  We discuss the results of a proton irradiation campaign of a GAGG:Ce
(Cerium-doped Gadolinium Aluminium Gallium Garnet) scintillation crystal,
carried out in the framework of the HERMES-TP/SP (High Energy Rapid Modular
Ensemble of Satellites -- Technological and Scientific Pathfinder) mission. A
scintillator sample was irradiated with 70 MeV protons, at levels equivalent to
those expected in equatorial and sun-synchronous low-Earth orbits over orbital
periods spanning 6 months to 10 years. The data we acquired are used to
introduce an original model of GAGG:Ce afterglow emission. Results from this
model are applied to the HERMES-TP/SP scenario, aiming at an upper-bound
estimate of the detector performance degradation resulting from afterglow
emission.
