Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 50 (2017)

Investigation of isoscalar and isovector dipole excitations in 20 O
N. Nakatsuka,∗1,∗2 H. Baba,∗2,∗2 N. Aoi,∗10 T. Aumann,∗3 R. Avigo,∗5,∗14 S. R. Banerjee,∗12 A. Bracco,∗5,∗14C. 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∗2tive to determine the isovector and isoscalar strength.
Further analysis using the distorted-wave Born approximation is in progress to determine the isovector andisoscalar strengths of the observed low-energy dipoleexcitations.

The electric dipole excitation is one of the mostbasic properties of atomic nuclei. Neutron-rich nuclei are predicted to have exotic electric dipole excitations owing to their small neutron separation energy
and excess neutrons. One example of such excitations in neutron-rich nuclei is the low-energy dipole
excitations found at excitation energies less than 10MeV. Recent experimental studies on stable nuclei revealed that some low-energy dipole excitations show
speciﬁc isospin character1) called isospin splitting. In
order to study the isospin properties of low-energy
dipole excitations in neutron-rich oxygen isotopes, weperformed an experiment at RIBF and measured thedipole resonances of the neutron-rich nucleus 20 O. The
beam was produced via projectile fragmentation of a345-MeV/nucleon 48 Ca beam on 9 Be target with athickness of 2.8 g/cm2 . Two secondary targets, a 5g/cm2 -thick gold target for coulomb excitation and a300 mg/cm2 thick liquid helium target for inelastic αparticle scattering, were used to obtain the isovector
and isoscaler dipole strengths independently. The γ
rays from the excited beam particles were detectedwith large volume LaBr3 crystals from INFN Milano2)
in combination with DALI23) . A preliminary dopplercorrected γ-ray spectrum of the α(20 O,20 Oγ) reaction is shown in Fig. 1 (a), and the spectrum ofAu(20 O,20 Oγ) reaction is shown in Fig. 1 (b). Preliminary ﬁts are presented by red solid lines, and two 1−
states are identiﬁed. A clear diﬀerence is observed between the two spectra. This suggests that the Coulomb
excitation and inelastic α-particle scattering have different sensitivities to the isospin and are actually eﬀec-(a) 20O+α
Fit Total
12 6.84 MeV
11 5.36 MeV

counts102

101

counts103

Preliminary(b) 20O+Au
Fit Total
12 6.84 MeV
11 5.36 MeV

102

200040006000

E γ (keV)

Fig. 1. Preliminary ﬁts of Doppler-corrected γ ray spectra:
20O+α (top panel) and 20 O+Au (bottom panel).

References
1) A. Bracco et al., Euro. Phys. J. A 51, 99 (2015).
2) A. Giaz et al., Nucl. Instrum. Methods Phys. Res., Sec.
A 729, 910 (2013).
3) S. Takeuchi et al., Nucl. Instrum. Methods Phys. Res.,
Sec. A 763, 596 (2014).

∗1

Department of Physics, Kyoto University
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∗2-7-