Standardization of iridium-192 gamma-ray sources in terms of exposure

Iridium-192, in the form of small platinum-or stainless-steel-clad seeds, is used for radiation therapy. Standardization of this radionuclide, for the quantity exposure was carried out by measuring groups of seeds in an open-air geometry, using the NBS standard graphite cavity ionization chambers, and transferring the exposure data to a re-entrant ionization chamber. Tables are provided from which the corrections for the graphite chamber have been calculated along with corrections for room scattering. Radiographs of the source arrays are shown and details of the re-entrant chamber source measurements and construction are provided. As assessment of the errors involved in establishing this standard leads to a statement of 2 percent for the overall uncertainty in the calibration of an iridium seed for the quantity exposure.