Single-particle structure in neutron-rich isotopes

理解原子核结构的一个基本方法是考虑其组成粒子质子和中子在它们之间的相互作用的影响下的运动。这些所谓的从头计算是困难的，但最轻的核，由于增加复杂性的数量越来越大的成分。考虑单个粒子在所有其他粒子产生的平均场中的运动的方法在近稳定核中取得了一些成功，当校正或剩余相互作用被包括在内时。这样的成功实际上是相当有限的，因为稳定系统的数量与束缚同位素的总数相比是非常小的，但尽管如此，这样的方法经常出现，甚至在学生的教科书中，给人一种持久性和坚实性的空气理论。然而，随着实验的日益复杂，新的影响已经被发现，只有在研究单粒子结构在很宽的范围内的中子过剩变得明显。在这些模型的背景下，单粒子结构的这种变化是令人惊讶的;例如，在某些情况下，它们甚至扰乱了为稳定系统所导出的“众所周知”的幻数序列。但它们也有更深层次的后果，因为核的潜在单粒子性质会显著影响其他性质，如核的形状和其他激发模式的存在和类型，如整个核的集体振动和旋转。这项资助计划旨在研究单粒子结构的两个方面。首先，调查可能导致正在发现的变化的机制。这些可以直接与两个核子之间的力联系起来。有人认为，这些变化中的一些是由于这种力的张量分量;如果得到证实，这是特别有趣的，因为这种分量在核结构中只有少数直接表现。其次，该提议旨在探测一个奇异核区域，在该区域中，基于从稳定系统粗略推断的单粒子结构的计算预测了一些有趣的新现象。这种计算是基于从稳定性推断出的单粒子水平，考虑到最近发现的壳结构的巨大变化，这需要受到质疑。该计划是利用A~100区域的转移来测量单粒子轨道，以限制该区域的计算，并使外推到更奇异的系统更加可靠。这些实验将使用涉及放射性束引发的碰撞中单个粒子转移的反应。这些材料将由阿贡国家实验室的CARIBU设施提供，该实验室使用一种基本上不受化学效应影响的方法对裂变碎片进行同位素选择，并将其加速到所需的能量。化学独立性很重要，因为它允许生产耐火元素束。这是研究A~100，Sr-Zr-Mo区所必需的独特特征。实验还将使用另一种独特的设备，新型HELIOS光谱仪。这是谱仪设计中的一个新概念，它使用超导螺线管来分析由重束在轻靶上引发的转移反应中的喷射离子。与传统方法相比，它在接受度、能量分辨率和易于识别粒子方面具有相当大的优势。该提案要求拨款建造该光谱仪的一个重要部分，一个检测重反冲离子的设备。

A fundamental way of understanding the structure of the atomic nucleus is to consider the motion of its constituent particles, protons and neutrons, under the influence of the individual interactions between them. These so-called ab-initio calculations are difficult in all but the lightest nuclei due to increasing complexity as the number of constituents gets larger. Approaches that consider the motion of individual particles in the average field generated by all the other particles have enjoyed some success in near stable nuclei, when corrections or residual interactions are included. Such success is actually rather limited as the number of stable systems is very small in comparison with the total number of bound isotopes, but despite this such methods appear frequently even in student textbooks, giving an air of permanence and solidity to the theories. However, with increasing experimental sophistication, new effects have been found that only become apparent when studying single-particle structures over a wide range of neutron excess. Such changes in single-particle structures are surprising within the context of these models; for example, in some cases they disturb even the 'well-known' sequence of magic numbers dervived for stable systems. But they also have deeper consequences since the underlying single-particle nature of a nucleus dramatically effects other properties such as the nuclear shape and the existence and type of other excitation modes like collective vibrations and rotations of the whole nucleus. This grant proposal aims to study two aspects of single-particle structure. Firstly to investigate the mechanisms which may be responsible for the changes that are being uncovered. These can be related directly back to the force between two nucleons. It has been suggested that some of these changes are due to the tensor component of this force; this is particularly interesting if substantiated as there are only a few direct manifestations of this component in nuclear structure. Secondly the proposal aims probe a region of exotic nuclei where calculations based on the single-particle structure extrapolated rather crudely from stable systems predict some interesting new phenomena. Such calculations are based on single-particle levels extrapolated from stability, which need to be questioned given the dramatic changes in shell structure which are being uncovered recently. The plan is to measure single-particle orbitals using transfer in the A~100 region to tie down calculations in that region and to make extrapolations to the more exotic systems more reliable. The experiments will use reactions involving the transfer of single particles in collisions initiated with radioactive beams. These will be supplied by the CARIBU facility at Argonne National Laboratory which performs isotopic selection of fission fragments using a method largely independent of chemical effects and accelerates them to the required energies. The chemical independency is important as it allows beams of refractory elements to be produced. This is a unique feature which is essential for studying the A~100, Sr-Zr-Mo region. The experiments will also employ another unique device, the novel HELIOS spectrometer. This is a new concept in spectrometer design which uses a superconducting solenoid to analyse ejectile ions from transfer reactions initiated with heavy beams on light targets. It has considerable advantages over traditional methods in terms of acceptance, energy resolution and ease of particle identification. The proposal requests funds to build an essential part of this spectrometer, a device to detect the heavy recoiling ions.