Development and Metrological Performance Evaluation of a Concentric Graphite Calorimeter for Absorbed Dose Realization from Brachytherapy Sources

The article describes the development and structural design of a concentric graphite calorimeter intended for the reproduction of the unit of absorbed dose to water from brachytherapy radiation sources. A physical substantiation is presented for the geometry of the calorimeter absorber, which allows minimizing the impact of dose gradients and thermal self-heating of high- activity sources. The results of comprehensive numerical simulation of dose field distributions and thermal processes within the calorimeter design are provided, performed using Monte Carlo methods (EGSnrc, PHITS) and finite element analysis (COMSOL). Correction factors accounting for the influence of vacuum gaps and structural inhomogeneities on the measurement results are evaluated. Experimental studies of metrological characteristics confirmed that the developed instrumental complex ensures the reproduction of the absorbed dose in graphite with a standard deviation of no more than 0.4% and an uneliminated systematic error of no more than 0.9% (at P = 0.99). The results obtained form the scientific and technical basis for improving the state primary standard of absorbed dose of photon radiation.