RUI: Stable Triaxial Deformation in A~165 and 110 Nuclei

RUI：A~165 和 110 核的稳定三轴变形

• 批准号: 0554762
• 负责人: Daryl Hartley
• 金额: $ 2.15万
• 依托单位: United States Naval Academy
• 依托单位国家: 美国
• 项目类别: Interagency Agreement
• 财政年份: 2006
• 资助国家: 美国
• 起止时间: 2006-06-01 至 2009-11-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0554762&HistoricalAwards=false
• 关键词: RUI; Stable; Triaxial; Deformation; 165

The atomic nucleus has been observed to exhibit several different shapes in different isotopes. In particular, they are known to have spherical, prolate (like a rugby ball), oblate (like a doorknob), and exotic octupole (like a pear) shapes. Each of these forms has at least one axis of symmetry for its mass distribution. That is, if the nucleus is rotated about its symmetry axis through any angle, it looks exactly the same. In fact, rotating nuclei about an axis perpendicular to this symmetry axis allows for the study of evolving shapes with decreasing spin and energy. This is done through the examination of long chains of gamma rays emitted by the rotating nuclei as they slow down. Theoretical calculations suggest that an unusual shape may be observed where there is no such axis of symmetry. The mass of this nucleus is distributed unequally along the three axes of length, width, and height. For this reason, these nuclei are said to be triaxial or asymmetric in shape. Although theory suggests this shape exists in many regions of the nuclear chart, direct experimental evidence of its existence is scarce. However, if a nucleus retains a triaxial shape and is rotated rapidly, a sequence of gamma-ray decays may be observed that is characteristic of a wobbling motion. One may envision the motion of a spinning, asymmetric top that precesses and wobbles as it slows down in order to picture the wobbling motion of a triaxial nucleus. Indeed, an isotope of lutetium (163Lu) was recently discovered to exhibit this wobbling motion and is perhaps the best example of an asymmetric nucleus to date. Other examples have been found in neighboring lutetium nuclei, but none have been observed in any other element. Is the wobbling motion confined to these nuclei, or is this shape more widely seen in the nearby region? What exactly are the necessary conditions for a nucleus to exhibit this unusual shape? These are some of the questions this proposal will try to answer. A search will begin for wobbling motion in the tantalum isotopes 165Ta and 167Ta. These nuclei have two more protons then the lutetium nuclei, which show evidence for triaxial shapes. Current theoretical investigations suggest that the number of protons should not greatly affect the presence of wobbling. Instead, theory predicts that nuclei having approximately 72 protons and 94 neutrons are the key factor; however, no evidence of wobbling is observed in any nuclei other than in lutetium (with 71 protons). The 165Ta and 167Ta have 73 protons as well as 94 and 96 neutrons, respectively. Therefore, they are prime candidates for possibly finding evidence of wobbling beyond the lutetium nuclei. These nuclei will be created in reactions that will leave them in very high-spin states and the gamma rays emitted will be detected with large arrays of gamma-ray detectors at Argonne National Laboratory and Yale University. In addition, a search for triaxial shapes in ruthenium nuclei with large neutron excess will begin when Argonne National Laboratory begins accelerating radioactive isotopes following the fission of a californium source. Once again, theory suggests asymmetric shapes for these nuclei at high spin and experiments will be performed to attempt to find conclusive evidence for this unusual shape. Undergraduate students from the US Naval Academy will be intimately involved with each project as they will participate in the experiments, analyze the data, and present results at various conferences. The opportunity to use world-class facilities and contribute to the frontiers of nuclear structure research will hopefully propel these students into scientific careers.

原子核被观察到在不同的同位素中呈现出几种不同的形状。 特别是，它们已知有球形，扁长形（像橄榄球），扁圆形（像门把手）和奇异的八极（像梨）形状。 这些形式中的每一种都至少有一个质量分布的对称轴。 也就是说，如果原子核绕其对称轴旋转任何角度，它看起来都是一样的。 事实上，围绕垂直于该对称轴的轴旋转原子核允许研究随着自旋和能量的减少而演变的形状。 这是通过检查旋转的原子核在减速时发出的长伽马射线链来完成的。 理论计算表明，在没有这种对称轴的地方可能会观察到一种不寻常的形状。 这个核的质量沿长、宽、高三个轴沿着不均匀地分布。 由于这个原因，这些原子核被认为是三轴或不对称的形状。 虽然理论表明这种形状存在于核图的许多区域，但它存在的直接实验证据很少。 然而，如果原子核保持三轴形状并快速旋转，则可以观察到一系列伽马射线衰变，这是摆动运动的特征。 人们可以想象一个旋转的、不对称的陀螺的运动，它在减速时会进动和摆动，以便描绘一个三轴原子核的摆动运动。 事实上，最近发现的一种镥的同位素（163 Lu）表现出这种摆动运动，可能是迄今为止不对称核的最好例子。其他的例子在邻近的镥核中也被发现，但在其他任何元素中都没有观察到。 这种摆动运动是局限于这些原子核，还是这种形状在附近区域更常见？ 原子核呈现这种不寻常形状的必要条件究竟是什么？ 这些是本提案将试图回答的一些问题。将开始对钽同位素165 Ta和167 Ta中的摆动运动进行搜索。 这些原子核比镥原子核多两个质子，显示出三轴形状的证据。 目前的理论研究表明，质子的数量不应该对摆动的存在有很大的影响。 相反，理论预测大约有72个质子和94个中子的原子核是关键因素;然而，除了镥（有71个质子）之外，在任何原子核中都没有观察到摆动的证据。 165 Ta和167 Ta分别有73个质子、94个中子和96个中子。 因此，它们是可能发现镥原子核外摆动证据的主要候选者。 这些原子核将在反应中产生，使它们处于非常高的自旋状态，发射的伽马射线将被阿贡国家实验室和耶鲁大学的大型伽马射线探测器阵列探测到。 此外，当阿贡国家实验室开始加速一个加利福尼亚放射源裂变后的放射性同位素时，将开始在具有大中子过剩的钌核中寻找三轴形状。 再一次，理论表明这些原子核在高自旋下的形状是不对称的，我们将进行实验，试图找到这种不寻常形状的确凿证据。 来自美国海军学院的本科生将密切参与每个项目，因为他们将参与实验，分析数据，并在各种会议上展示结果。 使用世界一流的设施，并有助于核结构研究的前沿的机会将有望推动这些学生进入科学生涯。