Detector response matrix calculation strategy for scintillation-based gamma-ray detectors in the GRID nanosatellite constellation

The Gamma-Ray Integrated Detectors (GRID) mission employs a nanosatellite constellation with scintillation detectors to monitor high-energy transients, mainly gamma-ray bursts (GRBs). Accurate detector response matrices (DRMs), which quantify the association between incident gamma-ray energy distributions and the measured spectra, are necessary for the spectral fitting and scientific analysis of detected GRBs. In this study, we present a DRM calculation strategy comprising two complementary approaches: (1) direct Geant4 Monte Carlo simulations with the actual incident direction of detected GRBs, which demonstrates high accuracy and is therefore used for analysis of scientifically significant events and (2) an interpolation method based on a precomputed database of simulated responses, which is faster and suitable for time-critical applications. We validated the DRMs obtained using both calculation methods via spectral analysis of in-orbit GRB data, demonstrating consistency with independent observations from the Fermi satellite. These results provide the essential DRMs required for GRB spectral analysis in the GRID mission, achieving reliable scientific outcomes. Furthermore, this clear and specific DRM calculation strategy ensures consistent results across detectors in the constellation and facilitates performance comparison, which is critical for multisatellite missions, such as GRID.