Signatures of octupole correlations in neutron-rich odd-mass barium isotopes

Octupole deformation and the relevant spectroscopic properties of neutron-rich odd-mass barium isotopes are investigated in a theoretical framework based on nuclear density functional theory and the particle-core coupling scheme. The interacting-boson Hamiltonian that describes the octupole-deformed even-even core nucleus, as well as the single-particle energies and occupation probabilities of an unpaired nucleon, are completely determined by microscopic axially symmetric $({\beta }_2,{\beta }_3)$-deformation constrained self-consistent mean-field calculations for a specific choice of the energy density functional and pairing interaction. A boson-fermion interaction that involves both quadrupole and octupole degrees of freedom is introduced, and their strength parameters are determined to reproduce selected spectroscopic data for the odd-mass nuclei. The model reproduces recent experimental results for both even-even and odd-mass Ba isotopes. In particular, for ${}^{145,147}\mathrm{Ba}$ our results indicate, in agreement with recent data, that octupole deformation does not determine the structure of the lowest states in the vicinity of the ground state, and only becomes relevant at higher excitation energies.

Figure 1

SCMF (${\beta }_2,{\beta }_3$) deformation energy surfaces for ${}^{142,144,146}\mathrm{Ba}$, obtained with the DD-PC1 nuclear functional [21] and a separable pairing force of finite range [22]. The energy difference between neighboring contours is 200 keV. Equilibrium minima are identified by open circles.

Figure 2

Low-energy positive- and negative-parity spectra of the even-even boson core nuclei ${}^{142,144,146}\mathrm{Ba}$. The $B\left(E2\right)$ (numbers along arrows within each band) and $B\left(E3\right)$ (interband, dashed arrows) values are given in Weisskopf units. Experimental values are taken from Refs. [29] (${}^{142}\mathrm{Ba}$), [5] (${}^{144}\mathrm{Ba}$), and [6] (${}^{146}\mathrm{Ba}$).

Figure 3

Evolution of low-energy negative-parity levels of the odd-mass Ba isotopes as functions of the neutron number. Experimental excitation energies are taken from Refs. [12, 13, 30].

Figure 4

Same as Fig. 3, but for the positive-parity levels.

Figure 5

Detailed comparison of the calculated and experimental low-energy positive- and negative-parity spectra of ${}^{143}\mathrm{Ba}$. Those theoretical energy levels that are made of one $f$-boson configuration are drawn in bold and in color blue. Data are taken from Ref. [13].

Figure 6

Same as Fig. 5, but for the nucleus ${}^{145}\mathrm{Ba}$. Experimental excitation energies are from Ref. [13].

Figure 7

Same as Fig. 5, but for ${}^{147}\mathrm{Ba}$ and with data from Ref. [30].