RIKEN Accel. Prog. Rep. 47 (2014)

HIGHLIGHTS OF THE YEAR

Evolution of collectivity in neutron-rich Ru nuclei†
P.-A. Söderström,∗1 G. Lorusso,∗1 H. Watanabe,∗2 S. Nishimura,∗1 P. Doornenbal,∗1 G. Thiamova,∗3F. Browne,∗1,∗4 G. Gey,∗1,∗3,∗5 H.S. Jung,∗6 T. Sumikama,∗7 J. Taprogge,∗1,∗8,∗9 Zs. Vajta,∗1,∗10 J. Wu,∗11Z.Y. Xu,∗12 H. Baba,∗1 G. Benzoni,∗13 K.Y. Chae,∗14 F.C.L. Crespi,∗13,∗14 N. Fukuda,∗1 R. Gernhäuser,∗16N. Inabe,∗1 T. Isobe,∗1 A. Jungclaus,∗9 D. Kameda,∗1 G.D. Kim,∗17 Y.-K. Kim,∗17 I. Kojouharov,∗18F.G. Kondev,∗19 T. Kubo,∗1 N. Kurz,∗18 Y.K. Kwon,∗17 G. Lane,∗20 Z. Li,∗11 A. Montaner-Pizá,∗21
K. Moschner,∗22 F. Naqvi,∗23 M. Niikura,∗12 H. Nishibata,∗24 A. Odahara,∗24 R. Orlandi,∗25 Z. Patel,∗26
Zs. Podolyák,∗26 H. Sakurai,∗12 H. Schaffner,∗18 G. Simpson,∗3 K. Steiger,∗16 H. Suzuki,∗1 H. Takeda,∗1A. Wendt,∗22 A. Yagi,∗24 and K. Yoshinaga∗27Pd

1

E(4 +)/E(2 +)One of the central features in our understanding ofthe atomic nucleus is the appearance of magic numbers. Isotopes in their proximity can be described interms of single-particle interactions with an inert core.
Most nuclei, however, lie sufficiently far from magic
numbers for collective behaviour to dominate over thesingle particle structure. In the prolate-oblate transition regions comparable energy minima corresponding
to different shapes can lead to shape coexistence and tostable intermediate shapes with different deformations
on each axis, so called triaxial nuclei.
An experiment was carried out using at RIBF using a 238 U beam with an energy of 345 Mev/u, andan average intensity of ∼ 10 pnA and a 555 mg/cm2
beryllium target. After the target, BigRIPS and ZeroDegree were used for separation and tagging of thenuclei of interest. The secondary beam was implanted
into the WAS3ABi silicon detectors. The β-delayed γrays were detected with the EURICA1) detector array.
The β-delayed γ-ray spectra associated with 116 Tc
and 118 Tc were analysed and interpreted in termsof observables of collectivity2) . The energy ratio

1Ru

3Mo
2.52607080NFig. 1. Experimental R(4/2) ratios. In addition to what
was shown in2) , recent data from3,4) have been added
to the Pd chain.
+R(4/2) = E(4+1 )/E(21 ) is one such observable, ranging approximately from the minimum at R(4/2) = 2
for spherical nuclei and the maximum R(4/2) = 3.33
for rigid rotors. The trends of this ratio as a functionof N for Mo, Ru and Pd chains, shown in Fig. 1, indicate that these elements are well deformed in thisregion. The Pd chain exhibits a relatively stable value
around the transitional limit, R(4/2) = 2.5, while theMo chain is closer to the deformed limit and the Ru
chain lies in between the former. For the most neutronrich Ru isotopes the beginning of a shape transition
towards sphericity can clearly be seen.
Calculations were also carried out for the even-even
108−118
Ru isotopes with the interacting boson modeland the algebraic collective model. The conclusions
are that the very neutron-rich nuclei still show many
features associated with triaxial γ-soft nuclei, represented by the O(6) symmetry, but are approaching aspherical structure, the U(5) symmetry, with increasing neutron number towards the N = 82 shell closure.

Condensed from the article in Phys. Rev. C, 88, 024301(2013)∗1 RIKEN Nishina Center∗2 Department of Physics, Beihang University∗3 LPSC, Université Grenoble-Alpes, CNRS/IN2P3∗4 University of Brighton∗5 ILL, Grenoble∗6 Department of Physics, Chung-Ang University∗7 Department of Physics, Tohoku University∗8 Universidad Autónoma de Madrid∗9 CSIC, Spain∗10 Atomki, Debrecen∗11 School of Physics, Peking University∗12 Department of Physics, University of Tokyo∗13 INFN Sezione di Milano∗14 Department of Physics, Sungkyunkwan University∗15 Dipartimento di Fisica, Università di Milano∗16 Technische Universität München∗17 Institute for Basic Science, Rare Isotope Science Project∗18 GSI Helmholtzzentrum für Schwerionenforschung∗19 Nuclear Engineering Division, Argonne National Laboratory∗20 Australian National University∗21 CSIC, University of Valencia∗22 Universität zu Köln∗23 Wright Nuclear Structure Laboratory, Yale University∗24 Department of Physics, Osaka University∗25 Instituut voor Kern en Stralingsfysica, University of Leuven∗26 Department of Physics, University of Surrey∗27 Department of Physics, Tokyo University of Science†

References
1) S. Nishimura: Prog. Theor. Exp. Phys., 2012, 03C006(2012)
2) P.-A. Söderström et al.: Phys. Rev. C, 88, 024301(2013)
3) H. Wang al.: Phys. Rev. C, 88, 054318 (2013)4) H. Watanabe al.: Phys. Rev. Lett., 111, 152501 (2013)- xvi -