Lifetime measurements and shape coexistence in Sr97

Abstract

Delayed γ rays from neutron-rich A = 97 fission fragments were measured using the Lohengrin spectrometer at the high-flux reactor of the Institut Laue-Langevin in Grenoble. Several lifetimes of excited states in 97 Sr were measured using the fast-timing technique. The nucleus 97 Sr exhibits shape coexistence and is located exactly at the border of the spherical (N 58) and deformed (N 60) ground-state deformation. It is of particular interest to study the shape-coexisting structures at the spherical-deformed border (N = 59). The determined lifetimes within this work are compared to an interacting boson-fermion model calculation that is based on the microscopic energy density functional to provide a better understanding of the spherical-deformed border in strontium isotopes.

An experimental view on shape coexistence in nuclei

“…The low-lying level structure of 97 Sr has a strong similarity to that of 99 Zr, possessing a I π = 1/2 + ground state, with a 3/2 + state at 167 keV and a 7/2 + state at 308 keV that are analogous to the spherical states in 99 Zr, as well as the 3/2 + [411], 3/2 − [541], and 9/2 + [404] configurations with their associated rotational bands. Lifetime measurements [314,315] for the spherical states and some rotational band members in 97 Sr and 99 Zr reinforced the previous determinations of the transitional quadrupole moments and are consistent with the assumed shapes. The 1/2 + , 3/2 + and 7/2 + states in 97 Sr were populated in the (d, p) reaction [278] with strengths considerably lower than expected for spherical shell model states, but this was attributed to the complicated 96 Sr ground-state wave function as mentioned above.…”

Boulay et al. Reply:

“…The 1/2 + 1 , 3/2 + 1 states of 99 Zr have similar B values and magnetic moments with those of 97 Zr and 97 Sr [2,5,6]. Based on this similarity and on the observation of the 1/2 + 1 state in the 96 Zr (d, p) 97 Zr study [7], it is claimed [1] that this state should be predominantly νs 1/2 , as provided by the IBFM-1 calculations of [3,4].…”

“…of these states in 95 Sr and 97 Sr. As the Sr and Zr isotones in this region are very similar, one may expect that these states of 99 Zr are more weakly populated in the (d, p) reaction than their analogs in 97 Zr. There are also recent IBFM-2 calculations [5,6] where the 1/2 + g.s. of 99,97 Zr and 97 Sr has νs 1/2 components of 21%, 16%, and 12%, respectively, compared to 1.5% in [2,9].…”

“…In our work this was achieved by empirically studying the influence of the various parameters. In the IBFM-2 calculations [5,6], the IBM-2 core Hamiltonian and the SPE were determined by microscopic calculations, leaving adjustable only the boson-fermion strengths. In the older calculations [3,4] the choice of the SPE and boson-fermion interaction parameters was made to reproduce the properties of the lowest three states of 97 Sr and 99 Zr, and was guided by zeroth order shell-model expectations for the quasiparticle structure of their w.f.…”

“…Overall, the relatively large number of model parameters was tuned to a rather small set of available experimental data. The calculations from [2,9] and [5,6] were tuned to a much larger set of experimental data [2,9], validating, within certain limits, the w.f. of the IBFM Hamiltonian.…”