宇宙中铁以上的重核是如何合成的?

How were the heavy chemical elements beyond iron made in the Universe?

针对21世纪尚未解决的11个重大物理问题之三,即"宇宙中从铁到铀的元素是如何产生的?"进行了系统的阐述,包括问题提出的背景和重要性、主要研究内容、国内外进展以及将来的发展趋势.介绍了产生宇宙中比铁重的元素(简称为超铁元素)的几个主要核合成过程,并总结了相关的研究目标.目前,尚需要精确测量天体核合成路径上关键核素的质量、寿命以及相关核反应的反应截面或者天体物理反应率等核物理输入量;开展天体元素或者同位素丰度的观测研究,以及星际X和γ射线等的卫星观测;发展天体物理模型以及核物理理论模型,最终将可靠的核物理输入量、核天体物理理论模型和天文观测数据相结合,以探索和解决宇宙中超铁元素的来源问题.

Nuclear astrophysics, which is an interdisciplinary branch of nuclear physics （micro scale） and astrophysics （macro scale）, addresses some of the most compelling questions in the universe. Research efforts have been devoted to many topics such as the origins of the chemical elements, the unique conditions of earth that makes life possible, and the for- mation and evolution of the sun, stars, and galaxies. Nuclear processes play an extremely important role in cosmic evolu- tion after the Big Bang, and they are the only known mechanisms that synthesize heavy elements, in addition to provid- ing the energy for stars to resist the force of gravity. Over the past 50 years, scientists have developed a deep understanding of Big-Bang primordial nucleosynthesis and the mechanisms for synthesizing heavy elements. However, the astrophysical models have yet to adequately reproduce the observed solar abundances of those elements beyond iron （referred to as ultra-iron elements）. It is widely believed that these ultra-iron elements were primarily synthesized via the slow neutron capture process （s-process） and the fast neutron capture process （r-process）. The fundamental s-process component is thought to originate from thermally pulsing low-mass AGB stars, with reaction pathways closed after the stable nuclides. About half of the heavy elements （up to bismuth） were produced via the s-process. The r-process is thought to occur in the explosive burning of core-collapse supernova and/or neutron-star mergers. Although the r-process site remains a mystery, experts believe that more than half of the ultra-iron elements （up to thorium and uranium） were produced via the r-process. In addition, there are 35 neu- tron-deficient stable isotopes, which are present in significantly less abundance in our solar system. These so-called p-nuclei are likely produced in Type II supernova, through the photo-dissociation （referred to as p-process or r-process） of existing s- or r-process seeds, or through the recently proposed neutrino-proton vp-process. Astrophysical models require a huge amount of nuclear-physics input data. The most essential data include nuclear mass, structure, decay and fission characteristics, and related nuclide cross-sections along the various nucleosynthesis paths. Thus far, the lack of systematic and precise nuclear inputs is one of the main reasons scientists have not been able to reproduce the observed solar abundances of ultra-iron elements. Published by Discover magazine in 2002, the Ameri- can National Research Council ranked the question, ＂how were the heavy elements from iron to uranium made？＂ as one of the 11 Greatest Unanswered Questions of Physics in this Century. More complete and precise nuclear physics inputs are therefore urgently needed to improve astrophysical models and decode the observations. This paper introduces the most important nucleosynthesis processes that are responsible for the ultra-iron elements production in the universe, while also summarizing the relevant key nuclear-physics inputs required in the astrophysical models. Finally, the frontier in this field study includes the origins of ultra-iron elements, and the future of nuclear astro- physics research in China is examined.