Determination of photon fluence spectra from the NPL Mobaltron 60Co unit

Absorbed dose to water calibrations of secondary standard hospital dosemeters are carried out at NPL in both X-radiation from 4 MV to 19 MV and in 60Co γ-radiation. Measurements are performed using the primary standard graphite calorimeter and by inserting the secondary standard and working standard chambers at a reference depth in either a water or graphite phantom. Details of the dose distributions and the responses of these chambers in each phantom for the NPL 60Co radiation quality are, however, unknown. A knowledge of the photon fluence spectra produced by the NPL 60Co unit is therefore required to enable this analysis to be carried out. This report describes the work that was undertaken to determine the photon fluence spectra incident on the front face of the water and graphite phantoms under standard measurement conditions. The spectra were validated by comparing calculated tissue-phantom ratios with recent experimental values. (author)