Study on the inhomogeneity of LYSO crystal boules grown by the CZ method for PET applications

Abstract

In this paper, Φ 72 × 200 mm LYSO:Ce and Ca-codoped LYSO:Ce crystal boules were grown by the CZ method. The growth conditions were stabilized by controlling the rotation rate to maintain a slightly convex solid–liquid interface shape. Samples 3.9 × 3.9 × 20 mm in size were cut from the crystal boules along the growth direction. The inhomogeneities, including the components and the optical and the scintillation properties, of the as-grown and air-annealed samples were studied. The component analysis showed an increasing trend of Ce and Ca concentrations along the growth direction of the crystals. By combining the defect study of the samples, appropriate ranges of doping concentrations on high-quality LYSO crystal growth were determined. The optical properties, including the emission, excitation and absorption spectra, of the samples were investigated. The spectra indicated the differences of Vo defect, Ce1/Ce2 ratio and Ce4+ content in the samples, which can be related to the scintillation performances. Scintillation studies showed that light output and energy resolution vary greatly from top to bottom of the as-grown LYSO:Ce. These properties can be improved to achieve better uniformity by air annealing and Ca codoping. The light output can reach 106.6% (relative to a standard sample) with a uniformity of ±2.6%, and the energy resolution can reach 9.4% with a uniformity of ±0.2% for the air-annealed Ca-codoped LYSO:Ce samples. In conclusion, this paper describes some factors that will affect the scintillation inhomogeneity and can guide the growth of high-quality LSO/LYSO crystals.