Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 49 (2016)

Study of the pygmy dipole resonance of 132 Sn and 128 Sn in inelastic
α-scattering
J. Tscheuschner,∗1,∗2 T. Aumann,∗1 D. S. Ahn,∗2 R. Avigo,∗3,∗4 H. Baba,∗2 K. Boretzky,∗5 A. Bracco,∗3,∗4C. Caesar,∗5 A. Camera,∗3,∗4 S. Chen,∗2,∗6 V. Derya,∗7 P. Doornenbal,∗2 J. Endres,∗7 N. Fukuda,∗2 U. Garg,∗8A. Giaz,∗3,∗4 M. N. Harakeh,∗9 M. Heil,∗5 A. Horvat,∗1 K. Ieki,∗10 N. Imai,∗11 N. Inabe,∗2N. Kalantar-Nayestanaki,∗9 N. Kobayashi,∗11 Y. Kondo,∗12 S. Koyama,∗11 T. Kubo,∗2 I. Martel,∗13M. Matsushita,∗14 B. Million,∗15 T. Motobayashi,∗2 T. Nakamura,∗12 N. Nakatsuka,∗2,∗16 M. Nishimura,∗2S. Nishimura,∗2 S. Ota,∗14 H. Otsu,∗2 T. Ozaki,∗12 M. Petri,∗1 R. Reifarth,∗17 D. Rossi,∗1 A. T. Saito,∗12H. Sakurai,∗2,∗11 D. Savran,∗5 H. Scheit,∗1 F. Schindler,∗1 P. Schrock,∗1 D. Semmler,∗1 Y. Shiga,∗2,∗10M. Shikata,∗12 Y. Shimizu,∗2 H. Simon,∗5 D. Steppenbeck,∗2 H. Suzuki,∗2 T. Sumikama,∗18 D. Symochko,∗1I. Syndikus,∗1 H. Takeda,∗2 S. Takeuchi,∗2 R. Taniuchi,∗11 Y. Togano,∗12 J. Tsubota,∗12 H. Wang,∗2O. Wieland,∗3 K. Yoneda,∗2 J. Zenihiro,∗2 and A. Zilges∗7Counts

In neutron-rich nuclei, the pygmy dipole resonancecan be visualized as a vibration of excess neutrons
against an isospin symmetric core in the nucleus, whichcorresponds to a dipole mode. Ongoing to the natureof the resonance, the phenomenon should be a functionof the neutron-skin thickness and neutron excess. As
the experimental data for this dipole mode are rare,even for stable nuclei1) , interesting open questions remain. One of them is the isospin character of the lowlying dipole strength. In an experiment with stable124Sn2) , it has been realized that a large fraction of
the pygmy strength is of isoscalar character. However,significant differences in the strength distribution withphoto-excitation have been observed.
In November 2014, an experiment to investigate theisoscalar character of the pygmy dipole resonancein 128 Sn and 132 Sn was performed with inelastic αscattering at RIKEN. The isotopes were produced witha high-intensity primary 238 U beam at 345 MeV/u impinging on a beryllium target. The secondary beamof approximately 200 MeV/u was directed towards
the liquid helium target, with a thickness of approximately 300 mg/cm2 . At the target position, theemitted γ-rays were measured by 8 large-volume 3.5”
× 8”LaBr3 :Ce crystals from the HECTOR-array atINFN Milano3) and 95 large-volume NaI DALI24) crystals.
The particle identification involved a combination of1041031021010500

1000 1500 2000 2500 3000 3500 4000 4500 5000
EγFig. 1. Doppler-corrected γ-energy spectra measured usingcrystals from the HECTOR-array, gated on incoming
and outgoing 132 Sn ions. The 2+ state at 4 MeV canbe identified.

energy loss, magnetic rigidity, and time-of-flight measurements using the BigRIPS and the ZeroDegree spectrometer5) . The registered hits in the γ-array are selected within a time window to suppress noise stemming from particle hits and the background. As a result, the preliminary energy plot for the γ-rays measured with the crystals from the HECTOR-array, emitted by the excited 132 Sn ions, is shown in Fig. 1. The
peak at about 4 MeV corresponds to the 2+ state,
which is the lowest excited state in the isotope..
With further investigation of the γ-energy spectra, thestrength of the (isoscalar) pygmy dipole resonances
can be determined. In addition, the obtained information will allow the isovector and isoscalar parts ofthe pygmy dipole resonance to be separated in dataof experiments already performed at GSI Darmstadt
with the R3 B setup.

Institut für Kernphysik, TU Darmstadt
RIKEN Nishina Center∗3 INFN sezione di Milano∗4 Dipartimento di Fisica, Universita degli studi di Milano∗5 GSI Helmholtzzentrum Darmstadt∗6 School of Physics, Peking University∗7 Institute für Kernphysik, Universität zu Köln∗8 Department of Physics, University of Notre Dame
∗9 KVI-CART Groningen∗10 Department of Physics, Rikkyo University∗11 Department of Physics, University of Tokyo∗12 Department of Physics, Tokyo Institute of Technoloy∗13 Departamento de Fisica Aplicada, Universidad de Huelva∗14 Center for Nuclear Study, University of Tokyo∗15 VECC India∗16 Department of Physics, Kyoto University∗17 Institut für Kernphysik, Goethe Universität Frankfurt∗18 Department of Physics, Tohoku University∗1∗2References
1) D. Savran et al., Prog. Part. Nucl. Phys. 70, 210 (2013).
2) J. Endres et al., Phys. Rev. Lett. 105, 212503 (2010).
3) A. Giaz et al., NIM A729, 910 (2013).
4) S. Takeuchi et al., NIM A 763, 596 (2014).
5) T. Kubo et al., Prog. Theor. Exp. Phys. 03C003 (2012).

- 39 -