大质量AGB星演化关键核反应18O(a,g)22Ne的实验研究

About a half of the abundances of elements heavier than iron comes from the so-called slow-neutron capture process(s-process). Asymptotic Giant Branch (AGB) stars provide the most important place for s-process to take place, with the 22Ne(a,n)25Mg reaction as one of the main neutron sources. In the beginning phase of AGB thermal pulse, 22Ne is produced by the 14N(a,g)18F(b+)18O(a,g)22Ne reaction sequence. While the first alpha capture reaction in the sequence is well understood, 18O(a,g)22Ne reaction is affected by considerable difficulties and uncertainties . Direct measurement of 18O(a,g)22Ne thermonuclear reaction rate is very challenging, which is complicated by several resonant states lying closely to the alpha threshold in 22Ne. To obtain the resonance paramters,we plan to measure the 18O(6Li,d)22Ne alpha transfer reaction to several resonant states in 22Ne by using the high-resolution Q3D magnetic spectrometer at Beijing HI-13 Tandem accelerator laboratory. The alpha spectroscopic factors and spin/parity can be obtained by analyzing the precisely-measured angular distributions, the deduced alpha decay widths and resonance strengths will enable for a reliable evaluation of the thermonuclear reaction rates of 18O(a,g)22Ne at AGB relevant temperatures.

宇宙中比铁重的元素约一半的丰度来源于慢速中子俘获核合成过程(s-过程）。渐进巨支（AGB）星是发生s-过程最合适的场所，而22Ne(a,n)25Mg 反应是大质量AGB星s-过程主要的中子源。AGB星中的22Ne主要通过热脉动早期14N(a,g)18F(b+)18O(a,g)22Ne反应链合成。对于该反应链中关键反应18O(a,g)22Ne，直接精确测量其反应率非常困难；22Ne alpha阈附近分布着若干条共振能级，关键共振参数仍极度缺失。本项目计划利用中国原子能科学研究院串列加速器上的高分辨 Q3D磁谱仪，通过测量18O(6Li,d)22Ne alpha转移反应，得到末态核22Ne alpha分离阈附近多条共振能级的精确角分布，进而得到自旋宇称和alpha谱因子，导出alpha分宽度和共振强度，实现对AGB星温度下18O(a,g)22Ne天体物理反应率的可靠评估。

慢速中子俘获过程(s-过程) 是合成比铁重元素的重要途径之一。22Ne(α, n)25Mg 反应是大质量AGB 星中s 过程主要的中子源，其中的22Ne 主要通过14N(α,γ)18F(β+)18O(α, γ)22Ne 反应链合成。该反应链中关键反应18O(α, γ)22Ne在天体物理感兴趣能区的截面非常低，其天体反应率主要来自于22Ne α 分离阈附近低能共振态的贡献，但目前相关能级的共振参数严重缺失。在HI-13 串列加速器的Q3D 磁谱仪上，通过测量18O(6Li,d)22Ne 反应的角分布，利用DWBA 分析确定了22Ne 分离阈附近共振能级Eα = 470 keV 的自旋宇称为0+，得到了22Ne激发态的alpha普银子，并计算了18O(α, γ)22Ne 的天体反应率。