1.Key Laboratory of Nuclear Data, China Institute of Atomic Energy, Beijing 102413, China 2.Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China 3.Dongguan Neutron Science Center, Dongguan 523803, China 4.Northwest Institute of Nuclear Technology, Xi’an 710024, China 5.State Key Laboratory of Particle Detection and Electronic, Beijing 100049, Hefei 230026, China 6.Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China 7.State Key Laboratory of Nuclear Physics and Technology, School of Physics, Peking University, Beijing 100871, China 8.Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China 9.University of South China, Hengyang 421001, China 10.Department of Engineering and Applied Physics, University of Science and Technology of China, Hefei 230026, China 11.Xi’an Jiaotong University, Xi’an 710049, China 12.Beihang University, Beijing 100083, China 13.School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, China
Fund Project:Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0401601), the National Natural Science Foundation of China (Grant Nos. 11790321, 11805282), and the Open Basic Research of State Key Laboratory of Intense Pulsed Radiation Simulation and Effect, China (Grant Nos. SKLIPR1515, SKLIPR1516).
Received Date:12 December 2018
Accepted Date:17 February 2019
Available Online:01 April 2019
Published Online:20 April 2019
Abstract:The Chinese spallation neutron source was completed in May 2018 and then subsequently commissioned. The Back-streaming white neutron beam line can be used in neutron nuclear data measurement, neutron physics research, and nuclear technology. In these experiments, it is necessary to know the neutron energy spectrum, the neutron flux, and the neutron beam profile of the neutron beam. In this paper, we present the preliminary measurements of these parameters. The neutron energy spectrum and neutron flux are measured by the time-of-flight method with a fission chamber equipped with 235U and 238U samples and a 6Li-Si detector. The neutron beam profile is measured by a scintillator-CMOS detection system. The preliminary experimental measurements of the beam line are obtained. Among them, the energy spectrum measurement range of white neutrons is from eV to more than 100 MeV, which also gives an uncertainty analysis; the neutron fluence rate gives the full power value of the two experimental halls; the collimated white neutron beam spot is given under a diameter of 60 mm. The future plan is also given. The results of these experimental parameters can serve as the foundation for the future nuclear data measurement and detector calibration experiments of the beam line. Keywords:China spallation neutron source/ back-streaming white neutron/ neutron time-of-flight method/ neutron beam characterization
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3.1.中子能谱和注量率测量
33.1.1.高能能区的中子能谱和注量率测量 -->
3.1.1.高能能区的中子能谱和注量率测量
1)实验装置 中子能谱和注量率测量系统的探测器为多层(5层)快裂变室[17-20], 如图4所示. 多层快裂变室内部是高纯度的235U和238U靶片, 镀层平均厚度约$280\;{\text{μ}}{\rm{g}}/{\rm{cm}}^2$, 镀层活性区直径40 mm, 极板间距10 mm. 裂变室主要由靶片、收集极、外壳、绝缘部件、导线、进气孔和出气孔等部分组成. 裂变室模型采用铜质的外壳和收集极. 收集极与靶片基板间、靶片基板与外壳间均用绝缘材料隔开, 通过改变绝缘垫片的厚度来控制靶片与收集极之间的距离, 即极板间距. 靶片接地, 收集极与MESYTEC-MSI-8前置放大器相连, 输出时间信号接300 V正高压. 裂变室工作气体为氩甲烷P10 (甲烷10%, 氩气90%), 1个标准大气压. 图 4 多层快裂变室 Figure4. The multi layer fission chamber.
探测器时间分辨和粒子分辨是对重要并需要兼容的指标, 初步选定极间距10 mm, 实验上可以有效分辨α粒子和裂变信号, 系统时间分辨好于35 ns. 探测器信号通过同轴电缆接入前置放大器和成形放大器, 然后进入读出电子学系统. 前置放大器使用Mesytec公司的MSI-8集成放大器, 它是一款紧凑型8通道成形放大器, 且具有集成的定时滤波放大功能. 由于模块化的设置, 每个通道可以单独选择前置放大和成形模块的类型, MSI-8可同时输入8路信号, 每路输入信号对应T(时间)和E(能量)两个输出信号, 如图5所示, 其中负信号为T信号, 正信号为E信号. T信号的上升时间约70 ns, 通过恒比定时可以得到好于20 ns的定时精度, 满足测量反角白光中子源的10 MeV以下中子的飞行时间的需求. 图 5 MSI-8的输出信号 Figure5. The output signal of MSI-8 module.
图6是多层快裂变室的实验测量时间信号, 左侧两个相隔较近的小信号就是两个束团的γ-flash, 右侧较大的信号是裂变碎片的信号. 通过分析不同的信号发现, 裂变碎片的能量信号和时间信号幅度都很大, α粒子的能量信号和时间信号幅度都较小, 而γ-flash的能量信号很大, 时间信号远小于裂变信号. 根据这一特征, 可以比较容易区分信号的类型. 图 6 裂变室的时间信号 Figure6. The time signal of the fission chamber.
图像处理内容包括图像预处理、系统性能标定和束流剖面判定尺寸判读等环节. 图像预处理包括: 尺寸标定、γ斑点(亮白点)去除和本底扣减等内容. 在此基础上分别完成了中子束剖面轮廓判定及尺寸判读、系统调制传递函数测量和中子能谱测量的初步分析. 系统性能标定主要指系统的空间分辨能力, 本实验中通过厚刀口法测量得到探测系统的调制传递函数, 从而得到系统空间分辨能力, 并利用狭缝法验证. 束流剖面判定尺寸判读基于Hough变换的圆检测算法, 得到不同能量对束流轮廓、尺寸的影响, 不同能量不同计数圈内非均匀性变化规律, 束剖面轮廓中心与“重心”偏差分析等结果. 对中子束轮廓判断、尺寸大小和均匀性进行了评估和定量分析, 并对中子重点能区的能谱(0.1—20.0 MeV)进行了初步实验研究. 在束斑直径理论值为Φ50 mm的实验厅-1 (距中子靶55 m), 实验测得的半高值(FWHM)对应的中子束斑值为Φ55 mm, 峰值强度约75%时对应的束斑直径为Φ50 mm, 与理论值一致. 在束斑直径理论值为Φ60 mm的实验厅-2 (距中子靶75.8 m), 实验测得的半高值(FWHM)对应的中子束斑值为Φ63 mm, 峰值强度75%时对应的束斑直径为Φ60 mm, 也与理论值一致. 进一步分析表明, 束斑的轮廓尺寸与中子能量无关, 这说明各种能量的中子在束斑内是均匀分布的, 中子束强度在已判定轮廓的80%范围内不均匀性小于10%, 且束剖面不均匀性与能量无关. 剖面上的全能区中子束强度分布图及其在x轴方向和y方向的投影如图14所示. 图 14 中子束斑剖面分布及其在x轴和y轴上的灰度值分布 Figure14. Neutron profile distribution and grey scale information on x and y axis