Methodology for simultaneous relative-calibration of multiple personal dosimeters in a ²²⁶Ra irradiation field

The reopening of the research reactor at Kyoto University required regular calibration of many personal dosimeters. However, it is not practical to subscribe to a calibration service for all the dosimeters. Traceability to the Japanese National Standard is transferred to a representative dosimeter with ¹³⁷Cs and ⁶⁰Co radioactive sources by the service. Here, we show a method to simultaneously confirm the accuracy of 100 dosimeters by using the transfer dosimeter in an irradiation field of a ²²⁶Ra radioactive source. The background in the 2.9-m × 4.0-m × 3.3-m chamber was 0.001 mSv day^-1, and thus had no effect on calibrations. The position dependence of dose equivalents from the source at the centre of the chamber was an inverse square of distances for the primary rays and was inconsequential for scattered rays. The angular dependence around a 44-cm-radius from the source was isotropic. After an irradiation time of 48 minutes, the average difference in readings between the transfer dosimeters and others arranged in the circle was 1.3%, and each difference was within the ± 10% error range stipulated by the manufacturer. These results demonstrate that this irradiation field is ideal for the metrological confirmations of multiple personal dosimeters. (author)