Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 51 (2018)

Observation of isoscalar and isovector dipole excitations in 20 O†N. Nakatsuka,∗1,∗2 H. Baba,∗2 N. Aoi,∗10 T. Aumann,∗3 R. Avigo,∗5,∗14 S. R. Banerjee,∗12 A. Bracco,∗5,∗14
C. Caesar,∗3 F. Camera,∗5,∗14 S. Ceruti,∗5,∗14 S. Chen,∗13,∗2 V. Derya,∗4 P. Doornenbal,∗2 A. Giaz,∗5,∗14
A. Horvat,∗3 K. Ieki,∗11 N. Imai,∗7 T. Kawabata,∗1 K. Yoneda,∗2 N. Kobayashi,∗8 Y. Kondo,∗9 S. Koyama,∗8M. Kurata-Nishimura,∗2 S. Masuoka,∗7 M. Matsushita,∗7 S. Michimasa,∗7 B. Millon,∗5 T. Motobayashi,∗2T. Murakami,∗1 T. Nakamura,∗9 T. Ohnishi,∗2 H. J. Ong,∗10 S. Ota,∗7 H. Otsu,∗2 T. Ozaki,∗9 A. T. Saito,∗9H. Sakurai,∗2,∗8 H. Scheit,∗3 F. Schindler,∗3 P. Schrock,∗3 Y. Shiga,∗11,∗2 M. Shikata,∗9 S. Shimoura,∗7D. Steppenbeck,∗2 T. Sumikama,∗6,∗2 I. Syndikus,∗3 H. Takeda,∗2 S. Takeuchi,∗2 A. Tamii,∗10 R. Taniuchi,∗8Y. Togano,∗9 J. Tscheuschner,∗3 J. Tsubota,∗9 H. Wang,∗2 O. Wieland,∗5 K. Wimmer,∗8,∗2 Y. Yamaguchi,∗7and J. Zenihiro∗2The electric dipole response, or E1 response, is
one of the most interesting properties of atomic nuclei. In medium to heavy neutron-rich nuclei, the electric dipole excitation is fragmented into a low-energy
region around the neutron separation energy, socalled Pygmy dipole resonance.1,2) Recent experimental studies on 40,48 Ca,6) 74 Ge,7) 124 Sn,8) 138 Ba,9) and140
Ce9,10) have demonstrated that low-energy dipole
excitations exhibit a speciﬁc isospin character, sometimes referred to as “isospin splitting.” They demonstrated that some dipole excitations, mostly in the
low-energy region, were populated by both isoscalarand isovector probes. In this work, the isospin character of low-energy dipole excitations in neutron-rich unstable nucleus 20 O was investigated, for the ﬁrst timein unstable nuclei. The experiment was performed at
Radioactive Isotope Beam Factory (RIBF). The 20 O
beam impinged on two diﬀerent reaction targets, a2.45(5) mm gold target as an isovector probe, and a317(28) mg/cm2 liquid helium target as an isoscalar
probe. The decay γ rays from the excited beam particles were detected with large volume LaBr3 crystals
from INFN Milano.3) Two low-energy dipole states at−energies of 5.36(5) MeV (1−1 ) and 6.84(7) MeV (12 ),previously known to be populated by the Coulomb
excitation,4,5) were consistently populated both by
the isoscalar and isovector probe. The decay schemeof those states were determined by the γ-γ coincidence analysis, and the decay branch via the 2+1 state(1.67 MeV) was observed.
In order to extract the cross sections and tran-sition strengths, a distorted-wave Born approximation (DWBA) analysis was performed by using theEcis97 code.12) As nuclear potential, we employed
the theoretically developed global optical potential described in Refs. 13–15). The transition strengths of
the 1− states were determined in the same manner,by including both the Coulomb and nuclear contributions in either system, with the assumption that theCoulomb potential contributed only to the isovector
dipole strength and the nuclear potential contributed
only to the isoscalar dipole strength. The HarakehDieperink dipole form factor11) was employed to determine the isoscalar dipole strength. The strengths
were determined so that the experimental cross sections from both the 20 O+α and 20 O+Au systems werereproduced by the same isoscalar and isovector dipolestrengths. The 1−
1 state (5.36(5) MeV) had an isoscalardipole strength of 2.70(32)% in ISD EWSR, while the1−2 state (6.84(7) MeV) had a strength of 0.67(12)%in Isoscalar dipole energy-weighted sum-rule fraction(ISD EWSR). These states, however, have comparable
isovector dipole strengths: B(E1)↑ = 3.57(20) × 10−2−2e2 fm2 for the 1−
1 state and B(E1)↑ = 3.79(26) × 10−22e fm for the 12 state. The results indicate that lowenergy dipole excitations in 20 O exhibit a dual character. The diﬀerence in isoscalar response suggests thatthese states have diﬀerent underlying structures.
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†Condensed from the article in Phys. Lett. B. 768, 387 (2017)
Department of Physics, Kyoto University∗2 RIKEN Nishina Center∗3 Institut für Kernphysik, Technische Universität Darmstadt∗4 Institut für Kernphysik, Universität zu Köln∗5 Istituto Nazionale di Fisica Nucleare Milan∗6 Department of Physics, Tohoku University∗7 Center for Nuclear Study, The University of Tokyo∗8 Department of Physics, The University of Tokyo∗9 Department of Physics, Tokyo Institute of Technology∗10 Research Center for Nuclear Physics, Osaka University∗11 Department of Physics, Rikkyo University∗12 Variable Energy Cyclotron Centre, The Indian Departmentof Atomic Energy∗13 School of Physics, Peking University∗14 University of Milan∗1- 30 -