Collaborative Research: RUI: Study of Exotic Nuclei and Neutron Detector Response

这个实验计划的主要目标是促进对奇异原子核的本质的理解，本科生的参与是一个不可或缺的组成部分。实验将在密歇根州立大学(MSU)的国家超导回旋实验室(NSCL)进行，使用模块中子阵列(MONA)和大面积多机构闪烁体阵列(LISA)，这两个阵列都是由MONA合作的几个机构的本科生建造和测试的。该奖项的PI和他们的本科生团队将研究接近稳定极限的轻中子核的结构，这将为测试当前的核结构理论模型提供数据。随着元素中同位素的核质量电荷比的增加，同位素变得弱结合或非结合，与稳定的同位素相比，它们表现出明显不同的结构。这些不稳定的核是由加速的核束与稳定的目标碰撞而产生的，而准确的计算机模拟蒙娜丽莎塑料闪烁体探测器中的中子散射对于从这些短暂的核的分裂中提取物理结果至关重要。对洛斯阿拉莫斯国家实验室最近进行的中子散射实验的分析将提高这些模拟的准确性，为即将在密歇根州立大学稀有同位素束设施(FRIB)进行的更高能量实验做准备。这项拟议工作的更广泛影响包括为本科生创造研究机会，并在设备开发和领导方面投资于MONA合作。该奖项支持的研究将包括进行两个新实验，分析当前和未来的数据，并为开发下一代快中子探测器做出贡献，用于FRIB的高刚性光谱仪。NSCL的两个最终MONA实验将于2020年进行，以研究丰含中子的原子核中的非束缚态。感兴趣的核是13Be和28F。这些实验将解决13Be的长期存在的能级结构问题，13Be位于核图中以结构变化而闻名的N=8区域，而对非束缚28F态的测量结合先前确定的来自28F的二体矩阵元素将有助于对28O的单中子分离能量施加限制。为了为即将到来的FRIB设施的实验做准备，2019年11月在洛斯阿拉莫斯中子科学中心(LANSCE)进行了一项实验，以测量快中子与塑料闪烁体探测器相互作用期间的中子散射和次级中子产生。这项工作将是对最近PI的中子散射工作的补充，这些工作暴露了当前基于GEANT4的蒙特卡罗模拟程序的一些故障。目标是改进与这些探测器的中子相互作用的建模，这将是提高探测器分辨率和过滤FRIB实验数据的关键。这项工作还服务于更广泛的社区，因为塑料闪烁体被广泛用于核物理和相关应用中的中子探测。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The main goal of this experimental program is to advance understanding of the nature of exotic atomic nuclei, with undergraduate student participation as an integral component. Experiments will be conducted at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU) using the Modular Neutron Array (MoNA) and the Large-area multi-Institutional Scintillator Array (LISA), both constructed and tested by undergraduate students from several institutions in the MoNA collaboration. The PIs of this award and their team of undergraduate students will investigate the structure of light neutron-rich nuclei near the limits of stability, which will provide data to test current theoretical models of nuclear structure. As the nuclear mass-to-charge ratio for isotopes of an element increases, the isotopes become weakly-bound or unbound, and they exhibit significantly different structure compared with their stable counterparts. These unstable nuclei are produced by collisions of an accelerated nuclear beam with a stable target, and accurate computer simulations of neutron scattering in the MoNA-LISA plastic scintillator detectors are critical for extracting physics results from the breakup of these short-lived nuclei. Analysis of recent neutron-scattering experiments conducted at Los Alamos National Laboratory will improve accuracy of these simulations in preparation for higher energy experiments soon to be conducted at the Facility for Rare Isotope Beams (FRIB) at MSU. Broader impacts of the proposed work include formative research opportunities for undergraduate students and investing in the MoNA Collaboration in terms of equipment development and leadership.Research supported by this award will consist of conducting two new experiments, analyzing recent and future data, and contributing to the development of the next generation fast neutron detector for use with the High-Rigidity Spectrometer at FRIB. Two final MoNA experiments at the NSCL will be performed in 2020 to study neutron-unbound states in neutron-rich nuclei. The nuclei of interest are 13Be and 28F. These experiments will resolve the longstanding issue of the level structure of 13Be, which is in the N=8 region of the nuclear chart known for structural changes, and measurements of unbound 28F states combined with previously determined two-body matrix elements from 28F will help place limits on the one-neutron separation energy of 28O. To prepare for experiments at the upcoming FRIB facility, an experiment was performed at the Los Alamos Neutron Science Center (LANSCE) in November 2019 to measure neutron scattering and secondary neutron production during the interaction of fast neutrons with plastic scintillator detectors. This work will add to the recent neutron scattering work from the PIs which exposed some failures of current GEANT4-based Monte Carlo simulation packages. The goal is to improve the modeling of neutron interactions with these detectors, which will be crucial for improving detector resolution and filtering of data for FRIB experiments. The work also serves a broader community since plastic scintillators are widely used for neutron detection in nuclear physics and related applications.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.