Transition from the seniority regime to collective motion

Abstract

BsE2d values from isomeric states near closed shells are discussed in the context of the behavior of seniority conserving transitions induced by even tensor operators. This result provides a signature for the transition from the seniority regime to collective motion that can be of use in identifying shell structure in exotic nuclei. One of the challenges of nuclear physics is to understand nuclear phenomena in terms of the underlying microscopic and collective structure. In this paper, we propose a signature of structure that can identify not only the proximity of a nucleus to closed shells, but can also give information on the underlying microscopic structure and the purity of shell model configurations in the yrast levels. The signature is based on aspects of the seniority scheme and of collective behavior that have long been known but, to our knowledge, have not been put together in this way. It provides a map of the structural transition from a noncollective seniority regime to collective motion and should be of greatest use in exotic nuclei where it is widely believed that traditional shell structure, magic numbers, and energy gaps may be different from those characteristic of nuclei near stability. The scope of this paper is to discuss this new approach, assess existing data relevant to it, and discuss new regions, mostly in exotic nuclei, where it may be useful. Transition probabilities, especially BsE2d values, give particularly valuable insights into the nature of nuclear collectivity and its evolution with N and Z. In almost all eveneven nuclei, BsE2d values connecting the lowest yrast states behave in a simple and systematic way, increasing both with increasing valence nucleon number and spin, as illustrated in Fig. 1(a). The BsE2d values exhibit a parabolic trend across a major shell, and higher spin BsE2d values scale with BsE 2;2 1 ! 0 1 d. For example, BsE 2;4 1 ! 2 1 d values can be approximated by BsE 2;4 1 ! 21 d< 1.5 BsE 2;2 1 ! 01 d, [see