Speaker
Description
For the collaboration:
S. Leoni et al., University of Milano and INFN sez.-Milano, Italy
B. Fornal et al., Institute of Nuclear Physics PAN, Krakow, Poland
N. Marginean et al., IFIN HH, Bucharest, Romania
C. Michelagnoli et al., ILL, Grenoble, France
R. V. F. Janssens et al., University of North Carolina at Chapel Hill, North Carolina, USA
M. Sferrazza, Universitè libre de Bruxelles, Bruxelles, Belgium
J. Wilson et al. , IPN-Orsay, France
T. Otsuka and Y. Tsunoda, University of Tokyo, Tokyo, Japan
We present a recent survey of decay properties of excited 0$^+$ states in regions of the nuclear chart well known for shape coexistence phenomena, focusing, in particular, on even-even nuclei around the Z=20 (Ca), 28 (Ni), 50 (Sn), 82 (Pb) proton shell closures and along the Z=36 (Kr), Z=38 (Sr) and Z=40 (Zr) isotopic chains [1]. The aim is to identify examples of extreme shape coexistence, namely, coexisting deformed and spherical (or close-to-spherical) nuclear states, with wave functions well separated in the Potential Energy Surface (PES) with coordinates in the deformation space. Such a wave function separation may result in a substantially hindered transition between the corresponding structures. This is in analogy to the 0$^+$ fission shape isomers in the actinides region and to the superdeformed (SD) states at the decay-out spin in medium/heavy mass systems. In the survey, the Hindrance Factor (HF) of the E2 transitions de-exciting 0$^+$ states or SD decay-out states is a primary quantity which is used to differentiate between types of shape coexistence.
It is found that a limited number of 0$^+$ excitations (in the Ni, Sr, Zr and Cd regions) exhibit large HF values (>10), few of them being associated with a clear separation of coexisting wave functions, while in most cases the decay is not hindered, due to the mixing between different configurations. A brief discussion will be devoted to the case of the relatively light $^{64,66}$Ni nuclei, where shape-isomer-like structures, of prolate deformed nature, have been observed at spin zero by performing gamma-spectroscopy investigation with different types of reaction mechanisms (i.e., proton and neutron transfer, neutron capture and Coulomb excitation) [2,3]. An analogous situation is expected to occur in $^{112-116}$Sn isotopes, for which preliminary results will be presented from experiments performed at IFIN-HH (Magurele, Romania) with ROSPHERE, and at Legnaro National Laboratory (Padua, Italy) with the AGATA tracking array. The experimental data will be interpreted in the light of state-of-the-art Monte Carlo Shell Model (MCSM) calculations [4], according to which the action of the monopole tensor force plays a relevant role in stabilizing and deepening isolated, deformed local minima in the PES, thus leading to a significant separation of the wave functions of states residing in these minima and, eventually, to shape isomerism.
References
[1] S. Leoni, B. Fornal, A. Bracco, Y. Tsunoda, and T. Otsuka, to be published in Prog. Part. Nuc. Phys.
[2] S. Leoni, B. Fornal, N. Marginean et al., Phys. Rev. Lett. 118, 162502 (2017)
[3] N. Marginean, et al., Phys. Rev. Lett. 125, 102502 (2020)
[4] Y. Tsunoda et al., Phys. Rev. C 89, 031301 (2014)
