Beta-Decay Experiments along the R-Process Path

宇宙中的大多数化学元素是由恒星中的核反应通过一系列不同的核合成过程产生的。这些过程中的大多数的基本特征都是众所周知的。然而，一个主要的悬而未决的问题是快速中子捕获过程(r过程)的性质，该过程负责合成大约一半比铁更重的化学元素，如金和铀。天文观测表明，自银河系历史早期以来，r过程一直很活跃，发生在爆炸的恒星环境中，例如超新星爆炸或两颗中子星之间的合并，但细节尚不清楚。理解r过程的一个主要困难是它涉及不稳定的同位素。该奖项支持的主要活动是一系列核物理实验，以测量与r过程研究直接相关的同位素的放射性衰变特性。该实验装置结合了用于β衰变测量的最先进的辐射探测器(BRIKEN)，以及日本(RIBF)和美国(NSCL)领先加速器设施的不稳定同位素束。在这个实验中获得的核数据将允许在天文观测和r过程的天体物理模型之间进行更精确的比较。这项提议的另一个目标是为学生，特别是本科水平的学生提供参与核物理前沿研究活动的机会，并为扩大美国从事科学职业的学生数量做出贡献。出于这个原因，该提案包括将在中央密歇根大学(CMU)校园内进行的研究项目，例如开发辐射探测器和使用r过程的天体物理模型进行计算工作。该奖项的主要科学成果将是使用安装在日本理研放射性离子束工厂的新BRIKEN探测器测量贝塔延迟中子发射概率(PN值)。BRIKEN装置包括一个放射性离子束的有源阻挡探测器，周围是一个由He-3正比计数器组成的高效中子探测器。目前计划的实验将测量从N=50壳层闭合到稀土元素的富中子同位素的PN值和衰变半衰期。这些实验将为核图中与解决r-过程核合成的重要问题有关的区域的r-过程模型提供新的核数据，例如导致A=130丰度峰中元素形成的天体物理条件。该奖项还将支持CMU研究人员参与密歇根州立大学国家超导回旋实验室(NSCL)的r过程实验。这包括一个项目，在N=82区域周围开发再加速的重同位素束流，有可能为r过程测量开辟新的机会。

Most of the chemical elements in the universe have been produced by nuclear reactions in stars through a series of different nucleosynthesis processes. The basic features of most of these processes are well understood. However, one of the major open questions is the nature of the rapid neutron capture process (r-process), which is responsible for the synthesis of roughly half of the chemical elements heavier than iron, such as gold and uranium. Astronomical observations indicate that the r-process has been active since the early stages of galactic history, and takes place in explosive stellar environments, such as supernovae explosions or mergers between two neutron stars, but the details are not known. A major difficulty to understanding the r-process is that it involves unstable isotopes. The main activities supported by this award are a series of nuclear physics experiments to measure the radioactive decay properties of isotopes directly relevant to r-process studies. The experimental setup combines state-of-the-art radiation detectors for beta-decay measurements (BRIKEN), and unstable isotope beams at leading accelerator facilities in Japan (RIBF) and the US (NSCL). The nuclear data obtained in this experiments will allow for a more precise comparison between astronomical observations and astrophysical models for the r-process. Another objective of this proposal is to provide students, especially at the undergraduate level, with opportunities to become involved in research activities at the forefront of nuclear physics, and contribute to expanding the number of students pursuing scientific careers in the US. For this reason, the proposal includes research projects that will take place at the campus of Central Michigan University (CMU), such as the development of radiation detectors and computational work with astrophysical models of the r-process.The main scientific outcome of this award will be the measurement of beta-delayed neutron emission probabilities (Pn values) with the new BRIKEN detector setup at the Radioactive Ion Beam Factory in RIKEN, Japan. The BRIKEN setup consists of an active stopper detector for radioactive ion beams, surrounded by a high-efficiency neutron detector composed of He-3 proportional counters. The experiments currently planned will measure Pn values, as well as decay half-lives, for neutron-rich isotopes spanning the regions from the N=50 shell closure up to the rare-earth elements. These experiments will provide new nuclear data for r-process models in regions of the nuclear chart relevant to address important questions of r-process nucleosynthesis, such as the astrophysical conditions that lead to the formation of elements in the A=130 abundance peak. The award will also support the participation of CMU researchers in r-process experiments at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. This includes a project to develop reaccelerated beams of heavy isotopes around the N=82 region, with the potential to open new opportunities for r-process measurements.