The UK primary standard calorimeter for photon-beam absorbed dose measurement

A description is given of the UK primary standard graphite calorimeter system. The calorimeter measures absorbed dose to graphite for photon radiations from to 19 MV x-rays, and is the basis of the NPL therapy-level absorbed dose to water calibration service. Absorbed dose to graphite from the photon calorimeter has been compared with three other standards: an ionization chamber and cavity theory, for gamma radiation; the NPL electron calorimeter, for 12 - 14 MeV electron beams; and the BIPM absorbed dose standard. The three standards agreed within 0.5% which is similar to the measurement uncertainties.