Lifetime measurements and the structure of some negative-parity states in ¹³⁴Ce

Abstract

The lifetimes of several negative-parity states in ¹³⁴Ce excited via the reaction ¹²²Sn(¹⁶O, 4n)¹³⁴Ce at an incident beam energy of 76 MeV were measured for the first time by means of the recoil-distance Doppler-shift technique. The small $B(E2)$ transition strengths deduced from the lifetime data at the bottom of the negative-parity yrast states show spherical shape, in qualitative accordance with predictions of the shell model. At higher excitation energy along the negative-parity yrast line, the experimental $B(E2)$ transition strengths indicate an apparent gain of collectivity with increasing spin. The quadrupole deformation derived from the $B(E2)$ values at the bottom of the negative-parity yrast band confirms the predictions of the cranked Nilsson-Strutinsky Bogoliubov calculations. The systematic evolution in structure of the negative yrast states in Ce isotopes and $N=76$ isotones is discussed.

Introduction

The neutron and proton bands in the $A\sim 130$ mass region, where proton and neutron excite within the same major shell, have been subject of a large number of experimental and theoretical studies [1], [2]. For high spin, a complete understanding of competition between triaxial and highly deformed or superdeformed (SD) structures in this region has not yet been achieved, which involve neutron intruder orbitals from above the $N=82$ (ν$i_{13/2}$, ν$f_{7/2}$, ν$h_{9/2}$) shell closure or proton intruder orbitals (π$h_{9/2}$) from below the $Z=50$ gap [2], [3], [4]. For low-medium spin, the proton $h_{11/2}$ and neutron $h_{11/2}$ quasiparticle energies, as well as the energies needed to break an $h_{11/2}$ proton or neutron pair, are quite similar. Therefore, there is a strong competition between proton and neutron excitations [5]. The systematics suggest that in the lighter even-even Ce isotopes ($A\le 132$) the proton excitations might be yrast, whereas in the heavier ones ($A>134$) the neutron excitations dominate at lower energies. These properties can be approached by studying the existence of two positive parity S bands in many of the even-even Ce nuclei, which have been attributed to the alignments of a pair of proton $h_{11/2}$ or neutron $h_{11/2}$ quasiparticles [6]. For the lighter of these ($A\le 132$), the alignment of protons precedes the alignment of neutrons [7], [8], [9], [10], [11]. As the $h_{11/2}$ neutron shell approaches half closure ($N=76$) at ¹³⁴Ce, both observed S-bands have a two-neutron feature [12], [13], [14]. For the heavier ¹³⁶Ce, the neutron alignment is favored over the proton alignment, which has been confirmed by g-factor measurements [15], [16].

In addition, the proton $h_{11/2}$ and neutron $h_{11/2}$ quasiparticle also contribute actively to the formation of the negative-parity yrast bands in Ce isotopes. In the light even-even Ce isotopes, these bands are considered to be based on the two-quasiproton configurations π$h_{11/2}{d}_{5/2}$ and/or π$h_{11/2}{g}_{7/2}$ [7], [8], [9], [10], [11], [17], whereas in the heavier ones ($A>134$) these bands/states are considered to be based on the two-quaneutron configurations (ν$h_{11/2}{d}_{3/2}$ and/or ν$h_{11/2}{s}_{1/2}$) [18], [19], [20]. For transitional ¹³⁴Ce nucleus, the $5^{-}$ and $7^{-}$ states had been suggested as ν$h_{11/2}{s}_{1/2}$ and ν$h_{11/2}{d}_{3/2}$ respectively according to systematics investigations [14]. Afterwards, all studies for the $5^{-}$, $7^{-}$ states and the sequences built on these states of ¹³⁴Ce adopted the neutron configurations assignment [21], [22]. However, due to the position of the proton Fermi level, two proton coupled band based on π$h_{11/2}{d}_{5/2}$ and/or π$h_{11/2}{g}_{7/2}$ configuration is not completely ruled out. In this situation, the comparison of theoretical and experimental reduced transition probabilities of γ-ray transitions depopulating such structures could facilitate the corresponding configuration assignments. Thus, the aim of the present study is to deduce by means of recoil-distance Doppler-shift (RDDS) lifetime measurements these transition strengths in ¹³⁴Ce and to use them for an investigation of the ambiguous structure along the negative-parity yrast band.