Measuring Solar Energetic Particles With a CubeSat-Scale Energetic Particle Telescope: Geant4 Based Design of the REPTile-3 Instrument

Solar Energetic Particles (SEPs) are present during increased solar activity, often associated with solar flares and coronal mass ejections (CMEs). Measuring and understanding these particles is important both for fundamental solar physics knowledge as well as the determination of radiation risks in interplanetary space. Solid-state particle telescopes are a useful tool to measure these particles. The Relativistic Electron and Proton Telescope integrated little experiment-2 (REPTile-2) was a solid-state energetic particle telescope that flew onboard the Colorado Inner Radiation Belt Experiment (CIRBE) and demonstrated a capability to measure electrons from 0.25 to 6 MeV and protons from 7 to 100 MeV with high energy and time resolution. REPTile-2 operated in a low-Earth orbit (LEO) and primarily measured radiation belt particles but was also able to measure SEPs during high-latitude passes. Because of REPTile-2's solid performance and its CubeSat-scale size, weight, and power, an opportunity arose to fly a modified REPTile-2, dubbed REPTile-3, on the Emirates Mission to the Asteroid Belt (EMA). In this paper, Geometry and tracking 4 (Geant4) Monte Carlo simulations are used to motivate changes to improve REPTile-3's ability to measure SEPs. Additionally, full instrument response functions and estimated count rates are used to understand the instrument's response to SEP fluxes. REPTile-3 is shown to be able to measure 1.2–35 MeV protons with ΔE/E < 9%, 35–100 MeV protons with ΔE/E < 50%, 0.1–5 MeV electrons with ΔE/E < 14%, 18–131 MeV helium ions with ΔE/E < 7%, and 131–200 MeV helium ions with ΔE/E < 50% with a 102° field of view (FOV).