Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 49 (2016)

Experimental study of isoscalar and isovector dipole resonances inneutron-rich oxygen isotopes
N. Nakatsuka,∗1,∗2 H. Baba,∗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,∗14A. 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 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 Y. Yamaguchi,∗7and J. Zenihiro∗2
Giant resonance is one of the most important phenomena for understanding quantum many-body systems. Neutron-rich nuclei are predicted to have exotic
giant resonances owing to their smaller neutron separation energy and excess neutrons. One of the exotic giant resonances in neutron-rich nuclei is a electric dipole
resonance found at excitation energies lower than 10MeV1) . The identiﬁcation of the isovector or isoscalar
resonances is of great interest to understand the natureof these resonances. In order to study the relationshipbetween the isovector and isoscalar dipole resonancesin neutron-rich oxygen isotopes, we performed an experiment at RIBF and measured the dipole resonancesof the neutron-rich nuclei 20 O, 22 O, and 24 O. Thesebeams were produced via projectile fragmentation ofa 345 MeV/nucleon 48 Ca beam on 9 Be targets withthicknesses of 2.8 g/cm2 , 2.8 g/cm2 , and 2.2 g/cm2 .
The γ rays from the excited beam particles were detected with large volume LaBr3 crystals from INFN
Milano2) in combination with DALI23) . Two diﬀerent targets, 5 g/cm2 natural gold target for coulomb
excitation and 300 mg/cm2 liquid helium target for
inelastic α particle scattering, were used to obtain theisovector and isoscaler dipole strengths, respectively.
A preliminary doppler-corrected γ-ray spectrum ofα(20 O,20 Oγ)α reaction is shown in Fig. 1, and thespectrum of nat Au(20 O,20 Oγ)nat Au reaction is shownin Fig. 2. A clear diﬀerence is observed between thespectrum of the diﬀerent target. This suggests that thecomparison of the coulomb excitation and the inelastic α particle scattering is actually eﬀective to distin-guish the isovector and isoscaler resonances. Furtheranalysis is in progress to search for the isovector andisoscalar nature of the excited states.

Fig. 1. Preliminary doppler-corrected γ-ray spectrum ofα(20 O,20 Oγ)α reaction∗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∗2Fig. 2. Preliminary doppler-corrected γ-ray spectrum ofnatAu(20 O,20 Oγ)nat Au reaction

References
1) V. Derya et al., J. Phys. Conf. Ser. 366, 012012 (2012).
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).

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