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Nowadays, it is possible to accelerate bunches of particles in the interaction of ultrahigh intensity (UHI) laser pulses with matter. Electrons, protons, ions, and high-energy photon beams can be produced in experiments and reach kinetic energies close to hundreds of megaelectronvolts for protons and gigaelectronvolts for electrons and for the associated Bremsstrahlung photons. At these energies, these beams can induce a large variety of nuclear reactions, which can be detected and studied using $\gamma $ -ray spectroscopy techniques. At standard accelerator facilities, scintillator detectors are commonly used to perform prompt $\gamma $ -ray spectrometry studies. However, during laser–matter interactions, high fluxes of X-rays (mostly soft) are generated, which lead to instantaneous huge energy deposits (~1 $\mu \text{J}$) in these scintillators. Depending on the laser characteristics (energy and pulse duration), the detector recovery time after these X-ray flashes can reach several milliseconds, which makes any prompt or “in beam” measurement impossible. The origin of this long-duration signal is investigated in the case of a LaBr3 crystal coupled to different photodetectors. While it was impossible using standard photomultiplier tubes to detect $\gamma $ -ray emissions before a few milliseconds after a laser shot, we could, using a hybrid photodiode, resolve single $\gamma $ -ray emission a few tens of microseconds after the laser shot. Furthermore, we have also shown that the LaBr3 scintillator presents an unexpected long-lived light emission (afterglow). Directions are suggested for future studies in order to minimize the effects of this afterglow emission. [ABSTRACT FROM AUTHOR]