Beta-delayed neutron measurements for nuclear astrophysics

在“快速中子俘获过程”(r过程)中，延迟中子发射体在恒星重元素的核合成中扮演着重要的双重角色。一方面，它们导致材料的贝塔衰退回到稳定的迂回。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：前驱物质的半衰期，以及从中子分支比可以推断出中子分离能量以上的β强度。 R-过程反应路径内的很大一部分同位素尚不能通过实验获得，它们位于(实验)“隐蔽土地”中。随着下一代碎片和气溶胶设施目前正在建设中，所有项目的主要动机之一是调查这些非常富含中子的同位素。目前，已知的单中子发射器约有600个，但只有其中三分之一的实验数据可用。然而，获得更多富含中子的同位素也意味着多重中子发射成为主导的衰变机制。到目前为止，已经发现了大约460个贝塔延迟的两个、三个或四个中子发射体，但其中只有30个获得了关于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)已经确定了这一紧迫性，并于最近在一项为期4年的“协调研究项目”中提出了这一主题，该项目的主题是“贝塔延迟中子发射数据参考数据库”。该项目将审查、汇编和评估现有的关于β延迟中子发射体的中子分支率和半衰期的数据。这将有助于为未来新同位素的发现确保一个可靠的数据库，并有助于约束天体物理和理论模型。TRIUMF在这方面发挥了带头作用，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关于未来测量和现有数据评估的广泛提案。 这项探索赠款申请旨在研究在不同的世界领先设施中使用几个探测器和新技术的贝塔延迟中子发射器。 一个方向是使用带有3He的中子探测器的“标准”方法来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 Campaign”计划在2014-16年间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参与了这项运动，并将提交关于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丰中子束，预计在几年后新的Ariel设施将出现这些束。我们建议对核结构和r过程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绕过中子探测的中子分支比。其中一种方法使用陷阱，通过β-反冲离子符合技术测量分枝比和中子能谱。这种方法最近已经成功地被Argonne/Livermore的合作所采用。这种方法的优缺点将在TRIUMF(Titan和TRINAT)的两个陷阱中进行调查。额外安装的中子探测器可以将这种方法扩展到多个中子发射器。

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.