EPR) laboratories, of the dose response coefficients (Kdr) of “self-calibrated” solid-state EPR dosimeters containing alanine as a radiation-sensitive material and Mn2+/MgO as an internal reference material, are reported. The intercomparison trial was divided into three steps. It started with the distribution of dosimeters among the participating EPR laboratories with the purpose of irradiating them with known doses of γ-rays and to estimate the Kdr. The percentage standard deviation (PSD) of the Kdr obtained at individual labs was in the range of 1.4–4.6%. The interlaboratory PSD of the Kdr was 8%, primarily pointing to variations in irradiation procedures and EPR spectrometer settings. Further investigations showed that the main source of the interlaboratory PSD is differences in the calibrations of irradiators and settings of EPR acquisition parameters. In order to provide reproducible estimates of the Kdr, low microwave power and modulation amplitude using a combination of sweep time and time constant that gives a distortion-free EPR spectrum should be utilized. In the third step following such a procedure, measuring the same irradiated alanine dosimeter at the respective laboratories, spectrometers (12 instruments of 6 different models and 3 producers) and 10 operators gave an interlaboratory PSD of 3.1%. In conclusion, EPR dosimetry using “self-calibrated” alanine dosimeters may be used as a secondary standard, although a careful calibration of the EPR spectrometer must be performed in order to further reduce the uncertainty.