Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 49 (2016)

Discovery of a µs isomer of 76 Co†
P.-A. Söderström,∗1 S. Nishimura,∗1 Z. Y. Xu,∗2 K. Sieja,∗3 V. Werner,∗4,∗5 P. Doornenbal,∗1 G. Lorusso,∗1F. Browne,∗1,∗6 G. Gey,∗1,∗7,∗8 H. S. Jung,∗9 T. Sumikama,∗10 J. Taprogge,∗1,∗11,∗12 Zs. Vajta,∗1,∗13H. Watanabe,∗1,∗14 J. Wu,∗1,∗15 H. Baba,∗1 Zs. Dombradi,∗13 S. Franchoo,∗16 T. Isobe,∗1 P. R. John,∗17,∗18Y.-K. Kim,∗19 I. Kojouharov,∗20 N. Kurz,∗20 Y. K. Kwon,∗19 Z. Li,∗15 I. Matea,∗16 K. Matsui,∗2
G. Martnez-Pinedo,∗5 D. Mengoni,∗17,∗18 P. Morfouace,∗16 D. R. Napoli,∗21 M. Niikura,∗2 H. Nishibata,∗22A. Odahara,∗22 K. Ogawa,∗1 N. Pietralla,∗5 E. Şahin,∗23 H. Sakurai,∗1,∗2 H. Schaffner,∗20 D. Sohler,∗13I. G. Stefan,∗16 D. Suzuki,∗16 R. Taniuchi,∗2 A. Yagi,∗22 and K. Yoshinaga∗24
Changes in nuclear shell structure far from stability
are largely associated with the monopole component of
the proton-neutron interaction. Thus, there is a large
ongoing experimental effort aiming to investigate howthese shell and sub-shell closures evolve for very exotic
nuclei at and below 78 Ni. The study of single neutronand proton particle and hole states outside 78 Ni is one
important way to gain information on this topic. Ina recent paper new experimental results on 76 Co, one
neutron-hole and one proton-hole in 78 Ni, were presented. Due to the purity of the excited states, this isa unique case to study the neutron-proton interaction
in a region with sparse experimental information.
The 76 Co nuclei were produced by in-flight fissionof a 345 MeV/u 238 U beam on a 3 mm beryllium target and then separated using the BigRIPS fragment
separator and the ZeroDegree spectrometer. At F11
the WAS3ABi1) silicon detector stack was used for implantation and β-decay correlation measurements andthe EURICA spectrometer was used for measuring theenergy and time of the γ rays. In total, approximatelyFig. 1. Proposed experimental level scheme of 76 Co compared to shell model calculations using a modified
LNPS interaction.

1000 76 Co ions were implanted in WAS3ABi during 10days of measurement.
In the experiment, two coincident γ rays of 192 and446 keV from the decay of a t1/2 = 3 µs isomeric stateof 76 Co were observed. The decay of the isomer was
assigned to an E1 transition with a reduced transition
probability of B(E1; 3+ → 2− ) = 1.79 × 10−8 W.u.
Shell model calculations carried out with an up-todate LNPS interaction2,3) including monopole changes
to assure the correct propagation of proton singleparticles energies showed the states to be about 70%−1−1−1pure structures of πf7/2⊗ νg9/2or πf7/2⊗ νp−1
1/2 hole
configurations for negative and positive parity states,−1respectively. Thus, the relative νg9/2
and νp−1
1/2 positions could be fine tuned by changing the strength−1of the πf7/2⊗ νp−1
1/2 monopole. The results of thesecalculations are shown in Fig. 1.
Furthermore, a β decaying 8− state was also observed in the data, consistent with the LNPS shellmodel calculations. These results will help constrain
further developments of theoretical models in theπf7/2 ⊗ νg9/2 region between 60 Ca and 78 Ni, wherescarce experimental data are available.

†Condensed from the article in Phys. Rev. C, Vol.92,051305(R) (2015)∗1 RIKEN Nishina Center∗2 Department of Physics, University of Tokyo∗3 Université de Strasbourg, IPHC
∗4 Wright Nuclear Structure Laboratory, Yale University∗5 Institut für Kernphysik, TU Darmstadt∗6 School of Computing, Engineering and Mathematics, University of Brighton∗7 LPSC, Université Grenoble-Alpes, CNRS/IN2P3∗8 ILL∗9 Department of Physics, Chung-Ang University∗10 Department of Physics, Tohoku University∗11 Departamento de Fı́sica Teórica, Universidad Autónoma deMadrid∗12 Instituto de Estructura de la Materia, CSIC
∗13 Institute for Nuclear Research, Hungarian Academy of Sciences∗14 School of Physics and Nuclear Energy Engineering, BeihangUniversity∗15 School of Physics and State key Laboratory of Nuclear
Physics and Technology, Peking University∗16 Institut de Physique Nucléaire, CNRS-IN2P3∗17 INFN Sezione di Padova∗18 Dipartimento di Fisica e Astronomia, Università di Padova∗19 Institute for Basic Science, Rare Isotope Science Project∗20 GSI∗21 INFN, LNL∗22 Department of Physics, Osaka University∗23 Department of Physics, University of Oslo
∗24 Department of Physics, Tokyo University of Science

References
1) S. Nishimura et al. RIKEN Accel. Prog. Rep. 46, 182(2013)
2) K. Sieja and F. Nowacki Phys. Rev. C 85, 051301(2012).
3) S. M. Lenzi, F. Nowacki, A. Poves, and K. Sieja Phys.
Rev. C 82, 054301 (2010).

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