Hybrid active-passive Galactic Cosmic Ray simulator: In-silico design and optimization

High-energy heavy-ion particle accelerators have long served as proxies for the harsh space radiation environment, enabling both fundamental life-science research and applied testing of flight hardware. Traditionally, monoenergetic high-energy heavy-ion beams have been employed for practicality, providing valuable datasets that underpin radiation risk and predictive computational models. However, such beams cannot fully reproduce the mixed-field nature of space radiation, motivating the development of realistic analogs for improved risk assessment and countermeasure evaluation in preparation for future deep-space missions to Moon or Mars. Spearheaded by developments at the NASA Space Radiation Laboratory, the GSI Helmholtzzentrum für Schwerionenforschung, supported by the European Space Agency (ESA), has established advanced space radiation simulation capabilities in Europe. Here, we present the design, optimization, and in-silico benchmarking of GSI's hybrid active-passive Galactic Cosmic Ray (GCR) simulator, together with a computationally optimized phase-space particle source for Geant4, which is available to external users for their own simulation studies and experimental planning.