Experimental investigation of dose calibrator response for 125 I brachytherapy solutions contained in 5 mL plastic syringes and 2 mL conical glass v‐vials as a function of filling mass

The effect of measurement geometry on the determination of the activity of solutions containing125 I for use in brachytherapy applications has been investigated for 5 mL plastic syringes and 2 mL conical glass dose vials as a function of filling mass. New dial settings for the syringes over a filling mass range of 1 to 3 g have been determined to be 497 ± 8 and 469 ± 8 (expanded, k = 2 , uncertainties) for the NIST Capintec CRC‐12 and Capintec CRC‐35R, respectively, with any effect due to the filling mass lying within the uncertainty in the activity calibration. A filling mass effect was observed in the dose vials, causing a 10.5% reduction in the chamber response from a 2 g filling mass to 1 g. Dial settings at 2 g were experimentally found to be 143 ± 2 and 135 ± 2 (expanded uncertainties) for the NIST Capintec CRC‐12 and Capintec CRC‐35R, respectively. The appropriate dial settings for the same vials with a 1 g filling mass were found to be 120 ± 2 and 114 ± 2 for CRC‐12 and CRC‐35R, respectively. Differences of up to ±45% in the activity determination were observed between values obtained with the manufacturer's recommended setting and the settings obtained experimentally for each specific geometry. Calibration factors were also determined for a Vinten 671 Radionuclide Calibrator, giving values of 0.226 ± 0.009   pA ⋅ MBq − 1and 0.231 ± 0.004   pA ⋅ MBq − 1(expanded uncertainties), respectively, for the 1 and 2 g dispensings. This study demonstrates that experimentally determined calibration factors for the exact measurement geometry are necessary when measuring radionuclides in configurations other than the manufacturer's standard geometry, especially when nuclides that emit low‐energy radiations are involved.