Evidence and Consequences of Ce $^{4+}$ in LYSO:Ce,Ca and LYSO:Ce,Mg Single Crystals for Medical Imaging Applications

Lu_2(1-x)Y_2xSiO₅:Ce (10 at% Y) single crystals co-doped with Ca²⁺ and Mg²⁺ were prepared by the Czochralski technique. It is shown that co-doping leads to significant improvements of the scintillation performances. Afterglow following X-ray excitation is reduced down to 200 ppm after 20 ms and light yield is increased from 28,000 ph/MeV up to 34,000 ph/MeV under ¹³⁷Cs-662 keV excitation. X-ray Absorption Near Edge Spectroscopy (XANES) was used to demonstrate that a significant part of the Ce ions are stabilized in the Ce⁴⁺ oxidation state in co-doped crystals. A new scintillation mechanism involving Ce⁴⁺ is proposed.