Cross section measurement of neutron capture reaction based on HI-13 tandem accelerator
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摘要: 利用BaF2晶体对γ射线探测效率高、时间分辨率好的特点，研制了国内首套由40个BaF2探测器单元组成的γ全吸收型探测装置，用于在线测量中子俘获反应截面。在HI-13串列加速器上建立250～850 keV的中子源，其0°角的源强约为5.09×106 n/（Sr·s），使用γ全吸收型探测装置，通过瞬发γ射线法测量了93Nb、197Au、natC和空样品的实验数据。根据BaF2探测器信号的特征，采用了基线补偿、软件阈值设置、时间窗限定、脉冲幅度积分增长率设置和快慢成分比设置等多种数字化波形分析方法，剔除噪声信号以提高效应本底比。以197Au样品数据为标准，natC样品数据为样品相关性本底，空样品数据为样品无关性本底，采用相对测量法得到了93Nb的中子俘获反应截面实验数据。通过与ENDF评价库数据的比较，验证了测量装置和技术方法的可行性。Abstract: The first gamma-ray total absorption facility composed of 40 BaF2 detector units has been constructed in China. Utilizing the property of high detection efficiency and good time resolution for gamma rays of BaF2 crystal, the facility will be used to measure on-line the cross section of neutron capture reaction. Neutron source of 250 keV～850 keV was set up on the HI-13 tandem accelerator, its source intensity at 0° was about 5.09 × 106 n/(Sr·s). The experimental data of 93Nb, 197Au, natC and empty samples were measured by the prompt gamma ray method with gamma-ray total absorption facility. According to the characteristics of BaF2 detector signal, a variety of digital waveform analysis methods were adopted to eliminate noise signal as much as possible to improve the effect background ratio, such as baseline compensation, software threshold setting, time window limitation, pulse amplitude integral growth rate setting and fast slow component ratio setting. The experimental data of 93Nb neutron capture reaction cross section was obtained by relative measurement method. The data of 197Au sample was the standard, the data of natC sample was the sample correlation background, and the data of empty sample was the sample independence background. The feasibility of the measurement facility and technical method was verified by comparing with the data of ENDF evaluation library.
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图 1 GTAF的五棱与六棱BaF2晶体形状
Figure 1. Shape of the pentagonal and hexagonal BaF2 crystal for GTAF
图 2 GTAF测量中子俘获反应截面实验原理图
Figure 2. Experimental diagram of measurement of neutron capture cross section based on GTAF
图 3 中子源的飞行时间与脉冲幅度积分值的二维谱
Figure 3. Time of flight versus integral of pulse height spectrum from neutron source
图 6 能量与脉冲幅度积分增长率的二维谱
Figure 6. Energy versus rate of increase of pulse height integral spectrum
图 8 BaF2探测器单元能谱的原始数据与处理结果的比较
Figure 8. Comparison with original energy spectrum and analytic results of BaF2 detector module
图 9 γ多重性的原始数据与处理结果的比较
Figure 9. Comparison with original data and analytic results of γ multiplicity
图 10 加和能谱的原始数据与数据处理结果的比较
Figure 10. Comparison with original data and analytic results of sum energy spectrum
图 12 中子俘获反应飞行时间效应谱
Figure 12. Effect time of flight (TOF) spectrum of neutron capture reaction
图 13 93Nb中子俘获反应截面测量结果
Figure 13. Measurement results of 93Nb neutron capture reaction cross section
表 1 在线实验样品参数
Table 1. Sample parameter of on-line experiment
sample density/（g·cm−3） diameter/mm thickness/mm purity/% measure time/min 93Nb 8.57 20 1 99.99 573 197Au 19.32 20 1 99.99 664 natC 2.3 20 1.5 99.99 626 blank 288 
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表 2 在线实验测量结果
Table 2. Measurement results of on line experiment
energy bin/MeV cross section/mb energy resolution/% uncertainty/% 0.249·0.302 45.14 6.37 22.3 0.303·0.374 42.93 7.06 25.8 0.375·0.475 41.68 7.91 27.3 0.476·0.624 39.64 8.99 30.6 0.625·0.857 36.55 10.43 33.6
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表 3 不确定度分析
Table 3. Analysis of uncertainties
source of
uncertaintiesstatistical uncertainty
of 93Nbstatistical uncertainty
of 197Austatistical uncertainty
of natCstatistical uncertainty
of blanknormalization of
beam integraluncertainty/% 10～15 7～10 10～15 7～10 3 source of uncertainties detection efficiency other backgrounds neutron energy standard cross section of 197Au total uncertainty/% 1 5 12.8～20.8 1～4 22.3～33.6
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