Design and performance of the Low-Energy Module for the Ziré instrument on the NUSES mission: A compact spectrometer for low-energy particles in space high-radiation environments

NUSES is a space mission scheduled for launch in late 2026 into a Sun-synchronous Low Earth Orbit at 550 km altitude. During its three-year nominal lifetime the satellite will test innovative methods for observing low-energy cosmic and gamma rays, astrophysical neutrinos, and for monitoring space-weather phenomena and lithosphere–atmosphere–ionosphere–magnetosphere couplings. Two complementary payloads are on board: Terzina, an optical Cherenkov telescope for extensive air showers, and Ziré, a particle detector covering an energy range from a few MeV up to hundreds. To extend Ziré’s sensitivity to lower energies, the Low-Energy Module (LEM) has been developed.The LEM is a compact (10 (Formula presented) 10 (Formula presented) 10 cm3) particle spectrometer capable of measuring electrons from 0.1 to 7 MeV and protons from 3 to 50 MeV along the satellite orbit. A drilled plastic scintillator acts as an active collimator, vetoing off-axis tracks, while five silicon spectrometers, completed by a plastic calorimeter and lateral veto, provide event-by-event identification. This layout yields an angular resolution of about (Formula presented) for protons and an electron threshold below 30 keV, enabling operations in high-radiation regions such as the South Atlantic Anomaly and the inner Van Allen belt.By tracking variations in trapped-particle fluxes the LEM will investigate precipitation episodes linked to geomagnetic storms, solar flares, thunderstorms, and possible seismic precursors, thus contributing both to space-weather forecasting and to the validation of particle-acceleration and models of Magnetospheric–Ionospheric–Lithospheric Coupling. Preliminary simulations and laboratory tests confirm the feasibility of the design and its expected performance.