Controllable Growth And Uniformity Optimization Of Large Size LYSO:Ce Scintillation Crystal For PET Application

Cerium doped lutetium yttrium silicate（LYSO:Ce）crystal has excellent comprehensive properties and is the most widely used scintillation crystal in Positron Emission Computed Tomography（PET）.Since LYSO:Ce is in great demand in PET systems,the development of large-size LYSO:Ce crystal growth technology is of great significance.At present,the control strategies,co-doping ions,technological parameters and growth interface during the growth of large-size LYSO:Ce crystal are mostly adjusted by artificial experience,which makes the crystallization environment,doping concentration and defects formation uncontrollable.Therefore,the quantity of the as-grown large-size LYSO:Ce crystals are unstable and the scintillation performances are inhomogeneous,which can result in low utilization of the crystals.In this paper,large-scale LYSO:Ce and LYSO:Ce,Ca crystals with controllable shape,quality and scintillation properties are grown by developing high-precision growth equipment,selecting suitable components,optimizing growth conditions,quantifying crystallization parameters and stabilizing a slightly convex growth interface.Furthermore,the inhomogeneities along the crystal growth direction are studied,the related mechanisms are concluded to guide the growth of high-performance,high-uniform crystals.The main research contents are as follows.Firstly,a crystal growth equipment with automatic diameter control technique is developed by introducing the meniscus model,developing expert PID control algorithm with self-turning parameters based on growth characteristics and constructing a distributed hardware with an information software.The self-developed equipment has high control precision with weight fluctuation less than 0.3 g,power fluctuation less than 8 W and diameter deviation less than 1 mm during the equal diameter stage.On this basis,a rapid Czochralski growth platform is developed,and the refined LYSO:Ce crystals with different host elements,co-doping ions and luminescent centers are grown and evaluated.The refined solution with better scintillation performance is applied to develop the large-size crystal.Then,large size LYSO:Ce and LYSO:Ce,Ca crystals are grown by the self-developed equipment.The key parameters of high-precision seeding,pulling speed optimization,crucible correction and protection are quantified.A controllable growth technology,which can stable the growth interface as a slightly convex shape,is established to control the crystallographic environment.The problems of color center suppression,temperature field design,oxygen balance and ion doping are solved.As a result,high quality LYSO:Ce and LYSO:Ce,Ca crystals with size ofΦ70 mm×200 mm are developed.In addition,the inhomogeneities of composition,defect,optical and scintillation properties along the growth direction of large size LYSO:Ce and LYSO:Ce,Ca ingots are studied.The effects of doping concentration,growth interface,air annealing and Ca²⁺co-doping on improving the scintillatiton performance and uniformity are concluded.Therefore,the sicintillation uniformity of the large size LYSO:Ce crystal is improved and the high performance indicators with non-uniformity of light yield less than±2.6%and fluorescence decay time 30.4 ns are achieved,which makes the utilization of crystal exceeding 80%.Finally,the high-performance crystals are used to develop the PET detector.The energy resolution can reach 10.9%and time resolution can reach 288 ps by coupling the air annealed LYSO:Ce,Ca crystal with silicon photomultiplier to develop PET detector prototype.Based on the digital PET detector,a crystal position adjustment scheme is proposed to improve the spatial resolution in the region of interest.In this dissertation,a high-precision crystal growth equipment with controllable growth technology and uniformity optimization theory is developed.Large-size LYSO:Ce and LYSO:Ce,Ca crystals with high performance and high uniformity are grown,which improves the industrial chain from scintillation crystals to PET systems,and provides a foundation for the development of new generation digital PET equipment.