Calibration of radiation protection dosimeters – influence of long-term stability

International measurement system is a chain of calibration laboratories, measurement standards and calibrations that make sure that all the measurements in the whole world are traceable to the SI units, and that all the measurements are comparable within the measurement uncertainty. Primary Standards Dosimetry Laboratories (PSDL) establish primary standards, based on the absolute measurements of SI units, and disseminate the units typically to the Secondary Standards Dosimetry Laboratories (SSDL) – by calibrating their secondary standards. SSDLs calibrate dosimeters owned by the end users, completing the measurement chain from the end user to the SI. Results of these calibrations are calibration coefficients with associated measurement uncertainties that can be used to correct the measurement results. However, a calibration coefficient is valid only at the time of calibration and in reference conditions. All measurements must be corrected for real measurement conditions, and instrument stability must be taken into account. The experience shows that the response of all dosimeter changes over time, due to different factors such as ambient environmental parameters (air pressure, temperature, and relative air humidity), battery level, dosimeter aging, or other stochastic or systematic effects that are unknown or can’t be corrected for. Stability checks can be performed using check sources or repeated calibrations or other measurements in reproducible conditions. They can be used to estimate the uncertainty contribution of the dosimeter long-term stability. Stability checks can also be used to detect dosimeter failure. Three secondary standards were followed for over 15 years in the SSDL of Vinca Institute, making sure that their stability is within the requirements of the standard ISO 4037. During this period, two failures of the standards happened, and this was reflected in large changes in response. In addition, two survey meters (one professional and one low cost) were monitored over 3 months, showing much larger uncertainty due to stability.