Monte Carlo sensitivity analysis of the Vinten 671 ionization chamber activity calibration coefficients

Accurate calibration coefficients (CCs) are essential for radionuclide calibrators employed in nuclear medicine and radiopharmacy applications. These coefficients depend on multiple experimental and modelling parameters, including sample composition, geometry, and positioning within the ionization chamber (IC). This work investigates the sensitivity of CCs for the National Physical Laboratory (NPL) Vinten 671 well-type IC, whose design is represented in the widely used NPL Fidelis calibrator. A comprehensive sensitivity analysis was conducted using the EGSnrc Monte Carlo (MC) model to evaluate the impact of both physical and computational parameters on CC determination. Variables examined included detector and vial geometry, nuclear decay data libraries, and simulation energy cut-off values. The results demonstrate that CCs are largely stable under minor variations in vial volume and vertical positioning but exhibit notable sensitivity to the nuclear data selection and the aluminium inner wall thickness of the chamber. Accurate modelling of the vial and holder, particularly in the axial plane, is crucial. Parameters such as nitrogen pressure and aluminium wall thickness, even with minimal changes, demonstrate a significant impact, with optimization offering potential improvements in agreement between experimental and simulated values. Future efforts should refine the EGSnrc model, with experimental validation enhancing model reliability.