γ-softBa146and the role of nonaxial shapes atN≈90

Abstract

Low-spin states in the neutron-rich, N = 90 nuclide 146 Ba were populated following β-decay of 146 Cs, with the goal of clarifying the development of deformation in Ba isotopes through delineation of their non-yrast structures. Fission fragments of 146 Cs were extracted from a 1.7-Ci 252 Cf source and mass-selected using the CARIBU facility. Low-energy ions were deposited at the center of a box of thin β detectors, surrounded by a high-efficiency HPGe array. The new 146 Ba decay scheme now contains 31 excited levels extending up to ∼2.5 MeV excitation energy, double what was previously known. These data are compared to predictions from the Interacting Boson Approximation (IBA) model. It appears that the abrupt shape change found at N = 90 in Sm and Gd is much more gradual in Ba and Ce, due to an enhanced role of the γ degree of freedom.

Low- Z boundary of the N=88 –90 shape phase transition: Ce148 near the critical point

“…The energy of the 2 + γ level peaks in 152 Sm, indicating its highest degree of axial symmetry, also reported in Ref. [47].…”

Evolution of deformation in neutron-rich Ba isotopes up to A=150

“…We do not see high-J states, apart from the 5 − level at 962.3 keV [9], which is populated from high-lying states and not directly fed by β decay. Ground state feeding seems to be very weak in 148 Ba compared to the value reported for 146 Ba (I β = 27%) [14].…”

“…Although other assignments are possible, as indicated in Table I, this state is interpreted as 0 + 2 , given the close resemblance to a similar state found in 146 Ba in Ref. [14]. (ii) The 980.2-keV level decays to 1 − 1 state through the 293.5-keV transition, to the 4 + 1 via the 557-keV one, and to the 2 + 1 via the 839.6-keV γ ray.…”

“…Levels and transitions of 146 Ba are taken from Ref. [14]. in terms of phenomenological quantities such as the ratio of the energies E(4 + 1 )/E(2 + 1 ), which results in a value of 3.1, very close to the axially symmetric rotor limit [30].…”

“…In both cases, the beam was post-accelerated to allow for a safe Coulomb excitation experiment, leading, for the first time, to the measurement of the B(E3; 3 + → 0 + ) transition probabilities [12,13]. The β decay of 146 Cs into 146 Ba was also measured with improved statistics, leading to a great extension of the level scheme up to 2.2 MeV [14].…”