Investigating the fluence of bright TGF events detected by the Atmosphere-Space Interactions Monitor

Terrestrial Gamma-ray Flashes (TGFs) are short flashes of high-energy photons produced by thunderstorms. They are intense phenomena that may have high photon fluxes with energies up to 40 MeV when observed from detectors in orbit. All instruments in space have suffered instrumental saturation during bright events, including CGRO-BATSE, RHESSI, Fermi-GBM, AGILE-MCAL and ASIM-MXGS. The effects include dead-time and pulse pile-up, which lead to an underestimation of the TGF fluences and, in some cases, incorrect photon energies. 
    A key asset of ASIM is that it has two detectors on the same platform: the High Energy Detector (HED, 300 keV to ~40 MeV) and the Low Energy Detector (LED, 50 keV to 400 keV). LED is only weakly affected, which makes it possible to estimate corrections to the HED measurements for even the brightest TGFs. With the method we propose, we estimate the loss of photons by combining the LED and HED measurements with GEANT4 Monte-Carlo simulations of the detector responses. 
    We applied the method to three TGF events. The first, TGF-200728, has about 0.15 counts per microsecond  per unit, and is not expected to experience saturation and is used as a sanity check for the method. The other events, TGF-181102 (1.5 counts per microsecond per unit) and TGF-181025 (2.8 counts per microsecond per unit), indicate that the HED misses at least 50% of the photon counts for the brightest TGF events.