A STANDARD FOR ABSORBED DOSE RATE TOWATER IN A 60CO FIELD USING A GRAPHITE CALORIMETER AT THE NATIONAL

A primary standard for the absorbed dose rate to water in a 60Co radiation field has been newly established at the National Metrology Institute of Japan. This primary standard combines the calorimetric measurements using a graphite calorimeter with the ionometric measurements using a thick-walled graphite cavity ionisation chamber. The calorimeter is operated in the constant temperature mode using AC Wheatstone bridges. The absorbed dose rate to water was determined to be 12 mGy s21 at a point of 1 m from the radiation source and at a water depth of 5 g cm22. The uncertainty on the calibration coefficient in terms of the absorbed dose to water of an ionisation chamber using this standard was estimated to be 0.39 % (k51)