RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-1. Nuclear Physics

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∗7
The pygmy dipole resonance in neutron-rich isotopes
can be pictured as a vibration of the excess neutrons
against an isospin symmetric core in the nucleus. Thisvibration corresponds to a dipole mode. Due to thenature of the resonance, the phenomenon should be afunction of the neutron-skin thickness and also of theneutron excess. The resonance has an isoscalar and anisovector part. To distinguish between both of themexperimentally one can use two diﬀerent probes. The
α-particle is isoscalar and will excite exclusively theisoscalar part in contrast to photon-excitation, which
excites mainly the isovector part of the resonance. As
the experimental data for this dipole mode is rare, evenfor stable nuclei1) , interesting open questions remain.
One of them is the isospin character and strength of the
low-lying dipole mode. In an experiment with the stable 124 Sn2) , it has been observed that a large fractionof the pygmy strength is of isoscalar character, however signiﬁcant diﬀerences in the strength distribution
with photo-excitation have been observed.
In October 2014, an experiment to investigate further the isoscalar character of the pygmy dipole resonance in 128 Sn, and 132 Sn was performed with inelastic α-scattering in inverse kinematics at RIKEN. The
isotopes were produced with a high-intensity primary238
U beam impinging with 345 MeV/u on a beryllium target. The secondary beam with approximately

220 MeV/u was transported towards the liquid-helium
target, with a thickness of approximately 320 mg/cm2 .
At the target position, emitted γ-rays were measured
by 8 large-volume 3.5” × 8”LaBr3 :Ce crystals fromHector-array at INFN Milano3) and 95 large-volume
NaI DALI24) crystals. The 8 LaBr3 :Ce crystals wereplaced at an angle θ of 30◦ and the DALI2 crystals covered a range in θ from about 45◦ to about 150◦ . Bothdetector types are distributed over 360◦ in φ. The registered hits in the γ-array are selected within a coincidence time-window with the beam to suppress background stemming from the intrinsic radiation of the8 LaBr3 :Ce crystals, and other random background.
The particle identiﬁcation and separation was doneusing BigRIPS and the ZeroDegree spectrometer.
Those devices are using a combination of energy loss,
magnetic rigidity, and time-of-ﬂight measurements5) .
The purity of the secondary 132 Sn-beam was about
28% with about 41 million registered events.
With further investigation of the γ-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 comparison with data of experiments already performed
at GSI Darmstadt with the R3 B setup, as in those experiments the isovector part is predominantly excitedby relativistic coulomb excitation.
The authors acknowledge the german support by
the Deutsche Forschungsgesellschaft (DFG), the grant
SFB 1245.

∗1

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∗2References
1) D. Savran et al.: Prog. Part. Nucl. Phys. 70, p.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).

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