Dose calibrator assay of iodine-123 and indium-111 with a copper filter

The accurate measurement of radionuclides in a dose calibrator can be affected by container geometry especially for low-energy photons. This study investigated the effectiveness of using copper filtration for selective absorption of low-energy x-rays during the measurement of 123I and 111In to minimize variations in measurement as a function of geometry.

Copper absorbers of varying thickness were used to define the absorption profile of low-energy x-rays in the 20- to 50-keV energy window with 125I and a gamma scintillation spectrometer. Iodine-123 and 111In standards were assayed in a dose calibrator, with and without a copper filter positioned in the ion chamber well, to determine new isotope calibration settings for these nuclides with the copper filter. Samples of 123I and 111In, of constant activity but various volumes, were assayed in 1-ml, 3-ml and 10-ml plastic syringes and a 10-ml glass vial with and without copper filtration to assess the influence of geometry on the accuracy of measurement.

A copper thickness of 0.6 mm removed 99.5% of 125I photons in the 20- to 50-keV window and significantly reduced the low-energy component of 123I and 111In. Activity measurements of 123I and 111In in the dose calibrator varied with container type and volume without copper filtration but were essentially unchanged when a copper filter was used.

A copper filter > or = 0.6 mm thick eliminates the need for geometry correction factors for measuring 123I and 111In with the dose calibrator.