Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 48 (2015)

Study of the pygmy-dipole resonances of 132 Sn and 128 Sn in inelastic
α-scattering

ZJ. 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,∗18 A. 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,∗19 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∗752
Pygmy-dipole resonance is commonly considered asa dipole mode of the nucleus related to a vibration
103
of excess neutrons against a core. As such, it should50
be related to the neutron richness of the nucleus as
102
well as its neutron-skin thickness. So far, the experSn-13249+
Sn-13248
imental information on this low-lying dipole mode isastonishingly scarce, even for stable nuclei1) . One in10
teresting open question is the isospin character of the46
low-lying dipole strength. In an experiment with the2.55
2.6
2.65
2.7
2.75 1stable 124 Sn isotope2) , it has been concluded that aA/Q
large fraction of the pygmy strength is of isoscalar charFig. 1. Particle identification plot of the secondary beamacter, however significant differences in the strength
after the liquid Helium target, gated on incoming 132 Sn
distribution compared with photoexcitation have beenions determined using the ZeroDegree spectrometer.
observed.
ber to achieve a high geometric acceptance.
In November 2014, the isoscalar mode of the pygmyAs a first step of the analysis, the preliminary par128
132
Sn andSn isotopes weredipole resonances inticle identification (PID) plot of Z versus A/Q canmeasured in inelastic α-scattering at RIKEN. The isobe
determined by a combination of the measured entopes of interest were produced with a high-intensity
ergy
loss, magnetic rigidity, and the time-of-flight238
U beam of 345 MeV/u impinging on aprimaryusingthe BigRIPS and the ZeroDegree spectromeberyllium target. The resulting secondary beam with5).
As a result, beam purity was dertermined toteran energy of approximately 200 MeV/u was directed
be18%
for 128 Sn and 26% for 132 Sn. As an extowards the liquid helium target with a thickness ofample, the resulting PID for the outgoing particles
approximately 300 mg/cm2 . The γ-rays, which are
for the 132 Sn experiment is shown in Fig. 1. Inejected at the target position, have been measured bythe plot, different charge states for 132 Sn can be ob8 large-volume 3.5”×8”LaBr3 :Ce crystals from Hector
served. However, the 129 Sn49+ -state has to be conINFN Milano3) and 95 large-volume NaI(Ti) DALI24)
sidered, because it has a similar A/Q-value as thecrystals. These crystals surrounded the target chamfully stripped 132 Sn ion (A/Q(129 Sn49+ )=2.633 andA/Q(132 Sn50+ )=2.640). The pygmy-dipole resonances∗1 Institut für Kernphysik, TU Darmstadt∗2 RIKEN Nishina Centercan be identified by the correlation of the identified∗3 INFN sezione di Milano
particles to the corresponding γ-rays. Finally, the∗4 Dipartimento di Fisica, Universita degli studi di Milano
strength of the isoscalar pygmy-dipole resonances can∗5 GSI Helmholtzzentrum Darmstadt
be determined. In addition, experiments already per∗6 School of Physics, Peking University∗7 Institute für Kernphysik, Universität zu Kölnformed at GSI Darmstadt with the R3 B setup will
∗8 Department of Physics, University of Notre Dameprofit, because with the result of this experiment, the∗9 KVI-CART Groningenisovector part of the resonance can be separated from∗10 Department of Physics, Rikkyo University
the isoscalar part.
∗11 Department of Physics, University of Tokyo∗12 Department of Physics, Tokyo Institut of Technoloy
References∗13p2Entries

Mean x3548250
2.627

Mean y

49.11

RMS x

0.03813RMS y

0.877450+

Departamento de Fisica Aplicada, Universidad de Huelva∗14 Center for Nuclear Research, University of Tokyo∗15 VECC India∗16 Department of Physics, Kyoto University∗17 Institut für Kernphysik, Goethe Universität Frankfurt∗18 National Supercoducting Cyclotron Laboratory,Michigan

State University∗19 Department of Physics, Tohoku University

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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