Lifetime measurements and the structure of some negative-parity states in 134Ce
Introduction
The neutron and proton bands in the 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 (ν, ν, ν) shell closure or proton intruder orbitals (π) from below the gap [2], [3], [4]. For low-medium spin, the proton and neutron quasiparticle energies, as well as the energies needed to break an 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 () the proton excitations might be yrast, whereas in the heavier ones () 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 or neutron quasiparticles [6]. For the lighter of these (), the alignment of protons precedes the alignment of neutrons [7], [8], [9], [10], [11]. As the neutron shell approaches half closure () 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 and neutron 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 and/or π [7], [8], [9], [10], [11], [17], whereas in the heavier ones () these bands/states are considered to be based on the two-quaneutron configurations (ν and/or ν) [18], [19], [20]. For transitional 134Ce nucleus, the and states had been suggested as ν and ν respectively according to systematics investigations [14]. Afterwards, all studies for the , 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 and/or π 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.
Section snippets
Experimental details
The 122Sn(16O,4n)134Ce reaction has been used to populate the excited states of 134Ce at an incident energy of 76 MeV. The 16O beam was delivered by the HI-13 tandem accelerator at the China Institute of Atomic Energy (CIAE). The beam intensity was limited to 2 pnA to avoid thermal stress of the plunger-target device in the process of whole experiment. The target consisted of 800 μg/cm2 122Sn evaporated on a 1.9 mg/cm2 181Ta foil facing the beam. A 9.9 mg/cm2 tantalum foil was used to stop the
Discussions
In previous papers [14], the and levels in 134Ce were considered to be the two-quasiparticle states with the configurations of ν and ν, respectively. In the neighboring isotones, the and states have been observed systematically having the same two neutron configurations. Furthermore, the and states in the light nuclei 126Sn [29] and 128Te [30] arise from the spherical shell-model states ν and ν, respectively. The absolute
Summary
In summary, the lifetimes of the low-lying negative parity yrast states in 134Ce have been measured by utilizing the recoil-distance Doppler-shift technique using the reaction 122Sn(16O, 4n)134Ce at an incident beam energy of 76 MeV. The lifetimes of six negative parity yrast states were derived by an analysis in coincidence mode within the framework of the differential decay-curve method. The small transition strengths at the bottom of the negative-parity yrast states show single
CRediT authorship contribution statement
T.X. Li: Data curation, Visualization, Writing – original draft. C.B. Li: Conceptualization, Writing – review & editing. Y. Zheng: Writing – review & editing. X.G. Wu: Supervision, Writing – review & editing. J. Zhong: Investigation. B.J. Zhu: Software. Q.W. Fan: Resources. Y.X. Gao: Visualization. Y.J. Jin: Software. G.S. Li: Investigation. L.H. Zhu: Validation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are grateful to the HI-13 tandem accelerator staff for providing stable 16O beam throughout the experiment. This work is supported by the National Nature Science Foundation of China under Grants No. 11975315, No. U1867210, U1932209; supported by the Leading Innovation Project under Grant No. LC192209000701, No. LC202309000201; supported by the Continuous Basic Scientific Research Project (Grant No. WDJC-2019-13); supported by China National Nuclear Corporation (Grant No. FA18000201).
References (46)
- et al.
Phys. Lett. B
(1988) - et al.
Nucl. Phys. A
(1996) Nucl. Phys. A
(1989)Nucl. Phys. A
(1997)Nucl. Phys. A
(2000)Nucl. Phys. A
(1996)Phys. Lett. B
(1980)Nucl. Phys. A
(1982)Nucl. Phys. A
(1984)Nucl. Phys. A
(2005)
Nucl. Phys. A
Nucl. Phys. A
Nucl. Phys. A
Nucl. Phys. A
Nucl. Phys. A
Nucl. Phys. A
Nucl. Phys. A
Nucl. Phys. A
Nucl. Phys. A
Phys. Lett. B
Phys. Rev. C
Phys. Rev. C
Z. Phys. A
Cited by (1)
Statistical fluctuations of negative parity levels in even mass nuclei
2021, Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics