Scintillation Properties And Mechanisms Of Pr³⁺-activated CLYC Crystals For N/γ Dual-mode Detection

With the advancement of neutron science and the development of neutron applications,the applications of neutron detection in national defense security,advanced manufacturing,nuclear medicine,and large scientific devices are becoming increasingly important.The widely used conventional 3He gaN-tube detectors are currently facing a global 3He resource crisis,and there is a serious risk of supply outage.The development of new scintillation materials that can realize neutron/gamma dual-mode detection is the main trend to solve this problem internationally.The elpasolite rare earth compound halide Cs2LiYCl6:Ce(CLYC:Ce)crystal is the best neutron/gamma dual-mode detection crystal with the best comprehensive performance.However,the self-absorption effect of Ce3+on its key corevalence luminescence(CVL)will cause the degradation of neutron/gamma discrimination performance of large size crystal.In this paper,CLYC was used as the research object to improve the performance of neutron/gamma discrimination through luminescence center substitution,and the corresponding scintillation mechanism was deeply studied.The main research contents and results were as follows:(1)A test system for neutron-gamma discrimination performance was independently built,the pulse shape discrimination(PSD)algorithm based on the charge comparison method was optimized.The accuracy and advancement of the system and algorithm were verified by comparing the neutron-gamma discrimination performance of 2-inch CLYC:Ce crystals and 3-inch NaI:Tl,Li crystals read out by photomultiplier tube(PMT)and Silicon photomultiplier tube(SiPM)arrays.A sampling method for fast and accurate evaluation of scintillation performance under low sampling rate was proposed,which provides a useful reference for the design of low-power detectors.The neutron/gamma discrimination performance of CLYC:Ce and NaI:Tl,Li crystals was comprehensively compared and evaluated based on the independent test system and algorithm.The work lays the foundation for the subsequent material performance evaluation.(2)Aiming at the problem of Ce3+self-absorption of CVL luminescence by CLYC:Ce crystal,a modification strategy of Pr3+ substitution for Ce3+was proposed to effectively avoid the absorption of CVL by the crystal,thus improving its neutron/gamma discriminative performance.The CLYC:Pr crystals were grown by the Bridgman method,and characterized by UV-Vis,PL/PLE,fluorescence lifetime,XEL and VUV.The scintillation luminescence mechanisms of CLYC:Pr crystals were revealed,including CVL luminescence,self-trapping exciton luminescence(STE)and Pr3+ luminescence.The CLYC:Pr crystals showed excellent neutron/gamma discrimination with a figure of merit(FoM)of 5.73.The energy resolution and light yield of CLYC:Pr crystals were comparable to those of CLYC:Ce.Decay time analysis showed that its excellent neutron/gamma discrimination performance was due to two reasons.First,Pr3+provided a higher proportion of fast components because it did not absorb CVL.The second is that the dual-band emission of Pr3+ provides both the fast component of 4f5d→4f emission and the slow component of 4f→4f emission,further amplifying the neutron/gamma waveform difference.Using the"Pr3+ activation strategy" is expected to develop a series of dual-mode detection crystals with excellent neutron/gamma discrimination performance.(3)The Pr3+ and Ce3+ co-doping strategy was used to further optimize the light yield of CLYC:Pr crystal and improve its comprehensive performance.The CLYC:Pr,Ce crystals were grown by the Bridgman method.The scintillation luminescence mechanism of CLYC:Pr,Ce crystals was revealed by UV-Vis,PL/PLE,fluorescence lifetime,XEL.It was found that Pr3+ and Ce3+ luminescence centers mainly showed independent competition.The CLYC:Pr,Ce crystals have high light yield,high energy resolution and excellent neutron/gamma discrlmination performance under the suitable concentration of Pr3+ and Ce3+ co-doping,which can play the advantages of the two luminescence centers at the same time.(4)A theoretical model of "The effect of particle action scale" was proposed to thoroughly analyze and reveal the origin of the difference in neutron-gamma waveforms in CLYC:Pr crystals.Monte Carlo softwares,such as ESTAR,SRIM and CASINO,were used to simulate and calculate the differences of gamma and neutron ionization densities and action scales in CLYC crystal.The physical processes of the four luminescence mechanisms of CLYC:Pr crystals under neutron/gamma excitation were discussed to reveal the sources of the differences in waveforms.Based on the above,a general strategy for designing excellent particle discrimination performance was proposed for the design of particle discrimination materials.