渐近巨星支（Asymptotic Giant Branch，简称AGB）星是中低质量恒星演化末期的必经阶段，其核合成过程是核天体物理学研究的重要领域之一。AGB星核合成所产生的核素丰度信息大量保存在陨石SiC颗粒中，对SiC颗粒中惰性气体的质谱分析发现²¹Ne/²²Ne丰度比与SiC颗粒大小有很强的关联性，如果能够对AGB星核合成的²¹Ne/²²Ne丰度比进行精确计算，就可以结合实验数据来探索SiC颗粒诞生地AGB星的性质，为深入研究AGB星核合成提供重要依据。其中，¹⁸O(α, n)²¹Ne和¹⁸O(α, γ)²²Ne分别是AGB星产生²¹Ne和²²Ne的主要途径，这两个反应率的大小比例直接决定了AGB星氦燃烧产生的²¹Ne/²²Ne丰度比。目前¹⁸O(α, n)²¹Ne的反应率误差较小，²¹Ne/²²Ne丰度比计算精度主要取决于¹⁸O(α, γ)²²Ne的反应率数据精度，因此对¹⁸O(α, γ)²²Ne反应进行精确测量具有重要意义。

此外，¹⁸O(α, γ)²²Ne反应在研究宇宙超铁元素起源问题中也具有重要意义。宇宙超铁元素起源一直是备受关注的重大科学问题之一。慢中子俘获过程（s-过程）被认为合成了一半左右的超铁元素，AGB星为s-过程提供了合适的天体场所以及所需中子。目前认为²²Ne(α, n)²⁵Mg反应是弱s-过程的主中子源，控制大质量AGB星中A=56-90的核素合成。在大质量AGB星氦燃烧壳层中，主中子源²²Ne(α, n)²⁵Mg反应所需的²²Ne种子核是通过¹⁴N(α, γ)¹⁸F(β⁺)¹⁸O(α, γ)²²Ne反应链产生的，因此¹⁸O(α, γ)²²Ne反应率是弱s-过程中子产额计算的关键输入量之一。

在AGB星氦燃烧典型温度（0.1-0.3 GK）下，¹⁸O(α, γ)²²Ne反应率主要由 470 keV附近的一个共振俘获过程决定，但由于该共振的共振能量、强度以及自旋宇称等参数仍存在很大的不确定性，导致0.1-0.3 GK内的¹⁸O(α, γ)²²Ne反应率误差较大（28-180%），远不能满足天体物理模型计算的需求。

1994年，Giesen等通过测量¹⁸O(⁶Li, d)²²Ne转移反应研究了²²Ne的高激发态，得到了470 keV共振能级的能量为10053±15 keV，自旋宇称为0⁺或1^-。同年，Mao等通过测量²⁰Ne(t, p)²²Ne转移反应得到该共振能级的能量为10072±10 keV。利用这两个实验结果计算出的共振能量分别为470±18 keV 和495±12 keV，差别较大。以现有数据计算，该共振能差别可导致0.1 GK温度下的¹⁸O(α, γ)²²Ne反应率改变240%，严重影响了反应率的数据精度。另外，470 keV共振强度较低，实验测量难度很大，至今仅有两个工作对该共振进行了直接测量。2003年，Dababneh等在地面实验室通过γ-γ符合测量得到共振强度为ωγ=0.48±0.16 μeV，但该数据是基于假设的γ分支比得到的；2022年，Dombos等在Sanford地下实验室通过直接测量得到共振强度为ωγ=0.26±0.05  μeV。二者差了近一倍，这也显著影响了¹⁸O(α, γ)²²Ne反应率数据精度。因此，有必要对¹⁸O(α, γ)²²Ne反应进行进一步的精确测量，减小关键共振数据的不确定性，得到精确的¹⁸O(α, γ)²²Ne反应率数据。

本工作是在位于中国锦屏地下实验室（CJPL）的锦屏深地核天体物理实验装置（JUNA）上完成的。利用JUNA强流加速器提供的0.5 mA的⁴He²⁺束流轰击¹⁸O同位素靶，使用全能量γ探测方法测量了E_α=470-787  keV能区的¹⁸O(α, γ)²²Ne产额曲线，对低能区5个共振能级进行了实验研究。首次精确测定了470 keV共振的能量为E_α=474.0±1.1 keV，精度比之前的数据提高了10倍以上。利用全能谱-单元谱联合解析方法首次得到了该共振的首次γ跃迁分支比，确定了该共振能级的自旋宇称为1^-，同时得到其共振强度为ωγ₄₇₀=0.25± 0.03 μeV，解决了以往工作中共振强度不一致的问题。此外，实验还得到了其余四个共振的共振强度分别为ωγ₅₇₀=0.62±0.10 μeV、ωγ₆₆₀=202±17 μeV、ωγ₇₅₀=500±44 μeV和ωγ₇₇₀=1093±99 μeV。根据本工作测量得到的最新实验数据，我们给出了新的高精度¹⁸O(α, γ)²²Ne反应率，将AGB星氦燃烧典型温度0.2 GK下的反应率误差从之前NACRE给出的-47+172% 降低至-11+13% 。

通过和国际理论天体物理学家Lugaro等合作，将本工作得到的最新¹⁸O(α, γ)²²Ne反应率应用到AGB星核合成计算中，发现由于反应率精度的显著提升，计算出的²¹Ne/²²Ne丰度比的误差从之前的~30%缩小至~8%。将新的天体模型计算结果和陨石SiC颗粒质谱分析结果进行对比，结果显示可初步确定不同尺寸的陨石SiC颗粒起源AGB星的性质，从而为利用SiC颗粒核素丰度分布研究AGB星核合成提供了一个重要的基准。

Asymptotic Giant Branch (AGB) stars are an essential stage at the end of the evolution of low- and intermediate-mass stars. The study of their nucleosynthesis is of great importance in nuclear astrophysics. A large amount of nuclide abundance information originating from AGB star nucleosynthesis is retained in the meteoritic SiC grains. Mass spectrometric analysis of noble gases in SiC grains shows a strong correlation between the ²¹Ne/²²Ne ratio and the size of SiC grains. If precise calculations of the ²¹Ne/²²Ne ratio produced by AGB star nucleosynthesis can be made, it will be possible to combine spectrometric analysis to study the properties of AGB stars and the birthplace of SiC grains, providing important evidence for further studies of AGB star nucleosynthesis. The main pathways of ²¹Ne and ²²Ne production in AGB stars are ¹⁸O(α, n)²¹Ne and ¹⁸O(α, γ)²²Ne, respectively. The ratio of their reaction rates directly determines the ²¹Ne/²²Ne ratio contributed by helium burning in AGB stars. Currently, the uncertainty in the ¹⁸O(α, n)²¹Ne reaction rate is relatively small, thus the precision of the calculated ²¹Ne/²²Ne ratio depends mainly on the uncertainty in the ¹⁸O(α, γ)²²Ne reaction rate. Therefore, the precise measurement of the ¹⁸O(α, γ)²²Ne reaction is very important and will be helpful to further understand the origin of the SiC grains.

In addition, the ¹⁸O(α, γ)²²Ne reaction is also of great importance in the study of the origin of heavy elements. The origin of cosmic ultra-iron elements has been regarded as one of the 11 Great unanswered Questions of Physics in this century. The slow neutron capture process (s-process) is thought to have synthesized about half of the ultra-iron elements, and AGB stars provide suitable astrophysical sites and the necessary neutrons for the s-process. The ²²Ne(α, n)²⁵Mg reaction is currently considered to be the main neutron source for the weak s-process, controlling the nucleosynthesis of nuclei with A=56-90 in massive AGB stars. In the helium-burning shell of massive AGB stars, the ²²Ne seed nuclei required for the main neutron source ²²Ne(α, n)²⁵Mg reaction are produced by the ¹⁴N(α, γ)¹⁸F(β⁺)¹⁸O(α, γ)²²Ne reaction chain. Therefore, the reaction rate of ¹⁸O(α, γ)²²Ne is one of the key input parameters for the calculation of the neutron yield in the weak s-process.

At the typical temperature range of helium burning (0.1-0.3 GK) in AGB stars, the ¹⁸O(α, γ)²²Ne reaction rate is mainly determined by a resonance capture process near 470 keV. However, due to the considerable uncertainties in the 470 keV resonance energy, strength, spin-parity, and other parameters, the reaction rate of ¹⁸O(α, γ)²²Ne at 0.1-0.3 GK still has a large uncertainty (28-180%), which is far from meeting the requirements of astrophysical model calculations.In 1994, Giesen et al. studied the excited states of ²²Ne through the ¹⁸O(⁶Li, d)²²Ne transfer reaction and determined this excited state at 10053±15 keV, with a spin-parity of 0⁺ or 1^-. In the same year, Mao et al. measured the ²⁰Ne(t, p)²²Ne transfer reaction and obtained a resonance energy of 10072±10 keV. These measurements gave two resonance energies of 470±18 keV and 495±12 keV, respectively. Such a difference in resonance energy leads to a 240% discrepancy in the ¹⁸O(α, γ)²²Ne reaction rate at 0.1 GK. In addition, due to the weak strength of the 470 keV resonance, only two works have directly measured this resonance. In 2003, Dababneh et al. obtained a resonance strength of ωγ=0.48±0.16 μeV from γ-γ coincidence measurements in a ground-based laboratory, but this value was based on an assumed γ branching ratio. In 2022, Dombos et al. directly measured the resonance strength at the Sanford underground laboratory and obtained a value of ωγ=0.26±0.05  μeV. The results of these two measurements differ by almost a factor of two, which significantly affects the precision of the ¹⁸O(α, γ)²²Ne reaction rate. Therefore, it is necessary to perform further precise measurements of the ¹⁸O(α, γ)²²Ne reaction in order to reduce its rate uncertainty considerably.

The experiment was carried out at the Jinping Underground Nuclear Astrophysics (JUNA) facility located in China. With a ⁴He²⁺ beam provided by the JUNA high-current accelerator, the ¹⁸O(α, γ)²²Ne yield curve was measured in the energy range of E_α=470-787 keV using the full-energy γ detection method. The energy of the 470 keV resonance was accurately determined for the first time as E_α=474.0±1.1 keV, with a precision more than 10 times higher than the previous data. Using the Bayesian analysis method, the primary γ-ray branching ratios of the resonance were determined for the first time by the combined analysis method of full-energy spectrum and single spectrum, and the spin-parity of the resonance state was determined to be 1^-. The resonance strength was determined to be ωγ₄₇₀=0.25±0.03 μeV, resolving the previous discrepancy in resonance strengths. In addition, the resonance strengths of the other four resonances were also determined to be ωγ₅₇₀=0.62±0.10 μeV, ωγ₆₆₀=202±17 μeV, ωγ₇₅₀=500±44 μeV, and ωγ₇₇₀=1093±99  μeV. Based on the JUNA data obtained in this work, a new precise ¹⁸O(α, γ)²²Ne reaction rate has been derived, reducing the associated uncertainties from previous -47+172% to -11+13% at the 0.2 GK of helium burning in AGB stars.

We have applied the new ¹⁸O(α, γ)²²Ne reaction rate to the AGB star nucleosynthesis models and studied its impact on the ²¹Ne/²²Ne ratio. The significantly improved precision of the ¹⁸O(α, γ)²²Ne reaction rate decreases the uncertainty of the ²¹Ne/²²Ne ratio from ~30% down to ~8%. The comparison of the predicted ²¹Ne/²²Ne ratio with analyses of SiC grains size distributions in meteorites has provided deeper insight into the properties of AGB stars that gave rise to the different sizes of SiC grains. This work provides an important benchmark for the study of AGB star nucleosynthesis using the abundance distribution of SiC grains.