核天体rp过程路径上A>80核素的精确质量测量

X-ray bursts are the most frequently observed thermonuclear explosions in the Galaxy. The main driving process of this astrophysical phenomenon is the rapid proton capture (rp) process, which involves many proton capture reactions on unstable nuclei. Nuclear masses are key parameters in the calculation of these reaction rates and high mass accuracy is generally required. At present, the masses of many related nuclei with A >80 are not well-known or even unknown. It becomes an important uncertainty in the model research of X-ray bursts. Based on the Large-scale scientific facility HIRFL-CSR, this project takes advantage of the international leading double time-of-flight (double TOF) detector technology and use 124Xe as the primary beam for the first time to carry out the mass measurements of a batch of nuclides with A>80 on the nuclear reaction pathway of rp-process. There are about 10 nuclides that the project intends to measure for the first time or to improve the mass precision. Basically, these nuclides’ mass precision can reach 50 keV or even 10 keV. Through the mass measurement of this project, the nuclear mass and nuclear reaction rate database required for astrophysics research can be updated. Combined with the X-ray burst model, the rp-process nucleosynthesis pathway as well as the X-ray burst ash composition can be more accurately defined. It also opens the way for HIAF to use a much heavier primary beam for double TOF mass measurements in the future.

X射线暴是银河系内最常见的热核爆发事件。驱动这一天体现象的主要核过程是快质子俘获(rp)过程，该过程涉及大量不稳定核素的质子俘获等反应，计算这些反应率需要精确的原子核质量数据。目前A>80核区的大量相关核素质量精度不够甚至质量未知，成为X射线暴模型研究中的重要不确定因素。本项目依托大科学装置HIRFL-CSR，利用国际领先的双飞行时间(双TOF)探测器技术，首次使用124Xe作为主束，开展对rp过程核反应路径上的一批A>80核素的精确质量测量。本项目拟首次测量和提高质量精度的核素有10个左右，质量精度基本可以达到50keV，部分可达10keV。通过本项目的质量测量，可以更新核天体物理研究所需的核素质量和核反应率数据库，结合X射线暴模型，可以更准确地限定rp过程核合成路径和X射线暴产物丰度。另外也可为未来HIAF使用更重的主束进行双TOF质量测量开辟道路。

对A=80-100缺中子核区原子核的研究是一个国际热点。这是因为该核区既涉及到原子核形变及壳层演化等核结构问题, 又涉及到关键的核天体核合成过程。本项目主要针对该核区，开展了一系列工作。1.提出了新的数据分析方法，可以准确区分存在周期谱重叠的基态和激发态。利用该方法，我们提高了69As,73Br,75Kr,79Sr和81Y的质量精度到keV量级，并且首次给出95Pd,93Rh,91Ru和87Mo的1/2-低位同核异能态的质量(激发能)。利用这些质量，在包含张量力作用下的壳模型帮助下，我们揭示并理解了A=80-100部分缺中子核素的1/2-低位同核异能态激发能的系统性变化规律，显示出张量力在该核区中子质子相互作用中起到了重要作用。2.利用新报道的本项目目标原子核(例如89Ru)高精度质量，研究了其对核天体rp核合成过程的影响。我们发现新质量很好地限制了有关核反应(例如88Tc(p,γ))反应率不确定度，消除了质量不确定性对A=89核素丰度的影响。3.利用双飞行时间探测器高精度测量了离子的速度信息并提取出了离子Betatron振荡的tune值和储存环的γt值，促进了有关加速器技术以及储存环上基于双TOF的新型等时性质谱术的发展。