基于SCA波形采样读出电子学的CSNS Back-n中子飞行时间测量

中国散裂中子源(CSNS)反角白光中子束线(Back-n)对中子核数据测量和核技术应用等多个领域均有重要意义。为监测其中子束斑轮廓、束流密度及束流能量，研制了由镀硼微网格气体(Micromegas)探测器构成的束流剖面监测装置，并通过测量中子的飞行时间(TOF)来获得能量信息。采用基于开关电容阵列(SCA)专用集成电路(ASIC)的波形采样电子学系统，实现了128路Micromegas探测器阳极条信号的低噪声放大、成形和波形数字化，在现场可编程逻辑门阵列(FPGA)芯片中实现了对信号过阈时间的实时测量，其量程为650 ns～10 ms，电子学时间分辨好于10 ns。在CSNS Back-n上开展实验，成功获得了中子束流剖面及10.65 μs～10 ms范围的飞行时间谱，对应的中子能量范围约为0.16 eV～0.14 MeV。利用钽、钴等吸收体进行了中子共振吸收峰的检验，验证了读出电子学系统的功能及飞行时间测量的正确性。

CSNS Back-n Neutron Time-of-flight Measurement Based on SCA Waveform-sampling Readout Electronics

The back-streaming white neutrons (Back-n) at Chinese Spallation Neutron Source (CSNS) are of great significance in many fields, such as neutron nuclear data measurement and nuclear technology application. A beam monitor composed of a boron-plated micro mesh gaseous structure (Micromegas) detector was carried out to monitor neutron profile, intensity and energy of the white neutron beam. The neutron time-of-flight (TOF) was measured to obtain energy information. A waveform sampling electronics system based on application-specific integrated circuit (ASIC) chips with switched capacitor arrays (SCAs) was applied to achieve the low-noise amplification, filtering and waveform digitization of 128-channel Micromegas detector anode strip signals. And the readout system also achieved real-time measurement of waveform-over-threshold moment in the field programmable gate array (FPGA) chip. The neutron TOF range is from 650 ns to 10 ms, and the time resolution of the electronics is better than 10 ns. Based on the experiments on CSNS Back-n neutron beam, the neutron beam profile and 10.65 μs 10 ms TOF spectrum were successfully reconstructed, and the corresponding neutron energy range is about 0.16 eV-0.14 MeV. The resonance absorption spectra of ⁵⁹Co and ¹⁸¹Ta absorbers were also measured. These testing results verify the function of the readout electronics system and the accuracy of the TOF measurement.