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

Abstract

Earth's outer radiation belt is filled with relativistic (∼100s of keV–3 MeV) and ultrarelativistic (∼>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 ∼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 ∼1.5–3 MeV electrons and 12% for ∼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.

The Medium Energy Electron Telescope (MEET): Geant4‐Based Instrument Design and Analysis

“…Comparison of the spherical cap simulations with full spherical runs was conducted for analysis of a similar solidstate particle telescope similar analysis by Krantz et al (2023). Compared to MEET their study analyzed higher energy particles, and therefore had thicker shielding.…”

“…A complete table of electron energy channels is reported in the Supporting Information S1. Using these electron energy channels we determine the geometry factor for each channel (e.g., Khoo et al, 2022;Krantz et al, 2023).…”

“…where E is the incident energy of the particles, r is the radius of the spherical cap, n i is the number of particles which register in that energy channel i, θ is the polar angle of the spherical cap (30°in this study), and N′(E) is the number of particles simulated over the spherical cap at the given energy. This method is described by Krantz et al (2023) and is a modification of the traditional spherical simulation method for geometry factors by Yando et al (2011) via the (1 cosθ)/2 factor. This accounts for the region on the sphere outside the spherical cap over which particles were not generated and assumes that no particles in this region contribute to the counts measured Journal of Geophysical Research: Space Physics 10.1029/2023JA032221 by the instrument.…”

“…Prior simulation studies to characterize instrument response have relied on computationally expensive spherical source simulations (e.g., Khoo et al, 2022) or only considered boresight response (e.g., Baker et al, 2021). Here, we utilized a spherical cap source over the FOV of the instrument to characterize its response to the full-angle FOV (Krantz et al, 2023). Using a spherical cap particle source is an efficient technique as it reduces the total number of simulated particles needed to quantify the response of an instrument.…”

“…This accounts for the region on the sphere outside the spherical cap over which particles were not generated and assumes that no particles in this region contribute to the counts measured Journal of Geophysical Research: Space Physics 10.1029/2023JA032221 by the instrument. The polar angle of the spherical cap is defined at the center of the front of the first detector between the telescope axis and the vector to the circumference of the spherical cap region, visualized in Figure 2 of Krantz et al (2023). The selected polar angle here defines a spherical cap region which includes trajectories outside the nominal telescope aperture through the entire span of the first tooth of the instrument, as shown in Figure 5, such that teeth-penetrating events are characterized.…”