Elsevier

Physics Letters B

Volume 704, Issue 4, 19 October 2011, Pages 270-275
Physics Letters B

Development of axial asymmetry in the neutron-rich nucleus 110Mo

https://doi.org/10.1016/j.physletb.2011.09.050Get rights and content

Abstract

The neutron-rich nucleus 110Mo has been investigated by means of γ-ray spectroscopy following the β-decay of 110Nb, produced using in-flight fission of a 238U beam at 345 MeV/nucleon at the RIBF facility. In addition to the ground-band members reported previously, spectroscopic information on the low-lying levels of the quasi-γ band built on the second 2+ state at 494 keV has been obtained for the first time. The experimental finding of the second 2+ state being lower than the yrast 4+ level suggests that axially-asymmetric γ softness is substantially enhanced in this nucleus. The experimental results are compared with model calculations based on the general Bohr Hamiltonian method. The systematics of the low-lying levels in even–even A110 nuclei is discussed in comparison with that in the neutron-rich A190 region, by introducing the quantity ES/E(21+), ES=E(22+)E(41+), as a global signature of the structural evolution involving axial asymmetry.

Introduction

One of the long-standing unsolved issues in nuclear structure studies is the rare occurrence of well-deformed oblate (disc-like) shapes in nuclear ground states, in contrast to the large abundance of prolate (cigar-like) deformations [1]. A simple explanation for this empirical fact can be suggested in Ref. [2] based on the properties of quantized orbits in deformed potentials. In actual nuclei, however, it is expected that a subtle interplay between the single-particle and collective degrees of freedom plays a significant role in the shape polarization. Under a certain condition that multiple energy minima coexist at prolate and oblate deformation in the potential energy surface [3], the two different shapes can compete, and presumably interact, leading to the nuclear shape being soft with respect to the γ degree of freedom, where γ represents a deviation from axial symmetry of the ellipsoidal shape; γ=0° and 60° for axially-symmetric, prolate and oblate shapes, respectively, and 30° for a maximally asymmetric nucleus that has three different radii in Cartesian coordinates. The absence of well-deformed oblate ground states in nature may be associated partly with such a transitional character of axially-asymmetric γ-soft nuclei. Hence, it is certainly necessary to explore deformed nuclei at the critical point of the prolate-to-oblate transition, if a proper understanding of the mechanisms underlying the spontaneous symmetry breaking (Jahn–Teller effect) in nuclear-shape deformation is to be reached.

In the present work, we have investigated neutron-rich Z40, A110 nuclei, in which the phase transitions from prolate, via γ-soft, to oblate shapes are predicted to occur with increasing number of neutrons [4]. A similar type of the shape evolution is suggested for Hf, W, and Os isotopes with A190 [5]. These regions are the ones where the Fermi surfaces for protons and neutrons concurrently lie at the upper halves of the respective major shells, and the rotation alignment of both types of nucleons in the high-j orbits is expected to enhance the oblate stability [6], [7]. The recent observation of a possible oblate-shape isomer in 109Nb68 [8] motivates one to probe the N=68 isotone 110Mo68, with the particular aim of studying its γ-soft nature as a fingerprint for the prolate-oblate shape transition.

Section snippets

Experimental procedures

Experiments were carried out at the RIBF facility [9], cooperated by RIKEN Nishina Center and CNS, University of Tokyo. Neutron-rich A110 nuclei were produced via in-flight fission of 238U86+ projectiles at 345 MeV/nucleon, incident on a beryllium target with a thickness of 3 mm. The average beam intensity was approximately 0.3 pnA during the experiment. The nuclei of interest were separated and transported through the BigRIPS spectrometer [10], [11], operated with a 6-mm-thick wedge-shaped

Results

Fig. 1 exhibits the level scheme of 110Mo, established by means of β-delayed γ-ray spectroscopy following the decay of 110Nb. Prior to the present work, the ground-state band in 110Mo has been known up to the 10+ state by measuring the prompt γ rays from the spontaneous fission of 248Cm [12]. In addition to the 214-, 386-, and 532-keV γ rays that belong to the ground-state band, seven new transitions have been unambiguously observed in a singles γ-ray spectrum measured in coincidence with β

Discussion

Fig. 5(a) exhibits the systematics of the low-lying levels in even 42Mo isotopes with neutron numbers ranging from 62 to 68, including the new result obtained for 110Mo68 in the present work. It can be seen that the excitation energies of the 21+ and 41+ states reach a minimum at N=64; an examination of the E(41+)/E(21+) ratio suggests that the maximum quadrupole deformation of the ground state occurs for 106Mo64 [12]. Meanwhile, the 22+ level falls down in energy as the neutron number

Conclusions

The level structure of 110Mo has been investigated following the β-decay of 110Nb, populated via in-flight fission of a 238U beam. In addition to the known levels of the ground-state band, several new levels, including a candidate for the 22+ quasi-γ-band state, have been identified. This is the most neutron-rich Mo isotope for which spectroscopic information on the low-lying level structure has been obtained. The observed level energies and the B(E2) ratio for a pair of γ rays from the 22+

Acknowledgements

We are indebted to the staff members of RIKEN Nishina Center for providing the uranium beams and to the BigRIPS team for tuning the secondary beams. H.W. thanks Prof. I. Hamamoto, Prof. Y. Sun, and Prof. K. Matsuyanagi for valuable discussions. This work was supported by the KAKENHI (Grant Nos. 19340074, 50126124, and 21340073), the RIKEN Presidentʼs Fund (2005), UK STFC and AWE plc., the DFG Cluster of Excellence Origin and Structure of the Universe and under DFG grant KR 2326/2. The numerical

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