Absorbed-dose calibrations in high-energy photon beams at the National Physical Laboratory: conversion procedure

The absorbed-dose calibration service from NPL is based on a primary-standard calorimeter that measures absorbed dose to graphite. Secondary-standard dosemeters are calibrated in absorbed dose to water in a ⁶⁰Co gamma-ray beam and in X-ray beams over a range of generating potentials from 4 MV to 19 MV. Two methods were used to convert the calibrations of working-standard ionization chambers from absorbed dose to graphite into absorbed dose to water. One method involved the use of published interaction data for photons and secondary electrons, and required a knowledge of the chamber construction. The second method involved the calculation of the ratio of absorbed dose in graphite and water phantoms irradiated consecutively in the same photon beam using the photon-fluence scaling theorem. The two methods were in agreement to 0.1%.