Understanding the scintillation efficiency of cerium-doped LSO, LYSO, YSO and LPS crystals from microwave study of photoconductivity and trapping

Photoconductivity and trapping effects were studied in various Ce-doped scintillating crystals (LSO, LYSO, YSO and LPS) by means of the microwave resonant cavity technique. In all the investigated crystals, at 300 K and under 355 nm excitation, transitions from the 4f ground state of Ce3+ ions to low energy levels of the 5d excited configuration give rise to a photoconductivity signal. Temperature- and excitation power-dependent measurements indicate that the photoionization mechanism results from the competition between thermal activation and two-photon (two-step) absorption. However, depending on the crystals, the relative weight of these two possible routes leading to electronic delocalization varies greatly. Moreover, four mixed Lu2−xYxSiO5:Ce (LYSO) crystals of different compositions and with various scintillation yields show different behaviors, in-between Y2SiO5:Ce (YSO) and Lu2SiO5:Ce (LSO). Our measurements concerning trapping and photoconductivity dynamics show that trap-related perturbations affecting the electronic transport during the scintillation process is a major source of losses for the light yield of these scintillating crystals.