II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Shape evolution of 106, 108, 110 Mo in the triaxial degree of freedom†

The properties of the 2+
2 band in even-even nuclei
are closely connected with the triaxial motion in thedirection of the γ degree of freedom, such as the γvibration, rigid triaxial rotor,1) or γ-unstable rotor.2)
The lowering of the known 2+
2 -state energy in neutronrich molybdenum isotopes (Z = 42) is interpreted as thedevelopment of these triaxial motions associated withthe ground-state shape. We studied the neutron-rich
106, 108, 110
Mo isotopes with higher statistics by measuring the β-delayed γ rays.
A neutron-rich cocktail beam was produced from thefragmentation of a 345-MeV/nucleon 238 U86+ beam.
The nuclides were separated and identiﬁed on the BigRIPS separator and delivered to F11. The ions andβ particles were detected by the WAS3ABi active stopper. A high-purity Ge array, EURICA,3) and fast-timing
LaBr3 (Ce) array were used to measure the energy andtime of γ rays.
Figure 1 shows B(E2) determined from the lifetime
measurement of the 2+
1 states using the LaBr3 (Ce) array. The quadrupole deformation parameters β2 of106, 108, 110
Mo were deduced to be 0.349(13), 0.327(10),and 0.305(7), respectively. The results were compared
Condensed from the article in Phys. Rev. C 101, 044311(2020)∗1 Department of Physics and Astronomy, Seoul National University∗2 RIKEN Nishina Center∗3 Department of Physics, Tohoku University∗4 School of Computing, Engineering, and Mathematics, University of Brighton∗5 Center for Computational Sciences, University of Tsukuba∗6 Department of Physics, University of Surrey∗7 National Physical Laboratory∗8 IRCNPC, School of Physics and Nuclear Energy Engineering,Beihang University∗9 Department of Physics, Osaka University∗10 Department of Physics, University of York∗11 Department of Physics, Peking University∗12 Department of Physics, University of Tokyo∗13 Department of Physics, University of Hong Kong
∗14 IPHC, CNRS/IN2P3, Université de Strasbourg∗15 LPSC, Université Grenoble-Alpes∗16 ILL∗17 GSI Helmholtzzentrum für Schwerionenforschung GmbH
∗18 Department of Physics, University of Sofia∗19 CSNSM, CNRS/IN2P3, Université Paris-Sud∗20 Departamento de Física Teórica, Universidad Autónoma deMadrid∗21 Instituto de Estructura de la Materia, CSIC
∗22 A.W. Wright Nuclear Structure Laboratory, Yale University∗23 Institut für Kernphysik, Technische Universität Darmstadt∗24 INFN Sezione di Milano†B(E2;2+1→ 0+1) [e2b2]

J. Ha,∗1,∗2 T. Sumikama,∗2,∗3 F. Browne,∗2,∗4 N. Hinohara,∗5 A. M. Bruce,∗4 S. Choi,∗1 I. Nishizuka,∗3S. Nishimura,∗2 P. Doornenbal,∗2 G. Lorusso,∗2,∗6,∗7 P. -A. Söderström,∗2 H. Watanabe,∗2,∗8 R. Daido,∗9Z. Patel,∗2,∗6 S. Rice,∗2,∗6 L. Sinclair,∗2,∗10 J. Wu,∗2,∗11 Z. Y. Xu,∗12,∗13 A. Yagi,∗9 H. Baba,∗2 N. Chiga,∗2,∗3R. Carroll,∗6 F. Didierjean,∗14 Y. Fang,∗9 N. Fukuda,∗2 G. Gey,∗15,∗16 E. Ideguchi,∗9 N. Inabe,∗2 T. Isobe,∗2D. Kameda,∗2 I. Kojouharov,∗17 N. Kurz,∗17 T. Kubo,∗2 S. Lalkovski,∗18 Z. Li,∗11 R. Lozeva,∗14,∗19 H. Nishibata,∗9A. Odahara,∗9 Zs. Podolyák,∗6 P. H. Regan,∗6,∗7 O. J. Roberts,∗4 H. Sakurai,∗2 H. Schaﬀner,∗17 G. S. Simpson,∗15H. Suzuki,∗2 H. Takeda,∗2 M. Tanaka,∗9 J. Taprogge,∗2,∗20,∗21 V. Werner,∗22,∗23 and O. Wieland∗240.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0Mo isotopes

5860

626466
Neutron number

SLy5+T
SLy468+Fig. 1. B(E2; 2+1 → 01 ) of the neutron-rich Mo isotopes.
The theoretical results calculated with SLy4 and SLy5+Tinteractions are shown.

with beyond-mean-ﬁeld calculations using SLy4 andSLy5+T interactions, for which the predicted groundstate shapes were oblate and prolate, respectively. Theprolate shape was indicated because the calculation withthe SLy5+T interaction reproduces both B(E2) and theenergies of the ground-state band.
110
The 2+
Mo was extended up to the 7+2 band instate. The energy staggerings of the 2+
2 bands in106, 108, 110
Mo are close to that of the axially symmetricrotor of the γ-vibrational state, rather than Davydov’s
rigid-triaxial rotor model or Wilets-Jean model for γunstable nuclei. A candidate of the two-phonon γ vibrational band with K π = 4+ , which has not been wellestablished yet, was found in 110 Mo. The K π = 4+ band
decays only to the γ-vibrational band, and the energyof the K π = 4+ state is 2.5 times larger than that of+the 2+2 state. Moreover, new 02 states were assigned in108
110
Mo andMo.
The spin and parity of parent nuclei were assigned
from the log f t values to be 4− and 2− for the groundstate in 106 Nb and 108 Nb, respectively. Two β-decaying
states were identiﬁed in 110 Nb, and their spin-parities
were asigned as 2− and 6− .
References

1) A. S. Davydov, G. F. Filippov, Nucl. Phys. 8, 237 (1958).
2) L. Wilets, M. Jean, Phys. Rev. 102, 788 (1956).
3) P. -A. Söderström et al., Nucl. Instrum. Methods Phys.
Res. B 317, 649 (2013).

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