面向位敏中子探测器的ZnS基透明陶瓷的制备与性能优化研究

Plenty of neutron scattering spectrometers require neutron detectors with large area, high detection efficiency and better spatial resolution to support the development of large-scale scientific facilities like Chinese spallation neutron source (CSNS) and others. The detectors based on 6LiF/ZnS(Ag) scintillator can replace the conventional detectors using 3He, which turn more expensive and lesser supply at the present time, so they are optimal detecors selected and applied by CSNS and other neutron sources. The scintillator detectors are economic to meet the requirement of large area, but the available form of 6LiF/ZnS(Ag) is an opaque mixture of powders with strong self-absorption and wide-angle scattering of emission. The self-absorption of emission can decrease the neutron detection efficency, and the scattering of emission can lead to emission crosstalk to limit the spatial resolution. The key problem can be solved to have higher detection efficiency and spatial resolution by using transparent single crystals and ceramics with very little self-absorption and scattering. By comparing with single crystals, the production of ZnS based transparent ceramics are more economic and easily but with similar optical properties, so by the thermal diffusion method, here we will implant Li/Ag ions in pure ZnS transparent ceramics obtained by chemical vapor deposition to produce Li/Ag doped ZnS based transparent ceramics. The related research cover the influencing mechanism of the distribution and concentration of Li/Ag on the material properties, the controlling rules about emission wavelength and the optimal process of thermal diffusion method. The result of the project can improve the knowledge about the fabrication and property of ZnS based transparent ceramics and offer novel neutron sensitive materials so as to provide the basis for future production of full 6Li lithiumization and large scale transparent ceramics, which are very useful for the development of high performance neutron scattering spectrometers.

中国散裂中子源等大型中子科学装置的发展需要大面积、高探测效率和高空间分辨率的中子探测器。虽然6LiF/ZnS(Ag)闪烁体可以解决传统大面积使用3He的价格昂贵和可供气量短缺的问题，但是目前可用的材料形态是不透明的粉体混合物，易于发生出射光的自吸收以及大角度散射造成的不同出射光间的串扰，从而限制中子探测效率和空间分辨率的提高。基于透明单晶或陶瓷在出射光自吸收和散射小的优势可以解决这个瓶颈问题。相比于单晶生长和改性的困难，透明陶瓷在性价比上具有优势。因此本项目拟在化学气相沉积法可制备纯ZnS透明陶瓷的基础上，通过热扩散法注入Li/Ag离子而获得掺杂的ZnS基透明陶瓷，研究影响陶瓷中Li/Ag的分布和浓度大小的机制，掌握发光波长的调控规律，并且优化热扩散工艺，从而获得新型中子敏感材料，深化有关陶瓷制备与性能的认识，为后继全6Li化和大尺寸透明陶瓷的制备奠定基础，促进高性能中子散射谱仪的建设。

中国散裂中子源（CSNS）是以中子散射谱仪为基础的大科学装置。高效的中子探测器是构成中子散射谱仪的关键部件。国内外公认基于6LiF/ZnS(Ag)闪烁体的中子位敏探测器是今后发展的重点，并且可以解决传统大面积使用He-3的昂贵和供应短缺的问题。由于目前所用的闪烁体是不透明的粉体，不但存在较强的出射光自吸收和大角度散射，而且粉层最大允许厚度仅0.4mm，限制了对中子的吸收，不利于中子探测效率和空间分辨率的提高。因此本项目提出了透明陶瓷化的设想，成功制备了天然锂掺杂的ZnS基透明陶瓷，最大尺寸为15x15x3.5mm^3，可见光范围内的平均直线透射率达到60%；并且发现作为扩散源，硫化物要优于纯金属、合金和氧化物。陶瓷的发光是绿光为主的可调宽带发光，可以实现多波段同时发射，不同发光波段来源于不同的缺陷能级。发光时间为微秒级别，和6LiF/ZnS(Ag)标准屏属于同一数量级。所得陶瓷在241Am源产生的α射线辐照下，相对光产额达到标准屏的47.8%。更重要的是这种天然锂掺杂（Li-6丰度近似为7%，而标准屏不小于95%）的陶瓷在中子束辐照下具有表征中子响应的高斯峰；相对光产额达到标准屏的25.8%，相对探测效率达到5‰。通过理论模拟进一步论证了Li掺杂的高浓度和立方结构的稳定性，而且证明了中子探测效率随Li-6浓度和厚度均是非线性增加，同时两者的耦合也是非线性的。最后，基于CSNS的BL20中子束线还建立了一套实验测试平台，用于新型闪烁体、探测器及其与标准屏的性能比较。总之，本项目验证了ZnS基透明陶瓷的存在以及在探测α射线和中子束线上的潜在应用。这种透明材料对出射光具有高透过率和近直线传输的特性可以降低自吸收和大角度散射的影响，有助于突破CSNS等中子源发展高性能中子探测器的瓶颈，提高中子探测效率和空间分辨率，更好地促进高性能中子散射谱仪的建设。