Beta-delayed neutron measurements for nuclear astrophysics

核天体物理学的β延迟中子测量

• 批准号: SAPIN-2014-00028
• 负责人: Dillmann, Iris
• 金额: $ 8.74万
• 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: 2016
• 资助国家: 加拿大
• 起止时间: 2016-01-01 至 2017-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=612386
• 关键词: Beta; delayed; neutron; measurements; nuclear

Beta-delayed neutron emitters play an important, two-fold role in the stellar nucleosynthesis of heavy elements in the "rapid neutron-capture process" (r process). On one hand they lead to a detour of the material beta-decaying back to stability. On the other hand, the released neutrons increase the neutron-to-seed ratio, and are re-captured during the freeze-out phase and thus influence the final solar r-abundance curve. Two important physical quantities can be measured at once with bn-emitters: the half-life of the precursor, and the beta-strength above the neutron separation energy can be deduced from the neutron-branching ratio. A large fraction of the isotopes inside the r-process reaction path are not yet experimentally accessible and are located in the (experimental) "Terra Incognita". With the next generation of fragmentation and ISOL facilities presently being built, one of the main motivation of all projects is the investigation of these very neutron-rich isotopes. Presently, about 600 one-neutron emitters are known, but only for a third of them experimental data is available. However, reaching more neutron-rich isotopes means also that multiple neutron-emission becomes the dominant decay mechanism. About 460 beta-delayed two-, three- or four-neutron emitters are identified up to now but for only 30 of them experimental data about the neutron branching ratios are available, most of them in the light mass region below A=30. The International Atomic and Energy Agency (IAEA) has identified the urgency and picked up this topic recently in a 4-year “Coordinated Research Project" on a “Reference Database for Beta-Delayed Neutron Emission Data”. This project will review, compile, and evaluate the existing data for neutron-branching ratios and half-lives of beta-delayed neutron emitters. This will help to ensure a reliable database for the future discoveries of new isotopes and help to constrain astrophysical and theoretical models. TRIUMF has taken a lead role in this and has submitted an extensive proposal with Canadian colleagues from McMaster University and the University of Guelph for future measurements and the evaluation of existing data. This Discovery Grant application aims to investigate beta-delayed neutron-emitters with several detectors and novel techniques at different world-leading facilities. One direction is the use of the “standard” method with 3He-filled neutron detectors to measure directly the branching ratio and half-lives. This research will be conducted with highly-efficient detector setups and state-of-the-art electronics. The "BRIKEN campaign" at RIKEN/ Japan proposes to perform experiments in 2014-16 with the most efficient neutron detector setup worldwide. Under my leadership TRIUMF is involved in this campaign and will submit proposals for measurements on astrophysical key nuclides. After the BRIKEN campaign the existing program at the present ISAC-1 facility at TRIUMF will be extended towards more neutron-rich beams which are expected with the new ARIEL facility in a few years. We propose to carry out high-precision branching-ratio measurements for nuclear structure and r-process studies. The second direction is the development of novel techniques to measure neutron-branching ratios circumventing the detection of the neutron. One of the proposed methods uses traps to measure the branching ratio and neutron energy spectra via the beta-recoil ion coincidence technique. This method has recently been successfully employed by an Argonne/ Livermore collaboration. The advantages and disadvantages of this method will be investigated with both traps at TRIUMF (TITAN and TRINAT). The additional installation of neutron detectors can expand this method to multi-neutron emitters.

β延迟中子发射体在“快速中子俘获过程”（r过程）中的恒星重元素核合成中发挥着重要的双重作用。一方面，它们导致材料β衰变回到稳定状态。另一方面，释放的中子增加了中子与种子的比率，并在冻结阶段被重新捕获，从而影响最终的太阳 r 丰度曲线。使用 bn 发射体可以同时测量两个重要的物理量：前驱体的半衰期，以及可以从中子分支比中推导出高于中子分离能的 β 强度。 r过程反应路径内的大部分同位素尚未通过实验获得，并且位于（实验）“Terra Incognita”中。随着下一代碎裂和 ISOL 设施的建设，所有项目的主要动机之一就是研究这些非常富含中子的同位素。目前，已知约 600 个单中子发射体，但只有其中三分之一的实验数据可用。然而，达到更多的富中子同位素也意味着多重中子发射成为主要的衰变机制。迄今为止，已识别出约 460 个 β 延迟二、三或四中子发射体，但其中只有 30 个可以获得有关中子分支比的实验数据，其中大多数位于 A=30 以下的轻质量区域。 国际原子能机构 (IAEA) 已经认识到了这一紧迫性，并在最近一项为期 4 年的“β 延迟中子发射数据参考数据库”“协调研究项目”中选择了这一主题。该项目将审查、编译和评估有关中子分支比和β延迟中子发射体半衰期的现有数据。这将有助于确保未来发现新同位素的可靠数据库，并有助于约束天体物理和理论模型。 TRIUMF 在这方面发挥了主导作用，并与麦克马斯特大学和圭尔夫大学的加拿大同事一起提交了一份广泛的提案，以进行未来的测量和现有数据的评估。 这项发现补助金申请旨在利用不同世界领先设施的多个探测器和新技术来研究β延迟中子发射体。 一个方向是使用“标准”方法和 3He 填充中子探测器来直接测量支化比和半衰期。这项研究将使用高效的探测器装置和最先进的电子设备进行。 RIKEN/日本的“BRIKEN 活动”提议在 2014-16 年使用全球最高效的中子探测器装置进行实验。在我的领导下，TRIUMF 参与了这项活动，并将提交天体物理关键核素测量建议。 BRIKEN 活动结束后，TRIUMF 目前 ISAC-1 设施的现有计划将扩展到更多的富中子束，预计几年后新的 ARIEL 设施将实现这一目标。我们建议对核结构和 r 过程研究进行高精度支化比测量。 第二个方向是开发绕过中子检测来测量中子分支比的新技术。所提出的方法之一使用陷阱通过β反冲离子符合技术来测量分支比和中子能谱。阿贡/利弗莫尔合作最近成功采用了这种方法。该方法的优点和缺点将通过 TRIUMF 的两种陷阱（TITAN 和 TRINAT）进行研究。额外安装中子探测器可以将该方法扩展到多中子发射器。