Low-spin particle-core and hole-core excitations in Ca41,47,49 isotopes studied by cold-neutron-capture reactions

Abstract

We present recent results on the structure of the one-valence-particle 41 Ca and 49 Ca nuclei and the one-valencehole 47 Ca nucleus. The isotopes of interest were populated via the cold-neutron-capture reactions 40 Ca(n, γ ), 48 Ca(n, γ ), and 46 Ca(n, γ ), respectively. The experiments were performed at the Institut Laue-Langevin, within the EXILL campaign, which employed a large array of high-purity germanium (HPGe) detectors. The γ decay and level schemes of these nuclei were investigated by γ -ray coincidence relationships, leading to the identification of 41, 10, and 6 new transitions in 41 Ca, 47 Ca, and 49 Ca, respectively. Branching ratios and intensities were extracted for the γ decay from each state, and γ -ray angular correlations were performed to establish a number of transition multipolarities and mixing ratios, thus helping in the spin assignment of the states. The experimental findings are discussed along with microscopic, self-consistent beyond-mean-field calculations performed with the hybrid configuration mixing model, based on a Skyrme SkX Hamiltonian. The latter suggests that a fraction of the low-spin states of the 41 Ca, 49 Ca, and 47 Ca nuclei is characterized by the coexistence of either 2p-1h (two-particle-one-hole) and 1p-2h excitations, or couplings between single-particle or single-hole degrees of freedom and collective vibrations (phonons) of the doubly-magic "core."

Gamma-ray spectroscopy of fission fragments with state-of-the-art techniques

“…In such a heavy-mass region, phonons cannot be fully treated by the shell model, as it would require full calculations in a configuration space that encompasses proton and neutron orbitals below and above 132 Sn. For this purpose, a new microscopic and self-consistent model (named "Hybrid Configuration Mixing" (HCM)) has been developed, with the aim of describing states with different degrees of collectivity [55,128,129]. The HCM model has no free parameters and is self-consistent in the sense that both single-particle states and phonons come out of Hartree-Fock (HF) and Random Phase Approximation (RPA) calculations performed with the Skyrme SkX interaction [130].…”

“…3 and 4). For example, studies performed with large HPGe arrays such as RISING [273] at the fragment separator at GSI, and EURICA [274] at the BigRIPS in-flight separator at RIKEN, pushed the investigation to the south of 132 Sn, by reaching high-spin isomers in 131 In [58], 134 In [275], 128 Cd [276,277], 129 Cd [278], and 130 Cd [76]. Similarly, studies were carried out to the east of 132 Sn, towards very neutron rich systems of 136,138 Sn [279], 136 Sb [280] (also observed at LOHENGRIN [89]), and 140 Sb [281] (see Fig.…”

“…6). In the case of the 6 + seniority isomers in 134,136,138 Sn, and for the 8 + isomers in 128 Pd and 130 Cd, the isomeric configuration is based on pairs of neutrons in the f 7/2 or on maximally aligned pair of proton holes in the g 9/2 orbit, respectively (see also Fig. 8).…”

“…At RIKEN, extensive searches of microsecond isomers were also performed in the low-mass fission peak: the γ decays of more than 50 microsecond isomers with half-lives of ∼0.1-10 μs were observed in very neutron-rich isotopes from 59 Ti to 126 Ag [79], using at first a limited γ array. Further detailed investigations, with the EURICA setup, provided additional information in the neighborhood of 78 Ni (e.g., on 76 Co [282]), and on nuclei in the upper edge of the low-A mass peak, such as 125,126,127,128 Pd (with three to zero neutron holes relative to the N = 82 shell closure, respectively), which are also relevant for the r-process nucleosynthesis [283][284][285].…”

Core-proton coupled nature of the 11/2+ state in 131 Sb probed by lifetime measurements

“…Particle-and hole-core coupled states around closed shells are of particular interest for nuclear structure, as reported, for example, in Refs. [2] and [3], for Ca isotopes and the 133 Sb nucleus, respectively. In the socalled weak coupling limit, the extra particle does not perturb the core and the total quadrupole strength of the multiplet obtained from the 2 + 1 state is conserved [4].…”