Synthesis,structure And Properties Of Novel High Density Scintillators Zr:GdTaO₄

Scintillators are a kind of luminescent materials capable of converting the energy of high-energy rays or particles into amounts of UV/visible photons.It is an important part of a radiation detector and plays an important role in the fields of high-energy physics,space exploration,nuclear medicine imaging,neutron detection,industrial detection,and so on.Scintillators usually require high density,fast luminescence decay,and high light yield.Among these properties,high density and large effective atomic number mean that the scintillator has a high stopping power to high-energy rays and particles,which can reduce the amount of scintillators required and thus reduce the size and construction cost of the detector,which is particularly important for scintillation detectors in the field of high-energy physics.GdTaO4(GTO)possesses a density of 8.94 g/cm3,which is higher than all inorganic scintillators currently used.Its radiation length is 1 cm and its light yield is about three times that of PbWO4(PWO).Thus,GTO has attracted people’s attention.However,the scintillation decay performance of GTO needs to be improved.Therefore,in this dissertation,combining the ideas of "defect engineering" and "bandgap engineering",the scintillation performance of GTO was optimized by Zr4+doping,and a new fast dense scintillator 0.3 at%Zr:GdTaO4(Zr:GTO)single crystal has been found.The main contents and results of this dissertation include:(1)Zr:GTO polycrystalline powders with different Zr4+doping concentrations were synthesized by solid-state reaction at high temperature,and their structure and luminescent properties were characterized.It was found for the first time that Zr4+can significantly enhance the luminescence of GTO polycrystalline powders,and accelerate the fluorescence decay.The reason why Zr4+can enhance the luminescence of GTO polycrystals may be that Zr4+doping increases the concentration of self-trapped excitons,which leads to the enhancement of self-trapped exciton luminescence.And Zr4+doping inhibits the process of trapping migrating electrons and holes via deep traps,which may help accelerate the luminescence decay of Zr:GTO.(2)The 0.0864 at%Zr:GTO,0.3 at%Zr:GTO,and 1.44 at%Zr:GTO scintillation single crystals were grown by the Czochralski method for the first time,and their structures and qualities were characterized.The segregation coefficient of Zr measured by LA-ICP-MS is 0.288.The dislocation morphologies of(100),(010),and(001)crystal faces for Zr:GTO single crystal are quadrilateral,which were determined by the chemical etching method.The dislocation morphology of Zr:GTO single crystal is related to the difference in crystal surface energy,which is explained by the ab initio method.(3)The fluorescent properties of Zr:GTO single crystals with different concentrations were studied.The results show that the transmissivity of Zr:GTO single crystals is about 75%,and the fluorescence spectra of Zr:GTO single crystals show the wide photoluminescence band at 420～620 nm.The results of the fluorescence lifetime show that the fluorescence decay of 0.3 at%Zr:GTO single crystals is faster than that of GTO and other Zr:GTO single crystals.(4)The light yield,energy resolution,and scintillation decay time of Zr:GTO single crystals with different Zr4+doping concentrations were studied.The results show that the light yield and the energy resolution of Zr:GTO single crystal are similar to that of GTO.In addition,0.3 at%Zr:GTO single crystal has the fastest scintillation decay,and the contribution of the fast scintillation decay is the largest.For 0.3 at%Zr:GTO single crystal,the fast scintillation decay is 6 ns(30.96%),the slow scintillation decay is 578 ns(69.04%),and the average scintillation decay is 401 ns.For GTO,the fast scintillation decay is 102 ns(16.25%),the slow scintillation decay is 3291 ns(83.75%),and the average scintillation decay is 2772 ns.The results show that Zr4+doping significantly improves the scintillation decay performance of GTO.(5)The luminescent mechanism for Zr4+accelerating the scintillation decay of Zr:GTO single crystal was studied.The femtosecond transient absorption spectra and the luminescence features of the single crystals show that the slow scintillation decay of GTO and Zr:GTO single crystal is related to self-trapped exciton(STE)luminescence.With the increase of Zr4+doping concentration,the component of STE luminescence in Zr:GTO decreases gradually.The thermoluminescence curves show that Zr4+doping inhibits the generation of deep traps in Zr:GTO,and reduces the number of original deep traps,which inhibits the process of trapping migrating electrons and holes via traps,then accelerates the scintillation decay of Zr:GTO.Besides,the increase of traps associated with oxygen vacancy defects or zirconium substitution doping defects is beneficial to further accelerate scintillation decay of Zr:GTO single crystals.The positron annihilation lifetime shows that Zr4+doping leads to the appearance of the second annihilation lifetime of Zr:GTO,and the first and second lifetime increases with the increase of Zr4+doping concentration.It is related to the positron trapping defect structure introduced by Zr4+doping(Zr’Ta)and the oxygen vacancy defect structure increased by charge compensation(F+).The zirconium substitution defects and oxygen vacancy defects produced by Zr4+doping are likely to produce fast scintillation decay luminescence center in Zr:GTO single crystal,which may be the origin of fast decay luminescence.(6)The thermodynamic properties at high temperature and mechanical properties at room temperature for Zr:GTO single crystal were studied.The results show that the anisotropic characteristics result in Zr:GTO single crystals cracking easily.Thermal expansion curves show that the phase transition occurs at 1650 K,and the annealing temperature should be lower than 1650 K to avoid crystal cracking.The thermal stress of Zr:GTO single crystal in the seeding stage and cooling stage was simulated.The results indicate that the shorter the crystal seed is,the smaller the thermal stress is.Therefore,in the case of satisfying the crystal growth condition,the length of the crystal seed should be as short as possible.Based on the consideration of time and cost,uniform cooling for 48 hours may be the optimal cooling scheme for Zr:GTO single crystal with a dimension of ?25 mm x 30 mm.The synthesis,structure,properties,and luminescent mechanisms of Zr:GTO crystals were studied in this dissertation.A new fast dense scintillator of 0.3 at%Zr:GTO single crystal with a density of 8.84 g/cm3,light yield of 600 ph./MeV,energy resolution of 68.92%,and fast scintillation decay of 6 ns has been found.The 0.3 at%Zr:GTO scintillator can be expected to be applied to electromagnetic calorimeters in the field of high-energy physics.