The rapid-neutron capture process ($r$ process) is identified as the producer of about 50\% of elements heavier than iron. This process requires an astrophysical environment with an extremely high neutron flux over a short amount of time ($\sim$ seconds), creating very neutron-rich nuclei that are subsequently transformed to stable nuclei via $\beta^-$ decay. One key ingredient to large-scale $r$-process reaction networks is radiative neutron-capture ($n,\gamma$) rates, for which there exist virtually no data for extremely neutron-rich nuclei involved in the $r$ process. Due to the current status of nuclear-reaction theory and our poor understanding of basic nuclear properties such as level densities and average $\gamma$-decay strengths, theoretically estimated ($n,\gamma$) rates may vary by orders of magnitude and represent a major source of uncertainty in any nuclear-reaction network calculation of $r$-process abundances. In this review, we discuss new approaches to provide information on neutron-capture cross sections and reaction rates relevant to the $r$ process. In particular, we focus on indirect, experimental techniques to measure radiative neutron-capture rates. While direct measurements are not available at present, but could possibly be realized in the future, the indirect approaches present a first step towards constraining neutron-capture rates of importance to the $r$ process.

中文翻译：

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用于限制与 r 过程重元素核合成相关的中子捕获率的新技术

快速中子捕获过程（$r$ 过程）被确定为产生约 50% 的比铁重的元素。这个过程需要一个在短时间内（$\sim$ 秒）具有极高中子通量的天体物理环境，产生非常富含中子的原子核，随后通过 $\beta^-$ 衰变转化为稳定的原子核。大规模 $r$ 过程反应网络的一个关键因素是辐射中子捕获 ($n,\gamma$) 率，实际上没有关于 $r$ 过程中涉及的极其富含中子的原子核的数据。由于核反应理论的现状以及我们对基本核性质如能级密度和平均 $\gamma$-衰变强度的了解不足，理论上估计 ($n, \gamma$) 率可能会因数量级而异，并且代表了任何核反应网络计算 $r$-过程丰度的主要不确定性来源。在这篇综述中，我们讨论了提供与 $r$ 过程相关的中子俘获截面和反应速率信息的新方法。特别是，我们专注于测量辐射中子捕获率的间接实验技术。虽然目前无法进行直接测量，但将来可能会实现，但间接方法是限制对 $r$ 过程具有重要意义的中子捕获率的第一步。我们讨论了提供与 $r$ 过程相关的中子捕获截面和反应速率信息的新方法。特别是，我们专注于测量辐射中子捕获率的间接实验技术。虽然目前无法进行直接测量，但将来可能会实现，但间接方法是限制对 $r$ 过程具有重要意义的中子捕获率的第一步。我们讨论了提供与 $r$ 过程相关的中子捕获截面和反应速率信息的新方法。特别是，我们专注于测量辐射中子捕获率的间接实验技术。虽然目前无法进行直接测量，但将来可能会实现，但间接方法是限制对 $r$ 过程具有重要意义的中子捕获率的第一步。