T. Otsuka,1,2,3, Y. Tsunoda,4,5 N. Shimizu,4,5 Y. Utsuno,6,4 T. Abe,2 and H. Ueno2
1 Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
2 RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
3 KU Leuven, Instituut voor Kern- en Stralingsfysica, 3000 Leuven, Belgium
4 Center for Nuclear Study, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
5 Center for Computational Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki, 305-8577, Japan
6Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
The shapes of heavy deformed nuclei have been believed, for seven decades, to be axially-symmetric as Bohr and Mottelson suggested from early days of nuclear physics1), providing textbook items. Very recent studies, however, demonstrate that this view may not be true, and that the nuclear shapes, if deformed, are triaxial more or less, not only in so-called gamma-soft nuclei but also in axially-symmetric deformed nuclei in the traditional scope. This feature is new, and was not reported even in our earlier work along the same line 2). Namely, recent advances have gone much beyond the outcome of the initial work 2). In fact, recent advanced studies with the state-of-the-art CI calculations by the QVSM (advanced version of MCSM) indicates that even a conventionally typical axially-symmetric prolate ground state of 154Sm is shown to be triaxial with gamma equal to 3.3 degrees. Such CI calculations successfully describe the structure of a number of heavy nuclei, with more developed triaxiality in other nuclei, for instance, 166Er. The driving forces for such triaxiality are two-fold from nuclear forces, making the triaxiality quite robust. One of them is the same monopole interaction producing the shell evolution in exotic nuclei, and is responsible for triaxiality with gamma equal to 7-10 degrees. The other is more universal, and makes the triaxial shape as equilibrium for virtually all nuclei, while the degree of triaxiality appears to be 3-5 degrees (but not zero). Gamma-soft nuclei, characterized by gamma being nearly 30 degrees, exist besides these nuclei. Because of the triaxiality, the so-called gamma band is shown not to be a gamma-phonon excitation a la Aage Bohr, but to be a K^P=2^+ band within the same triaxial members as the K^P=0^+ ground-band members. It is then of extreme interest to look for experimental approaches for possible verifications of this new picture. We will present some possible reactions from Relativistic Heavy-Ion Collisions (in LHC and RHIC), to multiple Coulomb excitations as a more conventional but powerful methodology. Besides them, direct excitations of the gamma band in nuclei, including those with small gamma angles like 154Sm, are of new interest. In such cases, the gamma band always exists, but may appear at higher excitation energy (> 2 MeV) with weaker but notable E2 strength, for smaller values of gamma. Thus, the search for possible signatures of a variety of the triaxiality will be discussed in this talk, as well as the new view of deformed shape itself. We will also ask the audience to think over possible other experimental approaches to such new perspectives of nuclear shapes and excitations.
References
1) Bohr, A. & Mottelson, B.R., Nuclear Structure (Benjamin, New York, 1975), Vol. II.
2) Otsuka, T., Tsunoda, Y., Abe, T., Shimizu, N., Van Duppen, P., Underlying Structure of Collective Bands and Self-Organization in Quantum Systems, Phys. Rev. Lett. 123, 222502 (2019).