Study of the pygmy-dipole resonances of 132Sn and 128Sn 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,∗4

C. Caesar,∗5 A. Camera,∗3,∗4 S. Chen,∗2,∗6 V. Derya,∗7 P. Doornenbal,∗2 J. Endres,∗7 N. Fukuda,∗2 U. Garg,∗8

A. Giaz,∗3,∗4 M. N. Harakeh,∗9 M. Heil,∗5 A. Horvat,∗1 K. Ieki,∗10 N. Imai,∗11 N. Inabe,∗2

N. Kalantar-Nayestanaki,∗9 N. Kobayashi,∗11 Y. Kondo,∗12 S. Koyama,∗11 T. Kubo,∗2 I. Martel,∗13

M. Matsushita,∗14 B. Million,∗15 T. Motobayashi,∗2 T. Nakamura,∗12 N. Nakatsuka,∗2,∗16 M. Nishimura,∗2

S. Nishimura,∗2 S. Ota,∗14 H. Otsu,∗2 T. Ozaki,∗12 M. Petri,∗1 R. Reifarth,∗17 D. Rossi,∗18 A. Saito,∗12

H. Sakurai,∗2,∗11 D. Savran,∗5 H. Scheit,∗1 F. Schindler,∗1 P. Schrock,∗1 D. Semmler,∗1 Y. Shiga,∗2,∗10

M. Shikata,∗12 Y. Shimizu,∗2 H. Simon,∗5 D. Steppenbeck,∗2 H. Suzuki,∗2 T. Sumikama,∗19 D. Symochko,∗1

I. Syndikus,∗1 H. Takeda,∗2 S. Takeuchi,∗2 R. Taniuchi,∗11 Y. Togano,∗12 J. Tsubota,∗12 H. Wang,∗2

O. Wieland,∗3 K. Yoneda,∗2 J. Zenihiro,∗2 and A. Zilges∗7

Pygmy-dipole resonance is commonly considered asa dipole mode of the nucleus related to a vibrationof excess neutrons against a core. As such, it shouldbe related to the neutron richness of the nucleus aswell as its neutron-skin thickness. So far, the exper-imental information on this low-lying dipole mode isastonishingly scarce, even for stable nuclei1). One in-teresting open question is the isospin character of thelow-lying dipole strength. In an experiment with thestable 124Sn isotope2), it has been concluded that alarge fraction of the pygmy strength is of isoscalar char-acter, however significant differences in the strengthdistribution compared with photoexcitation have beenobserved.In November 2014, the isoscalar mode of the pygmy-dipole resonances in 128Sn and 132Sn isotopes weremeasured in inelastic α-scattering at RIKEN. The iso-topes of interest were produced with a high-intensityprimary 238U beam of 345 MeV/u impinging on aberyllium target. The resulting secondary beam withan energy of approximately 200 MeV/u was directedtowards the liquid helium target with a thickness ofapproximately 300 mg/cm2. The γ-rays, which areejected at the target position, have been measured by8 large-volume 3.5”×8”LaBr3:Ce crystals from HectorINFN Milano3) and 95 large-volume NaI(Ti) DALI24)

crystals. These crystals surrounded the target cham-

∗1 Institut fur Kernphysik, TU Darmstadt∗2 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 fur Kernphysik, Universitat zu Koln∗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 Institut of Technoloy∗13 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 fur Kernphysik, Goethe Universitat Frankfurt∗18 National Supercoducting Cyclotron Laboratory, Michigan

State University∗19 Department of Physics, Tohoku University

A/Q2.55 2.6 2.65 2.7 2.75

Z46

48

50

52

p2Entries 3548250Mean x 2.627Mean y 49.11

RMS x 0.03813RMS y 0.8774

1

10

210

310p2

Entries 3548250Mean x 2.627Mean y 49.11

RMS x 0.03813RMS y 0.8774

50+Sn-13249+Sn-132

Fig. 1. Particle identification plot of the secondary beam

after the liquid Helium target, gated on incoming 132Sn

ions determined using the ZeroDegree spectrometer.

ber to achieve a high geometric acceptance.As a first step of the analysis, the preliminary par-ticle identification (PID) plot of Z versus A/Q canbe determined by a combination of the measured en-ergy loss, magnetic rigidity, and the time-of-flightusing the BigRIPS and the ZeroDegree spectrome-ter5). As a result, beam purity was dertermined tobe 18% for 128Sn and 26% for 132Sn. As an ex-ample, the resulting PID for the outgoing particlesfor the 132Sn experiment is shown in Fig. 1. Inthe plot, different charge states for 132Sn can be ob-served. However, the 129Sn49+-state has to be con-sidered, because it has a similar A/Q-value as thefully stripped 132Sn ion (A/Q(129Sn49+)=2.633 andA/Q(132Sn50+)=2.640). The pygmy-dipole resonancescan be identified by the correlation of the identifiedparticles to the corresponding γ-rays. Finally, thestrength of the isoscalar pygmy-dipole resonances canbe determined. In addition, experiments already per-formed at GSI Darmstadt with the R3B setup willprofit, because with the result of this experiment, theisovector part of the resonance can be separated fromthe isoscalar part.

References1) 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).

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,∗14

C. 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,∗8

M. Kurata-Nishimura,∗2 S. Masuoka,∗7 M. Matsushita,∗7 S. Michimasa,∗7 B. Millon,∗5 T. Motobayashi,∗2

T. Murakami,∗1 T. Nakamura,∗9 T. Ohnishi,∗2 H. J. Ong,∗10 S. Ota,∗7 H. Otsu,∗2 T. Ozaki,∗9 A. Saito,∗9

H. Sakurai,∗2,∗8 H. Scheit,∗3 F. Schindler,∗3 P. Schrock,∗3 Y. Shiga,∗11,∗2 M. Shikata,∗9 S. Shimoura,∗7

D. Steppenbeck,∗2 T. Sumikama,∗6 I. Syndikus,∗3 H. Takeda,∗2 S. Takeuchi,∗2 A. Tamii,∗10 R. Taniuchi,∗8

Y. Togano,∗9 J. Tscheuschner,∗3 J. Tsubota,∗9 H. Wang,∗2 O. Wieland,∗5 K. Wimmer,∗8 Y. Yamaguchi,∗7

and J. Zenihiro∗2

Giant resonance is one of the most important phe-nomena for understanding quantum many-body sys-tems. Neutron-rich nuclei are predicted to have ex-otic giant resonances due to their smaller neutron sep-aration energy and excess neutrons. One of the ex-otic giant resonances in neutron-rich nuclei is a dipoleresonance found at excitation energies lower than 10MeV1). The nature of these resonances is of great in-terest. One of the method to understand the nature ofthese resonances is to investigate if they are iso-vectoror iso-scalar resonances. In order to study the rela-tionship between iso-vector and iso-scalar dipole reso-nances in neutron-rich oxygen isotopes, we performedan experiment at RIBF and measure the dipole reso-nances of the neutron-rich nuclei 20O, 22O, and 24O.These beams were produced via projectile fragmenta-tion of a 345MeV/nucleon 48Ca beam on 9Be targetswith mass thicknesses of 2.8 g/cm2, 2.8 g/cm2, and 2.2g/cm2. Γ rays from the excited beam particles weredetected with large volume LaBr3 crystals from INFNMilano2) in combination with DALI23). Two differenttargets, 5 g/cm2 Au for coulomb excitation and 300mg/cm3 liquid helium for inelastic α particle scatter-ing, were used to obtain the iso-vector and iso-scalerdipole strengths respectively.

A preliminary particle identification (PID) plot of Zversus A/Z for the 24O beam is shown in Fig. 1. PIDwas performed by the Bρ-∆E-TOF technique using theBigRIPS. The Bρ information was reconstructed fromthe time difference between the left- and right-handsides of the plastic scintillator installed at the disper-

∗1 Department of Physics, Kyoto University∗2 RIKEN Nishina Center∗3 Institut fur Kernphysik, Technische Universitat Darmstadt∗4 Institut fur Kernphysik, Universitat zu Koln∗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 Department

of Atomic Energy∗13 School of Physics, Peking University∗14 University of Milan

sive focal plane. The achieved purity of 20O, 22O, and24O was 73%, 66%, and 51%, respectively. PID ofthe outgoing beams was performed by the same Bρ-∆E-TOF technique using the ZD spectrometer. Low-pressure multi-wire drift chambers4) were used to mea-sure Bρ of the outgoing beams. Figure 2 shows a pre-liminary PID plot of Z versus A/Z for the outgoingbeam where an 24O beam is on a Au target. The re-action products are clearly observed. The analysis ofγ rays is in progress.

Fig. 1. PID plot of the 24O beam setting

Fig. 2. PID plot of the 24O beam and the Au target setting

on ZD spectrometer

References1) 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)4) H. Miya et al. Nucl. Instrum. Methods Phys. Res., Sec.

B 317, 701 (2013)

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Ⅱ-1. Nuclear Physics RIKEN Accel. Prog. Rep. 48 (2015)RIKEN Accel. Prog. Rep. 48 (2015) Ⅱ-1. Nuclear Physics

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