Total Ionizing Dose Evaluation for Quantum Key Distribution Payload in Low Earth Orbit

Quantum key distribution (QKD) is a communication technology that exploits quantum mechanical principles to achieve unconditionally secure key exchange. As communication networks extend to space, implementing QKD on satellites is essential for long-distance, tamper-proof data transmission. However, satellite payloads are exposed to harsh radiation environments consisting of trapped protons and electrons, galactic cosmic rays, and solar energetic particles, which can cause cumulative degradation of electronic and optical components. This study evaluates the total ionizing dose (TID) in a satellite-based QKD system to assess its radiation tolerance and establish appropriate shielding design criteria. The space radiation environment corresponding to a low Earth orbit (LEO) at an altitude of 650 km was modeled using space environment information system (SPENVIS). Subsequently, detailed Monte Carlo simulations were performed with the geometry and tracking (GEANT4) toolkit, incorporating the actual structural geometry of the payload to obtain position-dependent shielding thickness and dose distribution. From the simulation results, the equivalent aluminum thickness required to maintain component-level dose within acceptable limits was derived. Based on this analysis, the radiation design margin (RDM) was calculated to ensure system reliability throughout the mission lifetime. The results show that applying an equivalent aluminum shielding thickness of at least 1 mm maintains the TID below 5 krad over a five-year mission, satisfying a RDM of two for commercial off-the-shelf components. Increasing the thickness to 2 mm improves reliability, but introduces a mass trade-off, providing a basis for shielding design. This analysis demonstrates the relationship between shielding configuration and total dose reduction, providing quantitative guidance for optimizing shielding mass and protection efficiency. This work contributes to the design of radiation-tolerant satellite QKD payloads by linking environment modeling and detailed shielding analysis. The presented methodology can serve as a practical reference for future spaceborne quantum communication missions operating in LEO and beyond.