Determination of the sensitive volume of a spherical ion chamber as a potential primary standard instrument for higher energy photon beams

The sensitive volume of cylindrical or parallel plate ion chamber used as a primary standard for air kerma is typically determined using mechanical tools. This study presents an alternative methodology to determine the sensitive volume for a spherical chamber using a high-resolution imaging system, such as a microcomputed tomography unit (microCT). These determinations were done independently at both the University of Wisconsin (UW) and at State University of Rio de Janeiro (UERJ). The internal dimensions and the collection electrode of a spherical chamber (model Exradin® A3, Standard Imaging) were determined with a resolution better than the manufacturer's nominal volume value of 3.6 cm³ for the A3. In this study, at UERJ the measured sensitive volume of the A3 chamber was found to be 3.52 cm³ and at UW, after a correction for the electric field was made for the volume determination, it was found to be 3.54 cm³. The measured sensitive volume was compared against the volume calculated from an ADCL air kerma calibration with ⁶⁰Co at the UW. The agreement between these two methods was within 0.65%. Note that the chambers at UW and UERJ are two independent chambers and thus, the volume would not necessarily agree.