Theoretical Study of Phase-Coexistence Phenomena in Rapidly Rotating Nuclei

原子核是一个有限的量子系统，在那里观察到许多现象，这些现象可以通过集体自由度和独立自由度之间的相互作用来理解。特别有趣的现象是最近在高自旋和高激发极端状态下观察到的。本研究利用一个简单的隧穿模型，研究了由核的形状和转动自由度引起的各种“相变”或“相共存“现象：第一种是超形变转动带的衰减。这种现象是极端变形状态向正常变形状态的转变。通过系统的计算，我们发现在超形变带的不同观测区域，势能面有很大的不同。此外，我们还将先前发明的隧穿模型推广到激发态，并通过引入ef，成功地再现了激发态超形变带的数目。 关于我们 第二个研究的是旋转轴倾角自由度的隧穿。已经观察到，高自旋的异构体，这是旋转轴周围的自旋轴，衰变到yraast旋转带，这是旋转轴垂直于自旋轴。这种衰减可以理解为三轴变形度或倾斜角自由度的过渡。我们成功地发展了一种计算与这些自由度有关的势能面的方法，并在同一基础上研究了两个自由度，最后，我们还考虑了集体形状自由度的耦合效应，研究了对研究高自旋高激发极端态很重要的核子有效质量。我们发现，在高自旋极限下，耦合效应大大减小，并引起核能级密度等重要变化。少

Nucleus is a finite quantal system and there observed many phenomena, which can be understand by an interplay between the collective and independent degrees of freedoms. Especially interesting phenomena are those observed recently at high-spin and highly excited extreme states. In this research we have studied various kinds of such phenomena related to the "phase-transitions" or "phase-coexistences"caused by the nuclear-shape and rotation degrees of freedoms by using a simple tunneling model.The first one is the decay-out of superdeformed rotational bands. This phenomenon is a transition of extremely deformed states to normally deformed states. Performing systematic calculations, we have found that the potential energy surfaces are considerably different in various observed regions of superdeformed bands. Furthermore, we have extended the previously invented tunneling model to excited states, and have successfully reproduced the number of excited superdeformed bands by including the ef … More fect ofdecay-out.The second research is the tunneling in the tilting-angle degrees of freedom of rotaitonal axis. It has been observed that the high-spin isomers, which are rotating around the symmetry-axis, decay to the yrast rotational bands, which are rotating around axis perpendicular to the symmetry-axis. Such decays can be understood as transitions either in the triaxial-deformation degrees or in the tilting-angle degrees of freedoms. We have successfully developed a method to calculate the potential energy surface with respect to these degrees of freedoms, and been studying two degrees of freedoms on the same footing.Finally, we have also investigated the nucleon effective mass, which is important to study the high-spin and highly excited extreme states, taking into account of the coupling effects of collective shape degrees of freedoms. We have found that the coupling effects reduces very much at high-spin limits, and it causes an important change to, for example, the nuclear level-density. Less