Low Energy Ionizing Radiation and Plasma Contributions to Radiation Dose in Materials at Sun-Earth Lagrange Points

The relatively low energy (few MeV or less) component of the charged particle environments beyond the Earth’s radiation belts represents a radiation source with potential impacts to materials used on the exterior surfaces of spacecraft for missions to the Sun—Earth Lagrange points. Sun-Earth L1 and L2 are of particular interest to mission designers since L1 provides opportunities for monitoring the Sun and incoming plasma and radiation events upstream of the Earth’s magnetosphere and L2 is a low temperature thermal environment far from Earth and the Moon useful for astrophysics missions. The low energy radiation and plasma environments are generally considered relatively benign to materials with little or no threat to spacecraft operations even though the total ionizing dose generated by electrons, protons, and alpha particles to exterior surfaces and volumes of thin materials over a mission life can be in the megarad to gigarad range. Radiation damage of materials by the low energy charged particle environments may be a concern for current and future spacecraft designs using very thin materials or bulk materials where the surface properties are mission critical. Some examples where radiation dose from low energy particles should be evaluated include radiation degradation of solar sail propulsion systems, changes in thermo-optical properties of thermal control coatings, radiation damage to thin sunshield materials, and modification of surface charging properties for materials used on the exterior surfaces of spacecraft with requirements to be electrostatically clean. This presentation will first discuss the characteristics of ionizing radiation and plasma environments relevant to thin materials including solar wind and low energy contributions from solar particle events at Sun-Earth L1, L3, L4, and L5 and the additional contributions of magnetosheath and magnetotail plasmas at Sun-Earth L2. Next, low energy charged particle damage mechanisms for materials will be summarized including total ionizing dose, hydrogen blistering, changes in thermo-optical properties, and sputter erosion yields. Finally, examples of modeling techniques used to compute the charged particle fluence to material surfaces and volumes of thin materials will be described along with methodologies for estimating total ionizing dose as a function of depth for materials in the low energy environment.