核天体物理学中存在着一个重要问题，即在氢燃烧环境中，CNO循环向更重质量核素方向的反应流是如何发生的。作为宇宙中的第一代恒星，也称为原初恒星或Pop-III恒星，它们由宇宙大爆炸后遗留的原初物质构成，播散了宇宙中首批重元素，从而为随后的恒星和星系发展创造了条件。在初始质量小于约1.2倍太阳质量的恒星中，pp反应链主导着氢聚变，提供了恒星能量的主要来源。然而，对于质量较大的恒星，CNO循环成为了主导过程。作为一种催化反应，CNO循环过程中的重原子核总量保持不变，而通过氢核的消耗和氦核的增加(即氢聚变成氦)来释放能量。这个过程持续进行，直到某个特定反应打破了这一连续性，导致物质向Ne-Na区域泄漏，或者恒星核心的温度和密度达到足够高，能够通过3α过程合成新的碳原子核。在这些反应中，能在较低温度下将CNO核素从循环中突破的关键反应即为19F(p, γ)20Ne反应。 虽然在金属度与太阳相似的恒星中，19F(p, γ)20Ne反应在生成更重核素方面的作用微乎其微，但在解释最贫金属恒星(即第一代恒星)中观测到的钙丰度现象时，这一反应可能扮演着关键角色。理论天体物理学家指出，在恒星氢燃烧过程中的热CNO循环突破可能是解决第一代恒星中钙丰度理论计算值与观测值不符问题的关键机制。通过对不同质量的第一代恒星的模拟研究，Clarkson和Herwig提出，若19F(p, γ)20Ne/19F(p, αγ)16O反应率比值相比于NACRE编评值提高10倍左右，则其天体模型能够解释第一代恒星观测到的钙丰度，突显了19F(p, γ)20Ne反应在解释宇宙中最古老恒星钙丰度来源问题中的重要性。 在核天体物理学感兴趣的能量范围内(Ec.m. < 1 MeV)，19F(p, γ)20Ne反应的实验数据非常有限，高能端主要难题源于竞争反应19F(p, αγ)16O产生的6.13 MeV γ射线的强度过高，导致了堆垒和死时间问题;而在低能端，主要由于库伦势垒的作用，反应截面极低，使得直接测量极为困难。早期实验采用小体积的NaI(Tl)探测器测量退激到20Ne第一激发态的γ射线，但因分辨率限制而受到堆垒现象的显著干扰。2008年，Couture等人通过使用一种基于HPGe和NaI探测器的符合测量技术，成功测量了能量范围最低至Ec.m. = 200 keV的产额，但由于实验条件限制，较低能区只给出了天体物理S因子的上限值。2022年，锦屏深地核天体物理实验进一步将该反应的直接测量推进至Ec.m. = 186 keV，并在225 keV处发现新共振，其R-矩阵外推结果显示，在第一代恒星的氢燃烧温度0.1 GK对应的伽莫夫能区，19F(p, γ)20Ne反应率比先前估计提高了5.4–7.4倍，为解决古老恒星中钙丰度问题提供了重要线索。然而，在最关键的伽莫夫能区内，19F(p, γ)20Ne反应缺乏直接测量数据，而R-矩阵外推的准确性可能受到反应截面剧烈变化的影响。因此，有必要对19F(p, γ)20Ne反应进行进一步的精确测量，将直接测量推进至其伽莫夫能区内。 本研究致力于19F(p, γ)20Ne反应在伽莫夫能区内的直接测量，使用了多项创新技术以提升实验精度。首先，基于锦屏深地核天体物理实验的经验，本工作利用同位素电磁分离技术完成了新的19F离子注入靶，使得对实验干扰最大的硼元素本底降低一个量级以上。此外，此次实验使用高颗粒度及4π全立体角覆盖的BGO探测器阵列—LAMBDA-II，此阵列在探测效率、能量分辨等关键性能指标上均位居国际同类装置的前列。在合肥核安全所的HINEG加速器上，利用最高2.8 mA的质子束流轰击19F靶，测量了Ep = 145–280 keV能量范围内19F(p, γ)20Ne反应的直接辐射俘获过程以及Ec.m. =225 keV共振的贡献。通过使用γ全能量解谱技术对获取到的γ能谱进行细致分析，本工作成功测定了19个不同能量点的反应产额，进而推导出了225 keV共振的强度以及直接过程的激发曲线与天体物理S因子。 本研究首次将19F(p, γ)20Ne反应的直接测量推进至第一代恒星典型温度0.1 GK对应的伽莫夫能区(Ec.m. = 76–146 keV)，低能区数据与JUNA实验的R-矩阵预测在误差范围内一致。这一成果不仅有望在低能端对S因子的外推提供更强的约束，而且基于本工作的实验数据以及JUNA实验的R-矩阵分析，新的19F(p, γ)20Ne反应率在0.1 GK附近的精度得到了显著提高，最多可达两倍。这些结果为我们深入理解第一代恒星中钙丰度之谜奠定了基础。 本论文还介绍了攻读博士期间负责的另外两个工作。一个工作是在锦屏地下实验室对 25Mg(p, γ)26Al反应低能共振的直接测量。这一反应在γ射线天文学和陨石中太阳前颗粒的26Al丰度研究中扮演着关键角色。根据此次实验结果，发现26Al的产量相比之前的研究增加了45%–79%。第二项工作是利用上海激光伽玛光源来测量p过程关键反应181Ta(γ, n)180Ta的总截面，所获得的激发函数在误差范围内与以往的研究结果一致，但缩小了不确定度，这对于深入理解p过程中的核素合成将起到关键作用。

In nuclear astrophysics, an important question pertains to how the reaction flow of the CNO cycle proceeds towards heavier nuclei within hydrogen-burning environments. The first generation stars, also known as primordial or Pop-III stars, originated from the pristine matter left over from the Big Bang. These stars played a pivotal role in dispersing the universe’s initial heavy elements, thereby setting the stage for the evolution of subsequent generations of stars and galaxies. In stars with initial masses less than approximately 1.2 solar masses, the pp chain predominates in hydrogen fusion, serving as the primary energy source. Yet, in more massive stars, the CNO cycle takes precedence. Functioning as a catalytic reaction, the CNO cycle involves a constant total mass of heavy nuclei, producing energy from the consumption of hydrogen nuclei and the accumulation of helium nuclei. This cycle persists until a particular reaction disrupts this sequence, resulting in material leakage into the Ne-Na region, or until the stellar core’s temperature and density reach a threshold sufficient for initiating the 3 process, thereby synthesizing new carbon nuclei. Among these CNO cycle reactions, the 19F(,γ)20Ne reaction stands out as the key breakout reaction capable of deriving CNO isotopes from the cycle at relatively low temperatures.

The 19F(p, γ)20Ne reaction plays a minimal role in generating heavier nuclei in stars with metallicity akin to the Sun’s. However, it may hold a key role in explaining the observed calcium abundance in extremely metal-poor stars (EMP stars), representing the first generation stars. Several astrophysicists have proposed in their respective studies that hot CNO breakout during the stellar hydrogen-burning process could be a crucial mechanism in resolving discrepancies between theoretical calculations and observed values of calcium abundance in the first generation stars. Through simulations of first generation stars of varying masses, Clarkson and Herwig suggested that if the reaction rate ratio of 19F(p, γ)20Ne to 19F(p, αγ)16O is increased tenfold over the NACRE compilation values, their model could account for the observed calcium abundance in the earliest stars, underscoring the significance of the 19F(p, γ)20Ne reaction in explaining the origins of calcium abundance in the universe’s oldest stars.

Experimental data for the 19F(p, γ)20Ne reaction within the energy range of interest in nuclear astrophysics (Ec.m. < 1 MeV) are severely limited. The primary challenge at higher energies stems from the competitive 19F(p, αγ)16O reaction, which produces intense 6.13 MeV γ-rays, leading to pile-up and dead time problems. At the lower energies, the exceedingly low reaction cross-section, a consequence of the Coulomb barrier, makes direct measurements extraordinarily difficult. Previous experiments utilized small-volume NaI(Tl) detectors to measure γ-rays de-exciting to the first excited state of 20Ne, but were significantly hindered by pile-up due to resolution limitations. In 2008, Couture et al. successfully measured the reaction yields down to Ec.m. = 200 keV using a coincident measurement technique with HPGe and NaI detectors, though limitations in experimental technique only provided upper limits for the astrophysical S- factor at lower energy ranges. The Jinping Underground Nuclear Astrophysics (JUNA) experiment further extended direct measurements down to Ec.m. = 186 keV in 2022, discovering a new resonance at 225 keV. R-matrix extrapolation from JUNA data indicated that in the Gamow energy range corresponding to hydrogen-burning temperatures of 0.1 GK in the first generation stars, the reaction rate for 19F(p, γ)20Ne increased by 5.4–7.4 times over previous recommend, offering crucial insights for resolving the calcium abundance mystery in the oldest stars. However, direct measurement data for the 19F(p, γ)20Ne reaction within the critical Gamow energy region are still lacking, and the accuracy of R-matrix extrapolations may be affected by dramatic variations in the re- action cross-section. Consequently, further precise measurements of the 19F(p, γ)20Ne reaction, extending directly into its Gamow window, are necessary.

This work is dedicated to direct measurements of the 19F(p, γ)20Ne reaction within the Gamow energy region, employing several innovative techniques to enhance experimental precision. Building on the experience from the JUNA experiment, this work utilized isotopic electromagnetic separation technology to develop a new 19F ion-implanted target, significantly reducing background from boron contamination, the most substantial experimental interference, by an order of magnitude. Additionally, the full solid-angle 4π BGO detector array, LAMBDA-II, was equipped, reaching international forefront levels in both resolution and detection efficiency. The experiment was performed using the HINEG accelerator at the Hefei Institute of Nuclear Safety, focusing on measurements of the direct process of the 19F(p, γ)20Ne reaction across an energy range of p = 145–280 keV and the resonance at Ec.m. = 225 keV with a proton beam up to 2.8 mA bombarding the 19F target. Through meticulous analysis of the acquired γ spectra using the full-energy peak deconvolution techniques, this work successfully determined the reaction yields at 19 energy points, thereby derived the strength of the 225 keV resonance, as well as the excitation curve and astrophysical factor for the direct process.

For the first time, this research has extended direct measurements of the 19F(,γ)20Ne reaction to the Gamow window (Ec.m.= 76–146 keV), for the typical of the temperatures in the first generation stars. The low-energy data agreed well with the JUNA experiment’s R-matrix predictions within error margins. This achievement not only provides stronger constraints for extrapolations of the factor at the low energies end but also significantly enhances the precision of the new 19F(p, γ)20Ne reaction rates around 0.1 GK, based on the present firmer experimental data and R-matrix analysis from the previous JUNA work, by up to a factor of two. These advancements offer pivotal information for a deeper understanding of the calcium abundance mystery in the universe’s oldest stars.

This thesis also presents two other works conducted during the PhD period. The first work is the direct measurement of the low-energy resonances in the 25Mg(p, γ)26Al reaction at the Jinping Underground Nuclear Astrophysics Experimental Facility. The 26Al is one of the most significant nuclides in γ-ray astronomy and presolar grains of meteorites. As a result, an increase of 45%–79% in 26Al yield is found compared to previous studies. The second work is the measurement of the total cross section of the significant p-process reaction 181Ta(γ, )180Ta at Shanghai Laser Electron Gamma Source. The excitation functions obtained in this work are consistent with previous studies within the uncertainties but with improved measurement precision. This improvement is vital for advancing our understanding of nucleosynthesis in the p-process.