  GAGG:Ce (Cerium-doped Gadolinium Aluminium Gallium Garnet) is a promising new
scintillator crystal. A wide array of interesting features, such as high light
output, fast decay times, almost non-existent intrinsic background and
robustness, make GAGG:Ce an interesting candidate as a component of new
space-based gamma-ray detectors. As a consequence of its novelty, literature on
GAGG:Ce is still lacking on points crucial to its applicability in space
missions. In particular, GAGG:Ce is characterized by unusually high and
long-lasting delayed luminescence. This afterglow emission can be stimulated by
the interactions between the scintillator and the particles of the near-Earth
radiation environment. By contributing to the noise, it will impact the
detector performance to some degree. In this manuscript we summarize the
results of an irradiation campaign of GAGG:Ce crystals with protons, conducted
in the framework of the HERMES-TP/SP (High Energy Rapid Modular Ensemble of
Satellites - Technological and Scientific Pathfinder) mission. A GAGG:Ce sample
was irradiated with 70 MeV protons, at doses equivalent to those expected in
equatorial and sun-synchronous Low-Earth orbits over orbital periods spanning 6
months to 10 years, time lapses representative of satellite lifetimes. We
introduce a new model of GAGG:Ce afterglow emission able to fully capture our
observations. Results are applied to the HERMES-TP/SP scenario, aiming at an
upper-bound estimate of the detector performance degradation due to the
afterglow emission expected from the interaction between the scintillator and
the near-Earth radiation environment.
