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中子辐射俘获截面是核天体物理、核反应堆设计、核医学以及核技术应用等领域中所需的关键数据. 目前, 受制于实验技术上的困难, 缺少半衰期短至数年或更短核素的中子辐射俘获截面测量数据. 本文采用一种间接测量短寿命核素反应截面的方法——替代反应法, 在中国原子能科学研究院北京HI-13串列加速器上利用89Y(p, α)反应作为替代反应, 产生与短寿命核素中子辐射俘获85Sr(n, γ)反应相同的复合核86Sr*进行替代反应法的实验研究. 通过使用硅探测器组成的望远镜阵列与HPGe探测器阵列符合测量, 得到了与特定能量角度的出射α粒子符合γ射线能谱, 从而获得了不同激发能下复合核的γ衰变的概率. 结合使用UNF唯象光学模型计算的复合核形成截面, 得到了中子能量范围En = 0.02—1.22 MeV的85Sr中子辐射俘获截面. 并且, 基于TALYS计算的自旋宇称分布, 建立了修正模型用于补偿直接反应和替代反应中自旋-宇称布居的差异, 改进了基于WeisskopfEwing近似下的间接测量结果.
Neutron capture cross sections, as important parameters for describing the probability of neutron-nucleus reactions, play a key role in multiple scientific fields. In astrophysics, neutron capture cross section data are essential elements for understanding stellar nucleosynthesis processes. In particular, in extreme environments such as supernova explosions and neutron star mergers, accurate neutron capture cross sections can reveal the secrets of heavy element formation. In the field of national security, neutron capture cross sections are crucial for the design of nuclear weapons and the security of nuclear materials. By accurately grasping the neutron capture characteristics of different nuclides, the nuclear reaction process can be optimized to ensure strategic security. In addition, in the simulation of nuclear power generation, neutron capture cross section data are the basis of reactor design and operational analysis. Through in-depth research on and precise measurements of neutron capture cross sections, the safety and efficiency of nuclear reactors can be improved, thus promoting the sustainable development of nuclear energy. At present, there is little research on the neutron capture cross sections of nuclides with half-lives of only a few years or even shorter, mainly due to the complexity of measurement techniques and the instability of the nuclides themselves. The neutron capture cross section data of these nuclides are crucial for reactor design, nuclear medicine applications, and nuclear waste treatment. Further research requires the development of more advanced detection techniques and theoretical models to accurately measure and predict their neutron capture behavior. The surrogate-reaction method, as an effective measurement means, plays an important role in studying reaction cross sections of short-lived nuclides. Its basic idea is to indirectly obtain the reaction cross section information of short-lived nuclides by measuring the specific particles emitted by stable nuclides. Specifically, when stable nuclides are bombarded by high-energy particles, nuclear reactions will occur and specific particles will be released. By accurately measuring the energies, angles, and numbers of these particles, the cross sections of short-lived nuclides in the corresponding reaction can be inferred. This method can not only overcome the technical difficulties in directly measuring short-lived nuclides, but also improve the accuracy and reliability of the measurement results, which provides important support for nuclear physics research. In addition, the surrogate-reaction method also shows broad application prospects in the fields of nuclear technology application and nuclear data assessment. The experiment is carried out on the Beijing HI-13 tandem accelerator at the China Institute of Atomic Energy. 89Y is bombarded with 22 MeV protons, and the 85Sr(n, γ) cross section is measured through the (p, αγ) reaction. The telescope array composed of silicon strip detectors can effectively identify the reaction products. By precisely measuring parameters such as the energies and angles of particles, the array can distinguish different nuclides, thus determining the outgoing particles. Combined with the γ-ray energy spectrum analysis of the HPGe detector, the (n, γ) reaction cross section data of 85Sr under the Weisskopf-Ewing (W-E) approximation are extracted. Due to the mismatch of the Jπ population between the existing alternative reactions and direct reactions, it is necessary to compensate for this mismatch and then correct the results. In order to obtain relatively reliable results, the Jπ population calculated by TALYS is used to revise the experimental data of the (n, γ) cross section. These results indicate that the cross section of 85Sr(n, γ) varies with neutron energy in a specific energy range, which is consistent with the trend of the existing international evaluation library data. This validates the effectiveness of cross section measurement as an alternative reaction method, thereby providing an important experimental basis for further exploring the nuclear reaction mechanism and nuclear data application. This method has reference significance for the cross section measurement of other nuclides. -
Keywords:
- neutron-capture cross section /
- surrogate reaction method /
- short-lived radioactive nuclei
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图 2 89Y(p, α)的双微分截面的θ-ϕ分布(黑色框内代表两组探测器能够接收到的部分)
Fig. 2. The θ-ϕ distribution of the double differential cross section for 89Y(p, α) (the black box indicates the detectable region covered by two sets of detectors).
图 3 各个探测器能量分辨率(以60Co的1332.5 keV特征峰的FWHM表征, G代表HPGe同轴型探测器, C代表Clover探测器, 后缀代表Clover探测器的模块编号)
Fig. 3. Energy resolutions of various detectors (characterized by the FWHM of the 1332.5 keV characteristic peak of 60Co, where G represents HPGe coaxial detectors, C represents Clover detectors, and the suffix denotes the module number of the Clover detector).
图 4 真空靶室及实验设备布局 (a)两组ΔE – E望远镜在真空靶室中的示意图; (b) 靶室放置在Gamma探测器阵列的照片
Fig. 4. The vacuum target chamber and experimental equipment layout: (a) Schematic representation of two sets of ΔE – E telescopes in vacuum target chamber; (b) physical diagram of the target chamber placed in the Gamma detector array.
图 5 在前角处(60°)望远镜探测器中的能量损失与剩余能量关联谱达成的粒子鉴别
Fig. 5. The identification achieved in the telescopes at 60° through the energy loss (ΔE) versus residual energy (E).
图 7 DSSD正背面的能量关联图 (a) 能量自归一前; (b) 能量自归一后
Fig. 7. Front-back energy correlation diagram of DSSD: (a) Before the Self-normalization; (b) after the self-normalization.
图 8 DSSD 刻度结果 (a)前角(60°) DSSD; (b)后角(–100°) DSSD
Fig. 8. DSSD energy calibration: (a) The DSSD at 60°; (b) the DSSD at –100°.
图 9 DSSD探测器能量分辨率 (a)后角(–100°)探测器; (b)前角(65°)探测器
Fig. 9. Energy resolution of the DSSD detector: (a) The detector at –100°; (b) the detector at 65°.
图 10 等效中子能谱 (a)对应前角(65°)望远镜探测器; (b)对应后角(–100°)望远镜探测器(后续不分析)
Fig. 10. The equivalent neutron energy: (a) The DSSD at 65°; (b) the DSSD at –100° (no further analysis).
图 11 对p +89 Y反应计算模拟的α粒子出射能谱 (a) 以反应道划分; (b) 以反应机制划分
Fig. 11. The calculated α emission energy spectrum of the p + 89Y reaction: (a) Classified by reaction channels; (b) classified by reaction mechanisms.
图 12 α符合HPGe γ能谱, 红线为本底
Fig. 12. Coincidence γ-ray spectrum, the red line is the background.
图 13 (a) ${E_{\text{n}}}$= (0.62±0.15) MeV时对应的α粒子计数; (b) ${E_{\text{n}}}$= (0.62±0.15) MeV时的后角符合γ能谱中1.076 MeV特征峰以及本底(红线); (c) 扣除本底后1.076 MeV的特征峰, 绿线为所作的高斯拟合
Fig. 13. (a) The α particle count corresponding to neutron energy point ${E_{\text{n}}}$= (0.62 ± 0.15) MeV; (b) the peak of 1.076 MeV gamma ray with neutron energy at ${E_{\text{n}}}$ = (0.62 ± 0.15) MeV and background obtained by using ROOT program (red line); (c) the peak of 1.076 MeV gamma ray after subtracting the background and the Gaussian fitting (green line).
图 14 归一化后的γ衰变概率随En的变化曲线(红线代表能量为1.076 MeV特征γ; 蓝线代表能量为1.153 MeV特征γ; 黑线为对各特征γ进行统计加权得到的γ衰变概率)
Fig. 14. Normalized γ decay probability as a function of neutron energy (red line represents characteristic γ with energy 1.076 MeV; blue line represents characteristic γ with energy 1.153 MeV; black line shows γ decay probability obtained by statistical weighting of each characteristic γ).
图 15 UNF计算n +85Sr复合核形成截面随中子能量变化曲线
Fig. 15. n + 85Sr compound nucleus formation cross section as a function of neutron energy (calculated by the UNF program).
图 16 W-E近似下替代反应法计算截面, 黑线为JENDL-5库数据, 蓝线为ENDF-6数据库
Fig. 16. Neutron-capture cross section as a function of neutron energy by W-E approximation. The black line corresponds to the data of JENDL-5 and the blue line corresponds to the data of ENDF-6.
图 17 由TALYS程序计算86Sr*各自旋宇称态的γ衰变概率
Fig. 17. The γ-decay probabilities of each spin – parity of 86Sr* calculated by the TALYS program.
图 18 TALYS程序计算的入射中子能量En = 0.02—1.22 MeV时中子俘获反应及本实验使用的89Y(p, α)产生的86Sr*自旋宇称态的布居概率
Fig. 18. Spin - parity population probabilities of 86Sr* population for the 89Y(p, α) and neutron capture reactions at En = 0.02—1.22 MeV.
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