Evidence for shape coexistence in98Mo

Abstract

A γγ angular correlation experiment has been performed to investigate the low-energy states of the nucleus 98 Mo. The new data, including spin assignments, multipole mixing ratios and lifetimes reveal evidence for shape coexistence and mixing in 98 Mo, arising from a proton intruder configuration. This result is reproduced by a theoretical calculation within the proton-neutron interacting boson model with configuration mixing, based on microscopic energy density functional theory. The microscopic calculation indicates the importance of the proton particle-hole excitation across the Z = 40 sub-shell closure and the subsequent mixing between spherical vibrational and the γ-soft equilibrium shapes in 98 Mo.PACS numbers: 21.60. Jz, 21.60.Fw, 23.20.En, 27.80.+w For decades, to clarify the nature of shape coexistence has been one of the major objectives in nuclear structure physics [1,2]. The phenomenon has been observed in various regions of the nuclear chart, from light The A ∼ 100 mass region also presents a unique laboratory for the evolution of nuclear shape and shape coexistence [6,7]. The interplay between single-particle and collective degrees of freedom leads to shape phase transitions along isotopic and isotonic chains [8]. The most dramatic examples for shape coexistence and shape transition occur in the Zr isotopic chain, as recently revealed for 94 Zr [9]. Especially in the N = 50−56 Zr isotopes the 0 + 1 state and the very low-lying 0 + 2 state are considered strongly mixed 0p-0h and 2p-2h proton configurations, where protons are promoted from the pf shell to the g 9/2 orbital, as also found in shell model calculations [6,10]. The structure of the low-lying 0 + 2 state in N ≥ 58 Zr isotopes is somewhat more complicated due to neutron contributions. In Mo isotopes, starting from N = 50 the nuclear shape gradually evolves from a sphere and, driven by the enhanced proton-neutron residual interac- * tim.thomas@ikp.uni-koeln.de tion, large deformation sets in at N ≈ 60 [11]. Situated in between, 98 42 Mo 56 is pivotal for understanding shape transitions in this mass region. In particular, the concept of shape coexistence can apply to this nucleus, where proton cross-shell excitations from the Z = 28 − 40 pf shell to the πg 9/2 orbit may play an important role [12]. In fact, experimentally, the first excited state of 98 Mo has been shown to be an coexisting isomeric 0 + state of different shape [13,14]. The mixing between the proton 2p-0h and 4p-2h configurations forms the first excited 0 + state and the ground state as revealed by the investigation of γ transitions depopulating 1 + states with equal strengths to both 0 + states [12], akin to the findings for 92 Zr [6].To address the important issue of the nature of lowlying structure in 98 Mo, we performed a γγ angular correlation experiment. In this paper, the results of this experiment are reported as well as the identification of shape coexistence and the role of a proton intruder configuration in 98 Mo. events were collected using a γγ coincidence tri...

Structural evolution inA≈100nuclei within the mapped interacting boson model based on the Gogny energy density functional

“…Note that, at variance with previous results [5], our calculations on the deformation energy surfaces with the Gogny-D1M EDF (see, Fig. 1) predict a single minimum around β ≈ 0.15 (see, Fig.…”

“…1, the energy surface corresponding to 96 Mo displays a nearly spherical minimum while for increasing neutron number the surfaces become γ-soft up to N =62. Previous Skyrme Hartree-Fock plus BCS calculations [5], based on the SLy6 parameter set [61], predicted two minima, one nearly spherical and the other triaxial, for 98 Mo. On the other hand, in our Gogny-D1M SCMF calculations no coexisting minima are found for 98 Mo, as well as for 100,102 Mo.…”

“…For the nucleus 98 Mo, already described using configuration mixing IBM calculations in Refs [5,39], we have obtained a pronounced deviation of the 0 + 2 excitation energy with respect to the experimental value. Note that, at variance with previous results [5], our calculations on the deformation energy surfaces with the Gogny-D1M EDF (see, Fig.…”

“…As a consequence of the subtle interplay between single-particle and collective degrees of freedom, nuclei in this region of the nuclear chart display a large variety of intriguing phenomena. Several experimental [2][3][4][5][6][7][8] and theoretical [9][10][11][12][13][14][15][16][17] studies have already been reported on the structure of those nuclei. In particular, the rapid structural change around the neutron number N = 60 has been carefully studied in Zr and Sr isotopes [2,7,12,14,15,[18][19][20].…”

Shape coexistence in the microscopically guided interacting boson model

“…Recently, the method of Ref. [31] was extended to handle shape coexistence [43] and applied to the neutron-deficient Pb [43] and Hg [44] nuclei and to some neutron-rich nuclei with mass A ≈ 100 [45,46].…”