The Australian Commonwealth standard of measurement for absorbed radiation dose

Abstract

As an agent of the Commonwealth Scientific and Industrial Research Organization, the Australian Atomic Energy Commission (AAEC) is responsible for the maintenance of the Australian Commonwealth standard of absorbed dose. A graphite calorimeter was designed and built at the AAEC Research Establishment to provide such a standard for the dose absorbed in carbon from y- and X-ray beams with maximum photon energies above 500 keV. The calorimeter measures the dose averaged over the volume of a thermally insulated absorber disc of diameter 20 mm, thickness 3 mm and mass 1.7 g. The absorber and associated adiabatic and temperature controlled jackets are contained within a small graphite phantom of diameter 15 cm. The depth to the centre of the absorber is adjustable between 2.1 and 10 cm. By using a modified vacuum chamber window, the minimum depth can be reduced to 5 mm for use with high energy electron beams. The calorimeter has been in operation for more than 12 months and no change in its response to a 60Co source has been detected in that time. The precision of dose measurements is ± 0.3 per cent at a dose rate of 4.2 mGy s-' (25 rad min-'). Graphite and water phantoms have been made to enable portable working standards of absorbed dose in carbon and water to be calibrated against the primary standard.