Design and Instrument Characterization of the High-Energy Resolution Relativistic Electron Telescope (HERT) Using Geant4 Simulation

Earth's outer radiation belt is filled with relativistic (similar to 100s of keV-3 MeV) and ultrarelativistic (similar to>3 MeV) electrons. The acceleration of electrons to these high energies are believed to be caused by inward radial transport and local acceleration, but the relative importance of the two mechanisms to electrons with different energies are still under considerate debate. In the post Van Allen Probes era, measurements of radiation belt populations must rely on small space missions such as CubeSats and SmallSats. The Miniaturized High-Energy-Resolution relativistic electron Telescope (HERT) is a compact (<= 3U) telescope designed for a CubeSat mission in geosynchronous transfer orbit (GTO). HERT's main objective is to provide high-energy-resolution measurements of outer belt electrons in an energy range of similar to 1-7 MeV to help differentiate the contribution of inward radial transport and local acceleration. Geant4 simulations were conducted to characterize the instrument responses. A novel method of using a spherical cap particle source in Geant4 simulation was developed for more efficient instrument characterization. Combined with Bow tie analysis, it is demonstrated that HERT will have an energy resolution of 5% for similar to 1.5-3 MeV electrons and 12% for similar to 3-7 MeV with current electronics design. In addition, using the AE9 model, we showed the instrument would have statistically sufficient count rates in the outer belt while not saturating the electronics. With a compact configuration and higher energy resolution in comparison to previous instruments, HERT will significantly contribute to the quantitative understanding of the radiation belt electron dynamics.