Estimation of Reference Gamma Radiation Field for Calibration of Radiation Monitors

Laboratories/facilities dealing with ionizing radiation encounter radiation field which are measured using suitable gamma radiation monitors, for area and personal monitoring. These radiation monitors are subjected to periodic calibrations to confirm that they are still fit for the purpose. In India, periodic calibration of these radiation monitors is carried out by NABL accredited calibration laboratories using source exposure devices containing radionuclide sources of 137Cs and/or 60Co. Calibration laboratories get the output from these source exposure devices standardized by RSSD, BARC, Mumbai. RSSD, BARC, provides a report on the measured output, in the form of air kerma rates at different distances, which is traceable to relevant national standards. The laboratory then uses the standardized radiation field in day-to-day calibration, by taking air kerma rate mostly at distance of 100 cm (fixed distance) as reference for estimating radiation fields at other distances by using inverse square law. The air kerma rate values obtained by the using inverse square law shows deviation from the measured values with an allowed deviation up to 5%. The impact of this deviation cannot be corrected thus is accounted in the form of uncertainty. This paper presents an average air kerma strength-based methodology which provides a simple and effective mechanism for estimating the conventional true value of air kerma rate at various distance which adds lower uncertainty to overall uncertainty, as compared to the method of using a fixed or reference distance (say 100 cm) based air kerma rate.