Beta-delayed neutrons for astrophysics: Understanding the most neutron-rich nuclides

天体物理学中的β延迟中子：了解中子最多的核素

• 批准号: SAPIN-2019-00030
• 负责人: Dillmann, Iris
• 金额: $ 10.42万
• 依托单位: TRIUMF
• 依托单位国家: 加拿大
• 项目类别: Subatomic Physics Envelope - Individual
• 财政年份: 2021
• 资助国家: 加拿大
• 起止时间: 2021-01-01 至 2022-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=739412
• 关键词: Beta; delayed; neutrons; astrophysics; Understanding

A large fraction of the heavy nuclei (elements heavier than iron) that are produced in the "rapid neutron capture process" (r process) are not (yet) experimentally accessible and are located in the so-called "Terra Incognita". With the next generation of radioactive ion beam (RIB) facilities presently under construction worldwide, the main motivation of all projects is the investigation of these very neutron-rich nuclei which are in or at the reaction path of the r process. However, until the new facilities will become operational within the next 5-10 years, the presently operating facilities are continuing to push the borders of known nuclei as far out as possible. Whereas the proton dripline has been reached for many elements up to mass A~200, the neutron dripline is only accessible up to A~40 (Na, Mg). Theoretical predictions show that on top of the 3000 known nuclei one can expect ~4000 more nuclei to be discovered, the largest fraction of them on the neutron-rich side. For these neutron-rich nuclei, a new decay mechanism appears when the neutron separation energy is lower than the reaction Q value: beta-delayed neutron (bdn) emission. Two important physical quantities can be measured with the investigation of bdn emitters: the beta-decay half-life and the beta strength above the neutron separation energy from the neutron-branching ratio. The more neutron-rich a nucleus becomes, the more important bn emission gets until it will be the dominant decay mechanism for almost all newly discovered neutron-rich nuclei. So far about 600 bn-emitters are known, but for only 300 of them the neutron-branching ratio has been measured. In 2016/17 we have (re)measured 270 bn-emitters alone in the BRIKEN campaign. For 183 of them (68%) the neutron-branching ratio was measured for the first time, and for 60 of them the beta-decay half-life for the first time. With this proposal I will continue and extend my ongoing program about the investigation of even more exotic bdn emitters in the next 5 years. In 2019/20 we are planning to run at least three more experiments at RIKEN, one led by my group. These measurements are aiming at a better understanding of the competition between delayed one- and multi-neutron emission. In addition, we will provide data for the very rare processes of delayed three- and four-neutron emission. After the BRIKEN campaign we aim to extend our experimental scope by moving our detection setup to another facility to try to investigate yet inaccessible nuclei like e.g. heavy isotopes at the N=126 shell closure with the multi-nucleon transfer. This promising reaction mechanism will allow for the first time access to those nuclei that are responsible for the third peak of the r-process around mass A~195. A detailed knowledge of the bdn process is an important key ingredient for a better understanding of even more neutron-rich nuclei that will not be accessible experimentally and have to be inferred from theoretical models.

在“快中子捕获过程”（r过程）中产生的重核（比铁重的元素）的很大一部分（尚）无法通过实验获得，它们位于所谓的“未知领域”。随着下一代放射性离子束（RIB）设施目前在世界范围内的建设，所有项目的主要动机是研究这些非常富中子的核，这些核处于或处于r过程的反应路径上。然而，在新设施在未来5-10年内投入运行之前，目前运行的设施仍在继续将已知核的边界尽可能地推向更远的地方。许多质量为A~200的元素都能达到质子滴线，而只有质量为A~40 （Na, Mg）的元素才能达到中子滴线。理论预测表明，除了已知的3000个原子核外，还会有4000多个原子核被发现，其中大部分位于中子富集的那一边。对于这些富中子核，当中子分离能低于反应Q值时，出现了一种新的衰变机制：β延迟中子（bdn）发射。通过对bdn发射体的研究可以测量两个重要的物理量：β衰变半衰期和β强度高于中子分支比的中子分离能。中子越丰富的原子核，bn发射就越重要，直到它成为几乎所有新发现的富中子原子核的主要衰变机制。到目前为止，已知的辐射源大约有6000亿个，但其中只有300个的中子分支比被测量过。在2016/17年度，我们仅在BRIKEN活动中就（重新）测量了2700亿排放者。其中183个（68%）是首次测量中子分支比，60个是首次测量β衰变半衰期。有了这个提议，我将继续并扩展我正在进行的关于在未来5年内调查更多奇异的bdn排放者的计划。在2019/20年，我们计划在理研再进行至少三个实验，其中一个由我的小组领导。这些测量的目的是为了更好地理解延迟单中子和多中子发射之间的竞争。此外，我们将提供非常罕见的延迟三中子和四中子发射过程的数据。在BRIKEN活动之后，我们的目标是通过将我们的检测装置移动到另一个设施来扩展我们的实验范围，以尝试研究尚无法进入的原子核，例如N=126壳闭合的重同位素与多核子转移。这个有希望的反应机制将使我们第一次能够接触到质量为A~195的r过程第三峰的原子核。对bdn过程的详细了解是更好地了解更多中子丰富的原子核的重要关键因素，这些原子核无法通过实验获得，必须从理论模型中推断出来。