Computational Modeling For The Vertical Bridgman Growth Of Babrcl:Eu Crystal

Abstract

In recent years, many new scintillator crystals for X-ray or gamma-ray detection have been discovered. They have great potential to be used in security devices or medical imaging devices. However, there are a couple challenges need to be overcome before these scintillator crystals can be commercialized. Firstly, the internal physical processes during the growth of these crystals are hard to be observed, making it difficult to control and optimize the processes. Secondly, cracking is a main issue that hinders the growth of high quality, large size scintillator crystals. Slow cooling, a conventional way to reduce thermo-elastic stress, fails to completely prevent cracking in scintillator crystals. In this thesis, we, together with our experimental collaborators, will demonstrate that computational modeling and advanced experimental tools can help researchers overcome these challenges and manufacture high quality, large size scintillator crystals. BaBrCl:Eu crystal and vertical Bridgman method are chosen as the candidate material and candidate crystal growth method in this thesis. Tremsin and coworkers developed a neutron imaging system to observe the vertical Bridgman growth process of scintillator crystals. Their measurements provided a direct observation of segregation and interface shape within a vertical gradient freeze system (VGF) that is large enough to exhibit the complex interplay of heat transfer, fluid flow, segregation, and phase change characteristic of an industrially relevant melt-growth process. We have applied continuum models to simulate a VGF growth process of BaBrCl:Eu crystal conducted in the neutron imaging system. Our models provide a rigorous framework in which to understand the mechanisms that are responsible for the complicated evolution of interface shape and dopant distribution in the growth experiment. We explain how a transition in the solid/liquid interface shape from concave to convex is driven by changes in radial heat transfer caused by furnace design. We also provide a mechanistic explanation of how dynamic growth conditions and changes of the flow structure in the melt result in complicated segregation patterns in this system. Onken and coworkers used neutron diffraction to measure the crystal structure evolution of BaBrCl:Eu at different temperature levels. Their results showed that the chemical stress induced by the lattice mismatch between Eu dopant and BaBrCl is responsible for the cracking of BaBrCl:Eu crystal during cooling process. We developed finite element models to analyze the chemical stress in BaBrCl:Eu crystal under different growth conditions based on the study of Onken and coworkers. To our knowledge, these are the first computations for chemical stress in bulk crystal growth process. Our results showed that the melt/crystal interface shape and the associated melt flows have a strong influence on the radial segregation outcome of Eu, which determines the chemical stress profile in the crystal. Counterintuitively, growing this crystal at slow growth rates can lead to high stress levels and tensile stress states near the cylindrical surface that promote cracking. However, a slightly faster growth rate can produce Eu radial concentration gradients that provide a protective, compressive force layer that would suppress cracking. Our results show that the chemical stress could be tailored by designing appropriate interface shapes and melt flows.