This answer states:
From the AstroMEV website:
Terrestrial gamma-ray flashes (TGFs) are high-energy photons originating from the Earth's atmosphere in association with thunderstorm activity. TGFs were serendipitously discovered by BATSE detectors aboard the Compton Gamma-Ray Observatory. TGFs have also been detected and further studied by the RHESSI, AGILE and Fermi satellites. Their emission extends up to 100 MeV and exhibits an e+ – e- annihilation line. TGFs were utterly unexpected and as of now they are not fully understood. They are believed to be the product of particles acceleration inside thunderstorms. As they are produced in the Earth’s atmosphere, they potentially have a tremendous impact on our understanding of thunderstorms and atmospheric electrodynamics in general.
100 MeV is quite large for a terrestrial process. Nuclear gamma rays are typically in the 100 keV to few MeV range for nuclear transitions, though there can be larger ones from exotic processes in nuclear collisions.
These are really high energy photons!
Question: How to satellites measure the energy of such high-energy single photons with reasonable certainty?
Please answer with something more than "they use gamma ray detectors". It takes a lot of mass to stop such a high energy photon, and to stop it in such a way that you can be reasonably certain you have contained all of the energy of the resulting shower of particles and photons produced, which adds even more mass and other constraints. Making it all work on a spacecraft that can run unattended in a space environment and withstand the vibrations of launch without losing accuracy or integrity is quite a feat.