Dosimetry Errors in Co-60 Gamma Cells Due to Transition Zone Phenomena

Measurements were made of the enhanced dose deposited by Co-60 gamma rays in aluminum next to gold after the radiation had been attenuated by variable thicknesses of paraffin wax, aluminum, steel or lead. The gold films varied in thickness from 0.65 to 250 ¿m. It was observed that the dose enhancement factor increased exponentially with the thickness of the attenuating material. Enhancement factors greater than 7.0 were observed for 66 g/cm2 of wax. The largest enhancement factor previously reported for a Co-60 source was slightly greater than 2.0. Using available calculations of differential photon intensity spectra and dose enhancement factors for monochromatic photon energies it was estimated that the enhancement could reach a maximum of ~10 at 100 g/cm2 and would not increase significantly for greater masses of attenuating material. The enhancement was experimentally found to be a maximum for ~6¿m gold films and as great as a factor of two for gold less than 0.65¿m thick. A simple empirical model reproduced the measured values within a few percent for all gold film thicknesses and wax attenuation thicknesses studied. Implications for testing electronic parts are discussed.