Beta-delayed neutron measurements for nuclear astrophysics

在“快速中子俘获过程”（r过程）中，β -延迟中子发射器在重元素的恒星核合成中起着重要的双重作用。一方面，它们会导致物质衰变回到稳定状态。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。用bn发射器可以同时测量两个重要的物理量：前驱体的半衰期和中子分离能以上的β强度可以从中子分支比中推断出来。** 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富含中子的同位素。目前，已知的单中子发射器约有600个，但只有三分之一的实验数据可用。然而，获得更多中子丰富的同位素也意味着多中子发射成为主要的衰变机制。到目前为止，已经发现了大约460种β -延迟的2、3或4中子发射体，但其中只有30种具有中子分支比的实验数据，而且大多数在A=30以下的轻质量区。**国际原子能机构（原子能机构）已经确定了这一紧迫性，并在最近关于“β -延迟中子发射数据参考数据库”的4年“协调研究项目”中选择了这一主题。该项目将审查、编译和评估现有的中子分支比和β -延迟中子发射体半衰期的数据。这将有助于确保为未来新同位素的发现提供可靠的数据库，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提交了一份广泛的提案，用于未来的测量和对现有数据的评估。**这项发现拨款申请旨在利用不同世界领先设施的几个探测器和新技术研究β -延迟中子发射器。**一个方向是使用“标准”方法与3he填充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和最先进的电子设备进行。日本理研所（RIKEN/ Japan）的“BRIKEN运动”计划在2014- 2016年使用世界上最高效的中子探测器装置进行实验。在我的领导下，TRIUMF参与了这项运动，并将提交天体物理关键核素测量的建议。**在BRIKEN活动之后，目前在TRIUMF的ISAC-1设施的现有计划将扩展到更多的富中子光束，预计几年后将与新的ARIEL设施一起使用。我们建议在核结构和r-过程研究中进行高精度的分支比测量。**第二个方向是开发新的技术来测量中子分支比，以绕过对中子的探测。其中一种方法是利用捕集器测量分支比和中子能谱，并利用离子重合技术测量中子能谱。这种方法最近被阿贡/利弗莫尔合作成功采用。这种方法的优点和缺点将在TRIUMF （TITAN和TRINAT）上进行研究。额外安装中子探测器可以将这种方法扩展到多中子发射器。

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.