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: $ 8.01万
• 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: 2018
• 资助国家: 加拿大
• 起止时间: 2018-01-01 至 2019-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=649301
• 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-缓发中子发射体在恒星重元素核合成的“快速中子俘获过程”（r过程）中起着重要的双重作用。一方面，它们导致材料β衰变回到稳定状态的绕道。另一方面，释放的中子增加了中子与种子的比例，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。用bn发射体可以同时测量两个重要的物理量：前体的半衰期，以及中子分离能以上的β强度可以从中子分支比推导出来。在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）“未知领域”。随着目前正在建造的下一代碎裂和ISOL设施，所有项目的主要动机之一是研究这些非常富含中子的同位素。目前，已知的单中子发射体大约有600个，但只有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个中子发射成为主要的衰变机制。到目前为止，大约有460个β延迟的两个、三个或四个中子发射体被确定，但只有30个中子分支比的实验数据可用，其中大多数在A=30以下的轻质量区域。** 国际原子能机构（原子能机构）已经确定了这一问题的紧迫性，并于最近在一个为期4年的“关于β延迟中子发射数据参考数据库”的“协调研究项目”中提出了这一问题。该项目将审查、汇编和评估β缓发中子发射体的中子分支比和半衰期的现有数据。这将有助于确保为今后发现新同位素建立可靠的数据库，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的目的是在不同的世界领先的设施中使用几种探测器和新技术研究β延迟中子发射体。一个方向是使用“标准”方法，用填充3 He的中子探测器直接测量分支比和半衰期。这项研究将使用高效的探测器设置和最先进的电子设备进行。RIKEN/ Japan的“BRIKEN活动”计划在2014-16年使用世界上最有效的中子探测器装置进行实验。在我的领导下，TRIUMF参与了这一活动，并将提交有关天体物理学关键核素测量的建议。在BRIKEN活动之后，TRIUMF现有ISAC-1设施的现有计划将扩展到更多的富中子束，预计新的ARIEL设施将在几年内实现这一目标。我们建议为核结构和r过程研究进行高精度的分支比测量。第二个方向是发展新的技术来测量中子分支比绕过中子的检测。所提出的方法之一是使用陷阱通过β-反冲离子符合技术测量分支比和中子能谱。这种方法最近已成功地采用了阿贡/利弗莫尔合作。将使用TRIUMF的两个阱（TITAN和TRINAT）研究该方法的优缺点。附加的中子探测器的安装可以将该方法扩展到多个中子发射体。