Activity cross-calibration of unsealed radionuclides utilizing a portable ion chamber

To present and evaluate an ion chamber-based method for the cross-calibration between sites of activity measurements of unsealed radionuclides. Notably, the method allows direct comparison of short lived (i.e., clinically used) radioisotopes and the cross-calibration of radionuclide activity meters (also known as "dose calibrators").

A portable ion chamber has been designed which is easily shipped between sites, e.g., between a standards laboratory and a nuclear medicine department. The cylindrical chamber accommodates a syringe filled with unsealed radionuclide. Low background and staff shielding are achieved by designing the ion chamber small enough to fit into the well of a dose calibrator. The chamber's sensitivity for the clinically important unsealed radioisotopes ^99mTc, ¹³¹I, and ¹⁸F was measured and compared to Monte Carlo calculations. The influence of syringe fill volume, positioning, and construction (wall diameter, length) was also investigated using Monte Carlo simulations. The chamber's linearity was measured over 5.5 orders of magnitude and its constancy tested over a period of >14.5 months. An overall uncertainty budget is presented.

Measured chamber sensitivity was 12.1 pA/100 MBq, 12.5 pA/100 MBq and 29.4 pA/100 MBq for ¹³¹I, ^99mTc, and ¹⁸F, respectively. The uncertainty budget for the ion chamber alone yields an overall uncertainty of less than 1%, with the greatest contribution arising from constancy and linearity (0.5% each). Strategies to further reduce uncertainties are discussed.

The investigation presented in this paper confirms the feasibility of the concept introduced here. To optimize its practical implementation, the concept would benefit from computerization for the purpose of data acquisition, evaluation, processing, and storage of measured values.