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

已经观察到原子核在不同的同位素中呈现出几种不同的形状。特别是，它们有球形、长形（像橄榄球）、扁形（像门把手）和奇异的八极形（像梨）。每一种形式都至少有一个质量分布的对称轴。也就是说，如果原子核绕它的对称轴旋转任何角度，它看起来都是完全一样的。事实上，围绕垂直于这个对称轴的轴旋转原子核，可以研究自旋和能量减少的演化形状。这是通过检查旋转原子核减速时发出的长链伽马射线来完成的。理论计算表明，在没有对称轴的地方可能会观察到一种不寻常的形状。这个原子核的质量沿长、宽、高三个轴分布不均匀。由于这个原因，这些核在形状上被称为三轴或不对称。虽然理论表明这种形状存在于核图的许多区域，但直接的实验证据很少。然而，如果原子核保持三轴形状并快速旋转，则可以观察到具有摆动运动特征的一系列伽马射线衰减。为了描绘一个三轴核的摆动运动，人们可以想象一个旋转的、不对称的陀螺在减速时进动和摆动。事实上，最近发现的一种同位素镥（163Lu）就表现出这种摇摆运动，可能是迄今为止不对称核的最好例子。在邻近的镥核中也发现了其他例子，但在任何其他元素中都没有发现。摇摆运动是局限于这些核，还是在附近地区更广泛地看到这种形状？原子核呈现这种不寻常形状的必要条件到底是什么？这些是本提案试图回答的一些问题。对钽同位素165Ta和167Ta的摆动运动的研究将开始。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，质子的数量不会对摇摆的存在产生太大的影响。相反，理论预测具有大约72个质子和94个中子的原子核是关键因素；然而，除了lutetium（有71个质子）之外，在任何原子核中都没有观测到摆动的证据。165Ta和167Ta分别有73个质子，94和96个中子。因此，它们是可能找到超越镥核的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在加利福尼亚源裂变后加速放射性同位素时，将开始寻找具有大量中子过剩的钌原子核的三轴形状。理论再次表明，在高自旋状态下，这些原子核的形状是不对称的，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。有机会使用世界一流的设施，并为核结构研究的前沿做出贡献，将有望推动这些学生进入科学事业。

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.