The Energetic Particle Detector Instrument on ELFIN

Abstract: We present the design methodology, implementation, and data products of the Energetic Particle Detector (EPD) instrument on-board the Electron Losses and Fields Investigation (ELFIN) dual SmallSat mission. ELFIN was designed to study the time-evolution of energetic particle distributions in Earth's magnetosphere, having operated in polar Low Earth Orbit (LEO) from 2018 to 2022. Each satellite's instrument payload contained a pair of detector heads to sense differential-directional fluxes of 50 keV to $\sim$5 MeV electrons (EPDE), and 50 keV to $\sim$2 MeV atomic ions (EPDI), with an energy resolution better than 50% ($ΔE/E&lt;0.5$). The satellites were spinning with a nominal period of 2.8 s in a plane containing Earth's background magnetic field, with particle counting intervals subdivided into 16 consecutive spin sectors. Each EPD was paired with a three-axis Fluxgate Magnetometer (FGM) sampled at 80 Hz for local pitch-angle determination. Each detector head operated on the principle of pulse-height analysis (PHA) in solid state media, collimated by a rectangular 22.5$<sup>\circ$ field of view and entrance aperture. The EPDE contained a stack of 6 silicon detectors (0.7 cm total thickness) with 0.15 cm$<sup>2$ str geometric factor, with ion contamination mitigated by an aperture foil and digital coincidence logic. The EPDI contained a stack of 2 silicon detectors (0.1 cm total thickness) with 0.03 cm$<sup>2$ str geometric factor, with electron contamination mitigated by a magnetic broom stage and digital anti-coincidence logic. The total assembled mass, volume, and power required for the EPD detector heads and circuit boards were 939 g, 10$\times$9.8$\times$8.4 cm$<sup>3$, and 2.3 W, respectively. We describe each EPD design element alongside instrument data products and processing.