Irradiation studies of a multi-doped Gd$_3$Al$_2$Ga$_3$O$_{12}$ scintillator

The characteristics of a Gd$_3$Al$_2$Ga$_3$O$_{12}$ crystal scintillator doped with cerium and co-doped with magnesium and titanium have been studied, mainly in view of using it for the Phase II upgrade of the LHCb electromagnetic calorimeter. Samples of the scintillator were irradiated with $\gamma$ ($^{60}$Co) to 2 kGy and with 24 GeV protons to 900 kGy. The proton fluence value was $\sim 3 \cdot 10^{15}$ cm$^{-2}$. It was found that $\gamma$-irradiation did not produce any change in the optical transmission of the crystals in the spectral range of the scintillation light, whereas a degradation after the proton irradiation was measurable. For the 1 cm thick sample, a loss of transmission of 3.6% was measured at the wavelength of maximum scintillation (520 nm), and the measured induced absorption coefficient at this wavelength was $\sim 3.6$ m$^{−1}$. The formation of radioisotopes in the crystal at proton irradiation has been analysed. The formation of isotopes was also simulated with the help of the FLUKA package. The simulation was found to be in a good agreement with experimental results. The results have been used to estimate the expected intensity of parasitic radio-luminescence at high-luminosity operation in a GAGG/W sampling electromagnetic calorimeter.