Development of a single crystal with a high index of refraction

Time-of-flight Positron emission tomography (TOF-PET) is one of the next-generation medical imaging methods, which requires scintillators with a very short decay time. However, the shortest scintillation decay times are typically 20–30 ns, and these values are not sufficient for TOF-PET. Cherenkov counters are used in high energy physics and they are expected to be applied in medical imaging due to their short decay time. Here, high-refractive index materials are necessary for Cherenkov radiators to reach a high light output. We measured refractive indices of Gd3Ga5O12 (GGG), Y3Ga5O12 (YGG) and Lu3Ga5O12 (LuGG) crystals grown by a micro-pulling-down ( μ - PD ) method. The GGG, YGG and LuGG crystals were found to have refractive indices of ~ 2.5 , ~ 2.3 and ~ 2.3 at 400 nm, respectively. Then we grew a 40 mm diameter GGG crystal by the Czochralski method, and the emission decay times of the GGG crystals irradiated with muons and gamma rays were 10 ± 1 ns and 10 ± 2 ns , respectively, using a photomultiplier tube (Hamamatsu R6231-100). Cherenkov light of the GGG crystal could be observed for the gamma-ray irradiation.