Study On Crystal Growth And Scintillation Properties Of Rare Earth Ions Doped LiCaAlF₆ Crystals

Scintillation materials can transform high-energy rays or high-energy particles into ultraviolet or visible fluorescent pulse,and act as the key materials for radiation detection.LiCaAlF₆ single crystal with trigonal structure is a scintillation material for neutron detection.The excellent neutron detection capability due to low effective atomic number(Z_eff=14)and density（ρ=2.989 g/cm³）,low sensitivity to gamma radiation background,⁶Li isotope,non-hygroscopic,single crystals of LiCaAlF₆:Eu and LiCaAlF₆:Ce have high light yields of 29,000 ph/n and fast scintillation decay time of28 ns under neutron radiation,respectively,which can satisfy the requirements of different applications.In this thesis,the crystal growth,homogeneity,optical and scintillation properties,defect structures,co-doping modification of rare earth ions doped LiCaAlF₆crystals are investigated.The main contents are divided into three parts:The suitable raw materials preliminary synthesis and crystal growth parameters are explored,a LiCaAlF₆:Eu single crystal withφ25 mm×90 mm is obtained by adding seed using the Bridgman method in non-vacuum atmosphere.The as-grown crystals are highly transparent,crack-and inclusion-free.X-ray diffraction pattern confirms a single phase of trigonal structure with space group of P 3?1c.The effective segregation coefficient of Eu in LiCaAlF₆ crystals is measured.The photoluminescence spectrum and X-ray exited radioluminescence exhibit Eu²⁺ions dominantly exist in LiCaAlF₆:Eu single crystal with a small amount of Eu³⁺ions.Underγ-ray radiation,single crystals of LiCaAlF₆:Eu have light yields of 23,000±2000 photons/Me V and single scintillation decay components of 1.64μs.The defect structures of LiCaAlF₆:Eu crystal grown in non-vacuum atmosphere is investigated by using three-dimensional thermoluminescence measurement.The phase analysis of opaque part at the end and transparent part in the middle reveal the impurities at the end of LiCaAlF₆:Eu crystal.The distribution of Eu ions along the axial and radial directions in the LiCaAlF₆:Eu crystal is studied,and the influence of Eu distribution on optical and scintillation properties for LiCaAlF₆:Eu crystal are compared.The distribution of Eu ions along the radial direction is more uniform than the axial direction.For axial direction,the closer to the seed,the worse scintillation efficiency becomes.LiCaAlF₆ single crystals doped with different Eu concentrations varied from 1 at%to 5 at%were grown by the Bridgman method in non-vacuum atmosphere.The highest scintillation light output of LiCaAlF₆:Eu single crystals is achieved when Eu concentration is between 2 and 2.5 at%.Co-doped LiCaAlF₆:Eu crystals with isovalent or aliovalent ions at the optimal concentration of 2at%Eu were grown.The effects of Cu⁺,Mg²⁺and Ba²⁺co-doping on optical properties,scintillation yield,scintillation decay,and the defect structures are studied.The results show that co-doped with Cu⁺,Mg²⁺and Ba²⁺ions will not change the luminescence mechanism,the luminescence centers are still Eu²⁺and Eu³⁺.The Cu⁺co-doping can improve the scintillation efficiency with 29,200±2000 ph/Me V light yield.But Mg²⁺and Ba²⁺co-doping have negligible effect on light yield.The causes of scintillation performance by co-doping with Cu⁺,Mg²⁺and Ba²⁺ions are studied.LiCaAlF₆:2at%Ce,LiCaAlF₆:3at%Sm and LiCaAlF₆:0.5at%Pr single crystals were obtained using the Bridgman method in non-vacuum atmosphere.Research on the luminescence mechanism and scintillation performance of other ion doped LiCaAlF₆crystals has been carried out.The Ce doped LiCaAlF₆ crystal showed the typical Ce³⁺ions 5d-4f emission.Underγ-ray radiation,single crystals of LiCaAlF₆:Ce have low light yields of 1,000±100 ph/Me V but fast decay time of 45 ns.It is found that the Sm doped LiCaAlF₆crystal have high transmittance of 90%,the emission center is Sm²⁺and Sm³⁺ions,and the emission range from 550 nm to 800 nm.The transmittance of Pr doped LiCaAlF₆ crystal is greater than 80%,and the emission center is Pr³⁺ions.Both crystals cannot detect the photopeak underγ-ray irradiation.