HERD on-orbit trigger system: Conceptual design and performance evaluation

This paper presents the conceptual design and expected performance of the on-orbit trigger system for the High Energy cosmic-Ray Detection (HERD) facility. HERD primary science objectives, which consist in the measurement of cosmic rays up to PeV energies and electrons up to tens of TeV, drive an innovative three-dimensional Calorimeter design with omnidirectional acceptance. This geometry, while enhancing the collection area, requires a specific trigger approach that differs from traditional telescope-style instruments. The HERD trigger system design complies with the hardware timing constraints. The trigger architecture is based on a physics-driven calorimeter geometrical partitioning that exploits the energy-dependent shower development to optimize the detection efficiency and background rejection. Several specialized trigger channels address HERD scientific goals: an inclusive High-Energy channel for cosmic-rays above 10 GeV; Low-Energy Gamma and Ultra-Low-Energy Gamma channels for gamma-ray detection below 10 GeV and 1 GeV ranges, respectively; a heavy nuclei channel selecting Z ≥ 3 nuclei; and dedicated calibration channels. Simulation results demonstrate that the system achieves > 98% efficiency for high-energy cosmic rays, while effectively suppressing charged-particle backgrounds in the gamma-ray channels. The total trigger rate is estimated to remain below 200 Hz under nominal orbital conditions, ensuring > 85% data acquisition efficiency. This design balances HERD competing requirements: high efficiency for rare high-energy events and selective background rejection for gamma-ray measurement, within the operational constraints of space-based observations.