TH-E-BRB-06: Characterization of Microionization Chambers for Small-Field Reference Dosimetry

Abstract

Purpose: Microchambers demonstrate anomalous behaviors not seen in larger-volume chambers. The purpose of this work is to perform an evaluation of several cylindrical microchambers to assess these performance issues and limitations through measurements of chamber response as a function of bias voltage and Monte Carlo transport in the presence of strong electric fields. Methods: Measurements were performed with five cylindrical microchambers, one scanning chamber, and two Farmer-type chambers. The response of each chamber to 60Co radiation was measured at opposing polarities for applied voltages between (10–600) V. Chamber reproducibility and polarity correction factors were evaluated. Ion recombination correct factors were calculated using several techniques accounting for initial and general ion recombination, ion diffusion, and charge multiplication. The electric field in each chamber was modeled using COMSOL Multiphysics Software and the data was exported and integrated into EGSnrc. Results: All microchambers displayed unique sets of saturation curves, which varied from chamber to chamber, even within chamber models. Ion recombination correction factors were inconsistent depending on the calculation technique used (with variations > 2%) and often fell below unity. Polarity correction factors were > 0.3% from unity and strongly voltage dependent. The responses of several microchambers varied inversely with the magnitude of the applied voltage, which may be due to strong electric fields in the presence of high-Z materials. To investigate the effect of the bias voltage, the transport of charged particles in the presence of calculated and COMSOL-generated electric fields was benchmarked against analytical calculations. Conclusions: Unlike the larger-volume chambers, each microchamber displayed a unique set of saturation curves highlighting several anomalous characteristics unique to microchambers. These results demonstrate that conventional TG-51 corrections may not be appropriate for reference dosimetry measurements. A technique was developed to investigate these behaviors using EGSnrc electric field simulations.