Dose Simulation for the MATROSHKA-R Experiment onboard the International Space Station Using a High-Fidelity Model of the Zvezda Module

The MATROSHKA-R (MTR-R) experiment conducted onboard the International Space Station (ISS) provided invaluable data on depth-dose distributions for critical human organs exposed to the low Earth orbit (LEO) radiation environments. In this study, a high-fidelity computational model of a tissue-equivalent spherical phantom and a refined geometric representation of the Zvezda module were developed using Geant4 Monte Carlo toolkit. In contrast to previous studies that relied on simplified cylindrical crew module models, our approach successfully and simultaneously reproduced the measured dose distributions in both the surface pocket detectors and the internal container detectors of the phantom. By correlating phantom depths with anatomical organ positions, organ-specific absorbed doses and dose equivalents were estimated. Simulations revealed that Galactic Cosmic Rays (GCRs) contribute over 60% of the total dose equivalent across all organs, with contribution increasing with depth and reaching up to 81% at specific internal positions. The simulated organ dose equivalents showed close agreement with empirical data, validating the spherical phantom's utility for organ-level radiation studies. This work provides a more realistic computational basis for interpreting MATROSHKA-R measurements and for future organ-level dosimetry studies in low-Earth orbit.