Non-yrast spectroscopy and high-spin states of reflection-asymmetric Th222 (travel and subsistence)

非对称光谱和反射非对称 Th222 的高自旋态（旅行和生存）

• 批准号: ST/H003894/1
• 负责人: John Smith
• 金额: $ 1.31万
• 依托单位: University of the West of Scotland
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2009
• 资助国家: 英国
• 起止时间: 2009 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=ST%2FH003894%2F1
• 关键词: Non; yrast; spectroscopy; spin; states

The nucleus at the heart of every atom is made up of two types of particles - neutrons and protons. It is well known that nature tends towards symmetry, and for that reason it may at first be thought that the neutrons and protons should stick together to form a spherical ball of nuclear matter. In many cases this is correct - many atomic nuclei are indeed spherical, but generally it is not true. In general, there is a complex interplay of interactions between the neutrons and protons which conspire to cause the nucleus to take on a non-spherical shape. Deformed shapes of atomic nuclei have been known and studied for many years. Nuclei often take on what is known as a prolate shape, similar to a rugby ball, or an oblate shape, like squashed sphere. In some areas of the nuclear chart, the neutrons and protons interact in such a way as to make the nucleus reflection asymmetric, giving it a pear shape. Such pear-shaped or 'octupole deformed' nuclei occur in different regions across the nuclear chart, but the most reflection-asymmetric shapes are found in the light-actinide nuclei - in the light radium (Z=88), thorium (Z=90), and uranium (Z=92) nuclei. In analogy with reflection-asymmetric molecules such as HCl, reflection-asymmetric nuclei have rotational excitations which form a sequence of positive- and negative-parity states in a rotational-like structure. Furthermore, because the pear-shaped nucleus has a 'pointed 'end, the 'lightening-rod effect' comes into play: the charged particles in the nucleus (protons) tend to gather in the pointed end of the pear shape where the radius of curvature of the equipotential surface is smallest. This causes the centre of charge to be displaced from the centre of mass, giving rise to an intrinsic electric dipole moment, giving rise to strong electric-dipole gamma-ray transitions. These experimental signatures of pear-shaped nuclei are well defined and have been observed in around twenty nuclei, in the light actinides, where octupole correlations are at their strongest. In almost all cases, the spectroscopic knowledge extends to knowledge of yrast states in the ground-state rotational 'octupole' band. There are only very few cases in which non-yrast states have been observed, and in no cases in the even-even isotopes has a second octupole band been observed. It would therefore be very interesting to make a comprehensive non-yrast high-spin nstudy of nuclei in this region. There are several other motivations for studying nuclei in this region: firstly, there are longstanding predictions of highly-elongated ('superdeformed') structures in the light actinides, and secondly, a new interpretation of the structure of reflection-asymmetric nuclei in terms of the condensation of rotationally-aligned octupole phonons has recently been put forward. An UWS-led experiment has been successfully proposed at Argonne National Laboratory in the UWS, to study the high-spin excitations of 222Th, and neighbouring nuclei such as 220Ra and 219Ra. Following the reaction of a 18O beam on a 208Pb target, these nuclei will be formed, and their de-excitation gamma rays will be detected with Gammasphere. However, instead of producing the thorium and radium nuclei to be studied, it is far more likely that the interaction of the O18 beam on the Pb208 target will induce fission accompanied by multiple gamma rays. In order to select the reaction products of interest from the background due to fission, the Washington University high-efficiency recoil detector HERCULES will be used. The results of the experiment will enable the high-spin properties of Th222 and neighbouring nuclei to be studied, which will be of significant interest in physics on a worldwide scale, and will help maintain the UK's longstanding position at the forefront of gamma-ray spectroscopy.

每个原子中心的原子核由两种粒子组成-中子和质子。众所周知，自然界倾向于对称性，因此，人们起初可能认为中子和质子应该粘在一起，形成一个核物质的球形球。在许多情况下，这是正确的-许多原子核确实是球形的，但通常不是真的。一般来说，中子和质子之间存在着复杂的相互作用，这导致原子核呈现出非球形的形状。原子核的变形形状已经被知道和研究了很多年。原子核通常呈现出类似橄榄球的扁长形状，或类似被压扁的球体的扁球形。在核图的某些区域，中子和质子以这样一种方式相互作用，使核反射不对称，使其呈梨形。这种梨形或“八极变形”的核出现在整个核图表的不同区域，但大多数反射不对称的形状出现在轻锕系元素核中-轻镭（Z=88），钍（Z=90）和铀（Z=92）核。与反射非对称分子（如HCl）类似，反射非对称核具有旋转激发，在旋转结构中形成一系列正宇称和负宇称态。此外，由于梨形核有一个“尖”端，“避雷针效应”开始发挥作用：核中的带电粒子（质子）倾向于聚集在梨形核的尖端，那里的等势面曲率半径最小。这导致电荷中心从质量中心移位，产生固有电偶极矩，产生强的电偶极伽马射线跃迁。梨形核的这些实验特征被很好地定义，并且在轻锕系元素中的大约20个核中被观察到，在轻锕系元素中，八极相关性最强。在几乎所有的情况下，光谱知识延伸到知识的yraast国家在基态旋转的“八极”带。只有极少数的情况下，非yraast状态已被观察到，在任何情况下，在偶数-偶数同位素有第二个八极带被观察到。因此，对这一区域的核进行全面的非静态高自旋研究将是非常有趣的。在这一区域研究原子核还有其他几个动机：首先，长期以来人们一直预测轻锕系元素中存在高度拉长（“超变形”）的结构，其次，最近有人提出了一种新的解释，即根据旋转排列的八极声子的凝聚来解释反射非对称原子核的结构。UWS领导的实验已经在UWS的阿贡国家实验室成功地提出，以研究222 Th和邻近核如220 Ra和219 Ra的高自旋激发。在18 O束与208 Pb靶反应后，这些核将形成，它们的去激发γ射线将被γ射线探测器探测到。然而，O 18射束与Pb 208靶的相互作用更有可能引发伴随着多个伽马射线的裂变，而不是产生要研究的钍和镭核。为了从裂变背景中选择感兴趣的反应产物，将使用华盛顿大学的高效反冲探测器HERCULES。该实验的结果将使Th 222和邻近原子核的高自旋特性得到研究，这将在全球范围内引起物理学的重大兴趣，并将有助于保持英国在伽马射线光谱学前沿的长期地位。