碘化铯闪烁晶体的余辉形成机理及其抑制

Thallium doped cesium iodide (CsI:Tl) crystal is one of outstanding and widely applied inorganic scintillators with many advantages, except its strong afterglow, which is easy to cause pulse pileup in high count rate detection,reconstruction artfacts in medical imaging and reduction of contrast and space resolution, so that its application in medical imanging, security inspectation and other high-speed X-ray flash imaging have been hindered.As afterglow is caused by the radiative recombination of carriers freed from traps filled during X-ray or Gamma ray irradiation, and the traps are closely related to the different point defects in the crystal,we'll focus our research on the energy traps resulted from Tl activation ions, other impurity ions and the intrinsic non-stoichiometry defects in the crystal. Based on the optical,electric, magnetic and thermol signals released from these defects, we are going to use our domestic afterglow tester as well as morden analytical instruments to explore the defect types, defect concentration, their energy levels and their relationship to the afterglow. The crystals will be grown with our modified vertical Bridgman method and some codopant may be introduced into CsI:Tl lattice to decrease the number of trapped carriors, which give rise to afterglow. The target of this project is to explain the underlying kinetic process,reveal the generation mechanism of the afterglow in CsI:Tl and propose the possible chemical recipe and growth condition to suppress the afterglow so that a new type of CsI:Tl crystals with low afterglow and high ligh yield can be obtained under the support of this research.

掺铊碘化铯（CsI:Tl）是一种综合性能优异且已经被广泛应用的无机闪烁体，但其余辉的存在极易造成高计数条件下脉冲信号的堆积、图像重建中的鬼影和图像衬度与空间分辨率的降低，从而严重制约了该晶体在核医学成像、安全检测和高速X射线闪光照相等领域的大规模应用。鉴于余辉与晶体结构中不同能级陷阱（或缺陷）之间的密切关系，本项目将以该晶体中存在的由掺杂剂铊离子、其它杂质离子和晶体内部的本证结构缺陷所诱发的陷阱能级为主要研究对象，利用这些缺陷或陷阱在光电磁热等性能上所释放出来的物理信号为线索，以自制的余辉测试仪和现代结构分析技术为手段，揭示出这些缺陷的结构、含量、能级、成因以及它们与晶体余辉之间的内在联系，阐明余辉的形成机理，并在此基础上探索出抑制晶体余辉的途径和工艺路线。晶体生长采用改进的坩埚垂直下降法，适当的共掺杂剂会被用来减少陷阱上载流子的浓度。目标是为生长出余辉低、光输出高的新型碘化铯晶体提供理论。

碘化铯晶体因余辉过高而限制了它在安全监测和核医学领域的广泛应用，本课题以坩埚下降法生长的CsI:Tl 晶体为研究对象，通过对影响余辉强度的铊离子浓度、杂质缺陷和结构缺陷等诸要素的深入分析，初步揭示出CsI:Tl 晶体余辉的形成机理，发现用Yb2+或Sm3+稀土离子为共掺杂剂能够有效抑制碘化铯晶体的余辉。在80ms的时间点下降了一个数量级，达到0.035%@80ms；同时对CsI:Tl晶体的光输出不仅没有任何消弱，反而还有所增强，达到90000± 6000Ph/MeV，晶体的能量分辨率还得到一定程度的优化，达到7.9%@511keV。这是迄今为止在抑制碘化铯晶体余辉方面所取得的最为积极的研究成果。根据实验结果提出Yb2+离子共掺杂CsI:Tl晶体的余辉得以大幅度降低的机理是，Yb2+离子在CsI:Tl晶体中通过捕获空穴形成Yb3+，阻止这些空穴与Tl原子结合形成Tl+离子发光中心，生成的Yb3+和Tl0阻止了[(TI+)Vk]芯的形成，达到了抑制余辉的效果。发表了8篇学术论文，申请了4项发明专利和1项使用新型专利。