铀矿测井中基于4H-SiC的瞬发裂变中子探测技术研究

The prompt fission neutron uranium logging technology is a real-time, fast, accurate and active method which can directly determinate uranium content for uranium ore layer. The helium-3 proportional counter is usually applied to detect prompt fission neutron, and the helium-3 gas is seriously in short supply. The methods of combining physical modeling and the fabrication technology about 4H-SiC Schottky diodes will be investigated, and its properties will be systematically addressed. The SiC sandwich neutron spectrometer which may be appropriate for prompt fission neutron uranium logging will be designed. High-sensitivity large-area 4H-SiC detector array will be developed. The thermal neutron detector and the epithermal neutron detector will be designed with the 4H-SiC detector array which can meet the demand of prompt fission neutron uranium logging. The waveform digitizing technology for nuclear pulse signal will be developed. The nuclear digital signal will be processed in FPGA through the pulse shaping, trigger module, the data buffer, the FIR filter and the baseline recovery module. According to the digital Constant Fraction Discriminator (dCFD) timing method, a time-calculating system will be designed and implemented as a solution in DSP. The thermal neutron time-spectrum logging system will be developed on FPGA+DSP. The 4H-SiC neutron detector will be designed, and will replace the helium-3 proportional counter. It will be carried out the research on the key technical of electronics circuit and the associated signal processing algorithms. The project will promote the development of prompt fission neutron detecting technology.

瞬发裂变中子铀矿测井技术是一种实时、快速、准确的直接测定矿层铀含量的主动测井方法。通常都使用3He正比计数管探测瞬发裂变中子，而3He气体资源严重短缺。本项目开展4H-SiC探测器的物理设计及制备工艺研究，建立其性能分析方法；设计适用于瞬发裂变中子铀矿测井的基于4H-SiC的6Li夹心谱仪；设计高灵敏大面积4H-SiC探测器阵列，研究4H-SiC探测器阵列构建热中子探测器和超热中子探测器的方法，满足瞬发裂变中子铀矿测井的需要；研究核脉冲信号的波形数字化技术，在FPGA中实现核脉冲信号的FIR滤波、数据缓冲、基线恢复、波形判选、阈值重构等算法；在DSP中实现数字恒比定时（dCFD）算法；设计基于FPGA+DSP的数字化中子时间谱探测系统。本项目将研制4H-SiC中子探测器取代通用的3He正比计数管，并研究其电子学电路和相关数据处理算法，推动瞬发裂变中子探测技术的发展。

该项目研究了铀矿测井中瞬发裂变中子的产生机理。使用一个小型的D-T中子加速器发射14MeV脉冲快中子流到钻孔周围的地层中，快中子流在地层中慢化而形成0.025eV～0.1eV的热中子，热中子与地层中的235U发生热裂变，从而产生2MeV高能快中子，也就是瞬发裂变中子。被测量的中子来自铀核裂变还是来自加速器中子源，以及被测量的中子是铀核裂变所释放的瞬发中子还是缓发中子，这两个问题是瞬发裂变中子测井装置能否准确测定地层中铀含量的关键所在。正确选择起始测量时间和收集时间是解决这两个问题的方法。该项目从探测器物理设计及制备、性能表征到具体中子场应用多个环节系统地研究了基于4H-SiC的中子探测技术，建立了探测器性能分析方法，揭示了其电荷收集率在快中子辐照下的变化规律，用Monte-Carlo仿真研究了夹心中子仪的结构和性能之间的关系。本课题使用形状与大小均基本相当的、上下排列的两个3He正比计数管制作双中子探测器。短源距中子探测器，也就是超热中子探测器，需包裹一层中子慢化体。没有包裹镉层和聚乙烯材料的中子探测器，是长源距中子探测器，用于探测热中子。本项目研究了基于4H-SiC探测器的热中子时间谱测量电路及其信号处理算法。基于数字恒比定时原理，设计了一套通用硬件电路系统来完成实时核脉冲数字恒比定时计算。基于符合计数器的原理，本项目采用数字化的方法实现4H-SiC中子探测器的两路输出信号的符合测量。本项目还完成了数字延迟单元、FIR滤波、数据缓冲、极零相消、抗混叠失真、单端信号转化为差分信号、电荷灵敏前置放大器、基线恢复、中子伽马甄别等算法和硬件电路设计。在完成计划任务书规定的研究内容的同时，我们还研究了裂缝型铀矿环境下的中子脉冲测井的数值模拟，初步研究了中子发生器的电源系统的关键技术。