Theoretical Study of Phase-Coexistence Phenomena in Rapidly Rotating Nuclei

原子核是一个有限的量子系统，观察到了许多现象，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态上观察到的那些现象。在这项研究中，我们用一个简单的隧道模型研究了与自由的核形和转动自由度引起的“相变”或“相共存”有关的各种现象。第一个现象是超形变转动带的衰减。这种现象是极端形变状态向正常形变状态的转变。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道模型推广到激发态，并通过引入EF…，成功地再现了激发超形变带的数目研究二是隧道在旋转轴倾角自由度上的隧道效应。观察到绕对称轴旋转的高自旋异构体衰变为沿垂直于对称轴旋转的yrast旋转带。这种衰变可以理解为三轴变形度或倾角自由度的转变。我们已经成功地开发了一种计算这些自由度的势能面的方法，并在相同的基础上研究了两个自由度的势能面。最后，我们还研究了核子的有效质量，这对于研究高自旋和高激发的极态是重要的，考虑了集体形状自由度的耦合效应。我们发现，在高自旋极限下，耦合效应大大减弱，并引起了例如核能级密度的重要变化。较少

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