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

每个原子核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，因此，起初人们可能会认为中子和质子应该粘在一起，形成一个由核物质组成的球形。在许多情况下这是正确的——许多原子核确实是球形的，但一般来说这是不正确的。一般来说，中子和质子之间存在着复杂的相互作用，这些相互作用共同使原子核呈现出非球形。原子核的变形形状已经被认识和研究了许多年。原子核通常呈长形，类似于橄榄球，或者呈扁圆形，像被压扁的球体。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”的原子核出现在原子核图的不同区域，但最不对称的形状是在轻锕系原子核中发现的——在轻镭（Z=88）、钍（Z=90）和铀（Z=92）的原子核中。与反射不对称分子（如HCl）类似，反射不对称核具有旋转激发，在类似旋转的结构中形成一系列正负宇称态。此外，由于梨形原子核有一个“尖”端，“避雷针效应”开始发挥作用：原子核中的带电粒子（质子）倾向于聚集在梨形的尖端，那里等势面曲率半径最小。这导致电荷中心从质量中心位移，产生本征电偶极矩，产生强电偶极伽玛射线跃迁。这些梨形原子核的实验特征被很好地定义，并且在大约20个原子核中被观察到，在轻锕系元素中，八极相关是最强的。在几乎所有情况下，光谱学知识扩展到基态旋转“八极子”波段的yast态知识。只在极少数情况下观察到非晶格态，在任何情况下，偶偶同位素都没有观察到第二个八极带。因此，对这一区域的核进行全面的非介子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，人们对轻锕系元素中高度拉长（“超变形”）结构的长期预测；其次，最近提出了一种关于反射不对称原子核结构的新解释，即旋转排列的八极声子的凝聚。UWS的Argonne国家实验室已经成功地提出了一个UWS主导的实验，以研究222Th以及邻近的原子核如220Ra和219Ra的高自旋激发。18O束流在208Pb靶上反应后，将形成这些核，用伽马球探测它们的去激发伽马射线。然而，O18束流在Pb208靶上的相互作用更有可能引起裂变，并伴随着多重伽马射线，而不是产生待研究的钍核和镭核。为了从背景中选择出由于裂变而产生的感兴趣的反应产物，将使用华盛顿大学的高效反冲探测器HERCULES。实验结果将使Th222和邻近原子核的高自旋特性得以研究，这将在世界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领域的长期领先地位。

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.