摘要
目前现有的反康普顿高纯锗γ谱仪主要采用模拟电路实现多路反符合,从而实现对康普顿散射本底的抑制。由于模拟电路复杂度高,调试不易,温度稳定性差,灵活性不足,长期稳定性与性能都有局限性。本文研制了数字式反康普顿高纯锗γ谱仪:将高纯锗主探测器、环形符合探测器及塞子符合探测器测量得到的每个射线事例信息(到达时间、能量、探测器ID等)以数据包流方式发送到计算机端(时间戳列表模式既list-mode),上位机端根据预设的配置参数对不同探测器的事例数据包进行组合处理实现符合\反符合等处理,并自动绘制出符合分辨曲线,从而确定最优符合分辨窗口τ值,实时得到原始高纯锗能谱、反康后高纯锗能谱、反符合能谱、符合分辨曲线等。系统采用了200 MHz同步时钟为每个事例数据包打上时间戳,保证有5 ns的时间同步精度,可满足高纯锗γ谱仪的反康普顿要求。采用了对称零面积梯形成形方式替代传统梯形成形在现场可编程门阵列(Field Programmable Gate Array,FPGA)芯片内部实现了快、慢成形双通道,实现优异的低频噪声抑制与基线估计效果。针对大体积高纯锗探测器开发了上升时间、多点能量沉积修正等算法,可有效提高系统能量分辨率。采用SQLite快速数据库存储所有探测器的事例数据包,可实现一次测量、多次重复使用的效果,避免了参数调整带来的重复测量时间浪费,体现了list-mode数字化测量设计的优势。经测试,本系统的康普顿减弱因子(康普顿边处)达9.8,反康后峰康比可达1 158:1,60Co的1 332.5 keV全能峰能量半高宽为1.70 keV(60%相对探测效率),反康前系统积分本底为1.674 s-1,反康后积分本底降为0.483 s-1,针对137Cs的最小可探测活度(MinimumDetectable Activity,MDA)反康前为20.5 mBq,反康后降低至11.3 mBq。
[Background] The existing anti-Compton high-purity germanium spectrometors mainly use analog circuits to realize multi-channel anti-coincidence and suppression of the Compton scattering background. Due to the high complexity of analog circuits, the debugging is not easy, and the temperature stability and flexibility are insufficient. Conditional changing caused by the delay in the long-term operation leads to the degradation of anti- Campton effect, and even failure of normal work in severe cases.[Purpose] This study aims to develop a digital anti- Compton high-purity germanium spectrometer based on time-stamped list mode.[Methods] The spectrometer combines high-purity germanium detector, ring-shaped detector and plug coincidence detector. Every incident event information of the detector (arrival time, energy, detector ID, etc.) is sent to the computer as an independent data packet, and the resolution curve is drawn automatically by the host computer software to determine the optimal resolution window τ value, and the original high-purity germanium energy spectrum is obtained in real time, and the anti-Compton high purity germanium energy spectrum, anti-Compton spectrum, coincidence resolution curve. A 200 MHz synchronous clock is employed to time-stamp each event data packet to ensure the time synchronization accuracy of 5 ns and meet the anti-Compton requirements of the high-purity germanium spectrometer. Instead of the traditional trapezoidal forming, the symmetrical zero-area trapezoidal forming method is used to realize fast and slow forming dual channels inside the FPGA (Field Programmable Gate Array) chip to achieve excellent low-frequency noise suppression and baseline estimation. For the large-volume high-purity germanium detector, a correction algorithms of rise time and multi-point energy deposition correction are developed, which effectively improve the system energy resolution. The SQLite fast database is employed to store the event data packets of all the detectors to realize the effect of one measurement and repeated use.[Results] The Compton reduction factor of this system reaches 9.8, the peak-to-Compton ratio can reach 1 158: 1, and the all-energy peak full width at half maximum (FWHM) of ^60Co's 1 332.5 keV is 1.70 keV (relative detection efficiency is 60%). The integral background was reduce from 1.674 s^-1 (before anti-Compton) to 0.483 s^-1 (after anti-Compton). The minimum detection activity (MDA) of ^137Cs at 1.2 h was reduced from 20.5 mBq before anti-Compton to 11.3 mBq after anti-Compton.[Conclusion] This proposed scheme of anti-Compton spectrometer avoids the waste of repeated measurement time caused by parameter adjustment, and embodies the advantages of list-mode digital measurement design.
作者
曾国强
卿松
欧阳晓平
严磊
白涛
师全林
张小林
代义华
胡传皓
何黎
ZENG Guoqiang;QING Song;OUYANG Xiaoping;YAN Lei;BAI Tao;SHI Quanlin;ZHANG Xiaolin;DAI Yihua;HU Chuanhao;HE Li(Nuclear Technology Key Laboratory of Earth Science, Chengdu University of Technology, Chengdu 610059, China;Northwest Institute of Nuclear Technology, Xi'an 710024, China)
出处
《核技术》
CAS
CSCD
北大核心
2019年第2期26-33,共8页
Nuclear Techniques
基金
国家重点研发计划项目(No.2017YFC0602101)
国家自然科学基金(No.41474159)资助~~
关键词
对称零面积梯形成形
数字反康普顿高纯锗γ
谱仪
上升时间-幅度补偿定时甄别器
时间戳列表模式
多点能量沉积修正
Symmetric zero-area trapezoidal shaping
Digital anti-Compton high-purity germanium spectrometer
Amplitude rising time compensation timing discriminator
Time-stamped list mode
Multi-site energy deposition correction
