(Mis)use of 133Ba as a calibration surrogate for 131I in clinical activity calibrators

Using NIST-calibrated solutions of 131Ba and 131I in the 5 mL NIST ampoule geometry, measurements were made in three NIST-maintained Capintec activity calibrators and the NIST Vinten 671 ionization chamber to evaluate the suitability of using 133Ba as a calibration surrogate for 131I. For the Capintec calibrators, the 133Ba response was a factor of about 300% higher than that of the same amount of 131I. For the Vinten 671, the Ba-133 response was about 7% higher than that of 131I. These results demonstrate that 133Ba is a poor surrogate for 131I. New calibration factors for these radionuclides in the ampoule geometry for the Vinten 671 and Capintec activity calibrators were also determined.