基于超越平均场方法对开壳原子核中反常B4/2比现象的研究

The B4/2 ratio (B(E2, 4+ yrast to 2+ yrast)/B(E2, 2+ yrast to g.s.)) is an important indicator of the structure and the basic excitation modes of nuclei. The range of the B4/2 ratio between the vibrational and rotational limits, given by the collective model of Bohr, covers the data observed in most open-shell nuclei. In recent years however, B4/2 values outside the range are discovered in a few open-shell nuclei, which are called ‘anomalous B4/2 ratios’. They are expected to relate with both collective and single-particle motions. The aim of this program is to find an explanation for the anomalous B4/2 ratios based on the coexistence or the interaction between the collective and single-particle modes. As a generalization of the generating coordinate method, beyond-mean-field calculations are to be carried out in a model space covering the collective mode and the single-particle mode simultaneously. The possible coexistence or interaction between the two modes in the low-lying states of the region with anomalous B4/2 ratios, and consequently, the possibility to understand the anomalous phenomena from this point of view, will be explored. The program is the first beyond-mean-field study attempting to treat the different excitation modes related to anomalous B4/2 ratios on the same foot, and, at the same time, the first systematic study on the anomalous B4/2 ratios observed so far. The outcome of this study is likely to shed light on the anomalous B4/2 ratios that are not understood theoretically up to now.

B4/2比（B(E2,4+晕态至2+晕态)/B(E2,2+晕态至基态)）是原子核结构及基本激发模式的重要表征。Bohr模型曾给出振动与转动极限间的B4/2比范围，能解释绝大多数开壳原子核的实验观测值。近来在少数开壳原子核中观测到上述范围外的B4/2比（反常B4/2比），其产生可能与集体激发及单粒子激发有关。本项目试图从两种激发模式的共存或相互作用的角度解释反常B4/2比的成因。作为对生成坐标方法的推广，本项目拟在同时考虑集体激发模式与单粒子激发模式的组态空间下进行超越平均场计算，以了解两种激发模式在反常B4/2比存在区域的低激发态中发生共存或相互作用的可能及其对B4/2比的影响，探索由这一角度解释反常B4/2比的可行性。本项目是在超越平均场框架下将可能与反常B4/2比相关的不同激发模式进行统一处理的首次尝试，亦是首个针对反常B4/2比的系统研究，有望为反常B4/2比的理论解释提供线索。

本项目基于推广的生成坐标方法，在同时考虑集体激发模式与单粒子激发模式的组态空间下，对形变原子核中集体运动与单粒子运动的相互影响进行了研究。研究工作主要涉及与形变及转动相关的集体激发模式，包括形状共存，轴对称核的集体转动，三轴原子核的奇特转动，以及原子核的剪刀模式。研究发现，这些集体模式与单粒子运动之间的相互影响在本质上往往可以归结为转动顺排效应。对于存在长-扁椭形状共存的轴对称核，扁椭准粒子的转动顺排可以引起晕线中由长椭到扁椭的形状跃变，并由此产生剧烈的回弯。对三轴原子核，转动顺排所带来的组态改变将会动摇基于特定组态的奇特转动模式存在的基础，从而限制它们的存在范围。此外，研究还发现同位旋矢量转动激发——剪刀模式同样也会与单粒子激发产生混合，并由此解释了近年来在剪刀模式转动谱中观测到的显著的旋称劈裂现象。剪刀模式与单粒子激发之间的混合是否同样源于转动顺排机制，目前仍有待进一步研究。本项目的研究工作揭示了单粒子运动在一系列近年来受到广泛关注的集体激发模式中所扮演的重要角色，深化了人们对原子核集体激发的理解。