The Graphite Calorimeter as a Standard of Absorbed Dose for Cobalt-60 Gamma Radiation

Absorbed dose to water in a cobalt-60 gamma-ray beam has been determined using a thick-walled graphite ionization chamber. The chamber was calibrated in a graphite phantom against a graphite calorimeter, and the graphite calibration factor was converted to a water calibration factor using published energy absorption coefficient ratios and a measured replacement factor. Comparisons between the graphite and water measurements were made at pairs of points that were scaled in position according to the ratio of electron densities, so that the photon spectra were the same for the two points in a given pair. Measurements performed in graphite over a wide range of phantom depths, field sizes, and source distances, showed that the calibration factor varies slowly with the phantom depth and field size, and probably has a negligible dependence on source distance. By comparison with the thick-walled chamber in a cobalt-60 gamma-ray beam, a secondary ionization chamber can be calibrated in terms of absorbed dose to water with an estimated uncertainty of about ± 1 percent.