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This article outlines the main results of a research and development cooperation between Philips Research Eindhoven; Peter the Great St. Petersburg Polytechnic University; Ioffe Institute, St. Petersburg; Utrecht University; and Philips Healthcare. It reviews the properties of garnet ceramics in the (Lu,Gd)3(Ga,Al)5O12:Ce system for medical imaging, especially time-of-flight positron emission tomography (PET). Thermally stimulated luminescence (TSL) peaks are attributed to impurities, verified by intentional codoping of samples. A lately developed method allows extraction of carrier lifetimes, thermal trap depths, and frequency factors from TSL and afterglow measurements. A detailed analysis reveals the presence of a distribution of trap depths, allowing a more accurate afterglow modeling. Activation energies of thermal ionization and trap depths obtained from TSL show the influence of Ga/Al substitution on thermal quenching and on trap position. The resulting nonmonotonic dependence of the conduction band edge with Ga content in (Lu,Gd) garnets is consistent with earlier predictions. Shallow traps determine both signal decay and short-term afterglow. The impact of signal height, rise, and decay times on coincidence resolving time and further on PET image quality is described by analytical models. [ABSTRACT FROM AUTHOR]

Scintillators based on Ce-doped garnets are regularly co-doped with Mg2+ or Ca2+ to form Ce ions in 4+ state and reduce undesired afterglow. However overly high Ce4+ concentration leads to poor light yield performance. In order to understand the reason for variation in luminescence efficiency of Ce3+- and Ce4+-doped garnets we investigate the differences in energy conversion processes in complex LuGd 2 Ga 3 Al 2 O 12 :Ce3+/Ce4+ ceramics by means of VUV synchrotron irradiation. At first we have established via transmission spectroscopy and X-ray absorption spectroscopy that LuGd 2 Ga 3 Al 2 O 12 :Ce, Mg sample contains cerium in the 4+ state only. Then we show with VUV spectroscopy efficient interaction of excitons with Gd3+ and Ce3+, and lack of exciton absorption edge in LuGd 2 Ga 3 Al 2 O 12 :Ce4+ excitation spectrum. Instead, Ce4+ exhibits charge-transfer absorption band in the range of exciton emission. We suggest that when Ce4+ concentration becomes too high, the exciton → Gd3+ → Ce3+ energy transfer path is hindered. It leads to high intensity of Gd3+ luminescence in Lu 1 Gd 2 Ga 3 Al 2 O 12 :Ce, Mg ceramics, but lowered Ce3+ X-ray excited luminescence. Fine balance between 3+ and 4+ Ce concentrations is necessary to achieve the best performance of garnet scintillators. • VUV excitation and X-ray absorption spectra of LuGd 2 Ga 3 Al 2 O 12 :Ce3+/Ce4+ under synchrotron radiation. • High Ce4+ concentration in garnet scintillators hinders the exciton.→ Gd3+ → Ce3+ energy transfer path • Electron-hole and exciton capture processes schematically depicted for Ce-doped garnets. [ABSTRACT FROM AUTHOR]