LiCaAlF6晶体的非真空生长与中子探测性能研究

The widely application of neutron detection technology promotes the development and progress of neutron detection materials. Traditional materials for neutron detection are deficient in number such as 3He, and not very well in some performance, such as LiI(Eu), so that they are difficult to meet the requirements of the newly developed detection technology. LiCaAlF6, as a new inorganic scintillator containing lithium-6 with a large neutron absorption cross-section, has low gamma-ray sensitivity due to its light composition and stable physical-chemical properties, is regarded a promising candidate for neutron detection. However, the strong dependence of luminescence efficiency on the activator, oxidation contamination, non-stoicheiometry and optical scattering centers formed during crystal growth deteriorate its scintillation properties. The motivation of this project is to eliminate the detriment of oxygen-containing impurities within the raw materials and growth process by means of chemical scavenger, reveal the formation mechanism of crystal defects and search for a growth technology for large size and transparent crystals. The absorption, conversion and transfer of neutron energy between the crystal lattice and luminescence centers will be studied by various spectrometer. The object of this project is to improve the luminescence efficiency and neutron/gamma discrimination ability by optimization the composition, doping ions and doping methods.

中子的应用加速了中子探测技术和中子探测材料的进步和发展，传统的中子探测材料因资源短缺（如3He气体）或性能缺陷（如LiI:Eu）而无法满足新型探测技术的应用需求。无机晶体LiCaAlF6不仅含有捕获截面大的Li-6元素，以及因密度小而具有的低伽马敏感性和稳定的物化性能等优点而可望成为新一代中子探测器材料。但该晶体的发光效率与激活剂的种类和浓度存在很大的依赖性，而且在生长过程中很容易发生氧化污染、组分偏析和引入光散射中心等而降低晶体的闪烁性能。本课题拟通过化学脱氧的方式排除原料和生长系统内的含氧杂质对晶体性能的危害，揭示缺陷的形成原因，探索出生长大尺寸和高透明度晶体的生长技术。通过研究晶体中不同发光中心的占位和替代作用阐明材料与中子之间的能量吸收、传递和转化机理，探索出最佳的组分配比、掺杂剂和掺杂方式，提高晶体的发光效率和中子/伽马甄别能力。

以闪烁晶体取代氦3气体实现中子探测是当今辐射探测技术的一个重要研究方向。本项目围绕LiCaAlF6晶体开展了制备技术和性能研究，在晶体生长方面，摒弃了传统的真空法生长技术，筛选出一种可有效脱除含氧杂质的化学试剂，创立了一套完全不用真空且能生长出优质LiCaAlF6晶体的非真空晶体生长技术，促使晶体的生长成本显著降低。同时通过对组分配比的探索，抑制了晶体中因组分偏析所造成的生长缺陷，获得了预制多晶LiCaAlF6原料的合成方法，即在还原气氛下通过固相反应LiF + (1-x)CaF2 + xEuF3/CeF3 + AlF3= Li(Ca1-x EuF3/CeF3x)AlF6+x来实现。在性能研究方面，通过对晶体缺陷、晶体组分及其与闪烁性能关系的研究，结合理论计算和结构分析，确定了掺杂离子Eu发光中心在LiCaAlF6晶体中占据Ca格位，Eu的分凝系数为0.56，最佳掺杂浓度为2at%Eu~2.5at%Eu，所获得的最高光产额可达25,300±2000 ph/MeV。但Ce在LiCaAlF6晶体中不仅分布不均匀（测得的分凝系数只有0.01），而且光产额较低，无法满足实用要求。LiCaAlF6:Eu晶体在Am-Be中子源激发下的信号响应明显强于LiCaAlF6:Ce晶体，说明前者更具有实用价值。尝试以Cu、Mg、Ca、Ba离子共掺杂LiCaAlF6:Eu晶体以提高其闪烁效率，发现Cu+离子的优化效果明显，其因在于提高了Eu在LiCaAlF6晶体中的有效分凝系数，并抑制了LiCaAlF6:Eu晶体中固有的浅能级陷阱，减少了陷阱浓度。而Mg2+和Ba2+同价离子共掺对LiCaAlF6:Eu晶体闪烁效率的提升无积极作用。实验发现LiCaAlF6:Eu晶体中的Eu离子存在2+和3+两种价态，虽然后者占比很低，但因Eu3+光衰减速度较慢，对晶体应用不利，说明晶体的生长环境存在较高的氧分压，今后须设法克服。