Ⅱ-1. RIKEN Accel. Prog. Rep. 51 (2018)

20 11 Observation of isoscalar and isovector dipole excitations in O† -neutron correlation in Borromean nucleus Li p, pn 1, 2 2 10 3 5, 14 12 5, 14 via the ( ) reaction N. Nakatsuka,∗ ∗ H. Baba,∗ N. Aoi,∗ T. Aumann,∗ R. Avigo,∗ ∗ S. R. Banerjee,∗ A. Bracco,∗ ∗ 3 5, 14 5, 14 13, 2 4 2 5, 14 C. Caesar,∗ F. Camera,∗ ∗ S. Ceruti,∗ ∗ S. Chen,∗ ∗ V. Derya,∗ P. Doornenbal,∗ A. Giaz,∗ ∗ 1, 2 3 3 2 4 3 3 1 3 11 7 1 2 8 9 8 Y. Kubota,∗ ∗ A. Corsi,∗ G. Authelet,∗ H. Baba,∗ C. Caesar,∗ D. Calvet,∗ A. Delbart,∗ M. Dozono,∗ A. Horvat,∗ K. Ieki,∗ N. Imai,∗ T. Kawabata,∗ K. Yoneda,∗ N. Kobayashi,∗ Y. Kondo,∗ S. Koyama,∗ 5 6 3 7 3 3 8, 2 2 7 7 7 5 2 J. Feng,∗ F. Flavigny,∗ J.-M. Gheller,∗ J. Gibelin,∗ A. Giganon,∗ A. Gillibert,∗ K. Hasegawa,∗ ∗ M. Kurata-Nishimura,∗ S. Masuoka,∗ M. Matsushita,∗ S. Michimasa,∗ B. Millon,∗ T. Motobayashi,∗ 2 9, 2 9, 2 10, 2 1 1 11 1 9 2 10 7 2 9 9 T. Isobe,∗ Y. Kanaya,∗ ∗ S. Kawakami,∗ ∗ D. Kim,∗ ∗ Y. Kiyokawa,∗ M. Kobayashi,∗ N. Kobayashi,∗ T. Murakami,∗ T. Nakamura,∗ T. Ohnishi,∗ H. J. Ong,∗ S. Ota,∗ H. Otsu,∗ T. Ozaki,∗ A. T. Saito,∗ 8, 2 12, 2 13, 2 11, 2 3, 2 9 2, 8 3 3 3 11, 2 9 7 T. Kobayashi,∗ ∗ Y. Kondo,∗ ∗ Z. Korkulu,∗ ∗ S. Koyama,∗ ∗ V. Lapoux,∗ ∗ Y. Maeda,∗ H. Sakurai,∗ ∗ H. Scheit,∗ F. Schindler,∗ P. Schrock,∗ Y. Shiga,∗ ∗ M. Shikata,∗ S. Shimoura,∗ 7 2 11 12, 2 14, 2 15, 2 2 6, 2 3 2 2 10 8 F. M. Marqu´es,∗ T. Motobayashi,∗ T. Miyazaki,∗ T. Nakamura,∗ ∗ N. Nakatsuka,∗ ∗ Y. Nishio,∗ ∗ D. Steppenbeck,∗ T. Sumikama,∗ ∗ I. Syndikus,∗ H. Takeda,∗ S. Takeuchi,∗ A. Tamii,∗ R. Taniuchi,∗ 3, 2 15, 2 7 1 2 12, 2 2 4 9 3 9 2 5 8, 2 7 A. Obertelli,∗ ∗ A. Ohkura,∗ ∗ N. A. Orr,∗ S. Ota,∗ H. Otsu,∗ T. Ozaki,∗ ∗ V. Panin,∗ S. Paschalis,∗ Y. Togano,∗ J. Tscheuschner,∗ J. Tsubota,∗ H. Wang,∗ O. Wieland,∗ K. Wimmer,∗ ∗ Y. Yamaguchi,∗ 3 16 3 12, 2 15, 2 2 2 E. C. Pollacco,∗ S. Reichert,∗ J.-Y. Rouss´e,∗ A. T. Saito,∗ ∗ S. Sakaguchi,∗ ∗ M. Sako,∗ and J. Zenihiro∗ 3, 2 2 2 12, 2 2 15, 2 2 C. Santamaria,∗ ∗ M. Sasano,∗ H. Sato,∗ M. Shikata,∗ ∗ Y. Shimizu,∗ Y. Shindo,∗ ∗ L. Stuhl,∗ 8, 2 15, 2 12, 2 12, 2 2 2 T. Sumikama,∗ ∗ M. Tabata,∗ ∗ Y. Togano,∗ ∗ J. Tsubota,∗ ∗ T. Uesaka,∗ Z. H. Yang,∗ The electric dipole response, or E1 response, is sition strengths, a distorted-wave Born approxima- J. Yasuda, 15, 2 K. Yoneda, 2 and J. Zenihiro 2 one of the most interesting properties of atomic nu- tion (DWBA) analysis was performed by using the ∗ ∗ ∗ ∗ clei. In medium to heavy neutron-rich nuclei, the elec- Ecis97 code.12) As nuclear potential, we employed 1) tric dipole excitation is fragmented into a low-energy the theoretically developed global optical potential de- Since a theoretical prediction was made by Migdal, opening angle of two valence cos θY was re- region around the neutron , so- scribed in Refs. 13–15). The transition strengths of a hypothetical bound state of two neutrons, dineu- constructed from momentum vectors of all the particles 1,2) tron, has attracted much attention. The neutron- involved in the reaction. The obtained cos θY distribu- called Pygmy dipole resonance. Recent experimen- the 1− states were determined in the same manner, tal studies on 40,48Ca,6) 74Ge,7) 124Sn,8) 138Ba,9) and by including both the Coulomb and nuclear contribu- neutron correlation caused by the dineutron is ex- tion is shown in Fig. 1. The geometrical acceptance 140Ce9,10) have demonstrated that low-energy dipole tions in either system, with the assumption that the pected to appear in weakly bound systems, such as of the experimental setup was corrected by performing 11 excitations exhibit a specific isospin character, some- Coulomb potential contributed only to the isovector the Borromean nucleus Li. There have been exten- a Monte-Carlo simulation. The asymmetric distribu- 11 times referred to as “isospin splitting.” They demon- dipole strength and the nuclear potential contributed sive studies to search for such a correlation in Li. tion indicates an admixture of different parity states 11 strated that some dipole excitations, mostly in the only to the isoscalar dipole strength. The Harakeh- E1 strengths deduced from Coulomb dissociation cross and the dineutron correlation in Li. The asymme- low-energy region, were populated by both isoscalar Dieperink dipole form factor11) was employed to de- sections have been used by employing the E1 clus- try obtained in the present work is weaker than that 2) and isovector probes. In this work, the isospin charac- termine the isoscalar dipole strength. The strengths ter sum rule to characterize their correlation. How- in the previous work employing the neutron removal 9) ter of low-energy dipole excitations in neutron-rich un- were determined so that the experimental cross sec- ever, the model dependence was not negligible owing reaction by using a carbon target. We presume that 9 stable nucleus 20O was investigated, for the first time tions from both the 20O+α and 20O+Au systems were to the Li core excitation and the final-state interac- the dineutron correlation was overestimated in the pre- tions.3) The kinematically complete measurement of vious study because of the sensitivity of the probe; the in unstable nuclei. The experiment was performed at reproduced by the same isoscalar and isovector dipole Fri Jan 19 05:55:00 2018 20 the quasi-free (p, pn) reaction was thus performed with probe used in the previous study is only sensitive to Radioactive Beam Factory (RIBF). The O strengths. The 1− state (5.36(5) MeV) had an isoscalar 1 11 14 17,19 beam impinged on two different reaction targets, a dipole strength of 2.70(32)% in ISD EWSR, while the Borromean nuclei Li, Be, and B at the RIBF so the nuclear surface, where the dineutron correlation is as to determine the neutron momentum distributions expected to develop. 2.45(5) mm gold target as an isovector probe, and a 1− state (6.84(7) MeV) had a strength of 0.67(12)% 2 3 317(28) mg/cm2 liquid helium target as an isoscalar in Isoscalar dipole energy-weighted sum-rule fraction that provide more direct information of the ground- ×10 state correlation.4) probe. The decay γ rays from the excited beam par- (ISD EWSR). These states, however, have comparable 600 2 The measurement required a high luminosity to have ticles were detected with large volume LaBr3 crystals isovector dipole strengths: B(E1) =3.57(20) 10−

3) 2 2 ↑ × 2 as much statistics as possible. For this purpose, the /0.1 from INFN Milano. Two low-energy dipole states at e fm for the 11− state and B(E1) =3.79(26) 10− 5) acc

2 2 ↑ × 15-cm-thick liquid hydrogen target MINOS was in- ε 400 energies of 5.36(5) MeV (1−) and 6.84(7) MeV (1−), e fm for the 1− state. The results indicate that low- 1 2 2 6) previously known to be populated by the Coulomb energy dipole excitations in 20O exhibit a dual charac- troduced. The SAMURAI spectrometer contributed Fit 4,5) to minimize experimental biases originating from the 200 excitation, were consistently populated both by ter. The difference in isoscalar response suggests that Counts/ This work 9) the isoscalar and isovector probe. The decay scheme these states have different underlying structures. geometrical acceptance. A missing-mass setup com- Ref. (scaled) posed of the neutron detector WINDS,7) the recoil pro- of those states were determined by the γ-γ coinci- 0 + ton detector RPD, and the gamma-ray detector array -1 -0.5 0 0.5 1 dence analysis, and the decay branch via the 21 state References Opening angle cosθ DALI28) was newly configured for realizing the quasi- Y (1.67 MeV) was observed. 1) A. Bracco et al., Euro. Phys. J. A 51, 99 (2015). 11 Fig. 1. Opening-angle distribution cos θY for Li. The In order to extract the cross sections and tran- 2) D. Savran et al., Prog. Part. Nucl. Phys. 70, 210 free (p, pn) measurement. (2013). As a measure of the dineutron correlation in 11Li, the blue open and black closed marks represent the data taken in the present and previous works,9) respectively. 3) A. Giaz et al., Nucl. Instrum. Methods Phys. Res. A 1 † Condensed from the article in Phys. Lett. B. 768, 387 (2017) ∗ Center for Nuclear Study, University of Tokyo 1 729, 910 (2013). 2 ∗ Department of Physics, Kyoto University ∗ RIKEN Nishina Center References 2 4) E. Tryggestad et al., Phys. Lett. B 541, 52 (2002). 3 ∗ RIKEN Nishina Center ∗ CEA, Saclay 3 4 1) A. B. Migdal, Sov. J. Nucl. Phys. 16, 238 (1973). ∗ Institut f¨urKernphysik, Technische Universit¨atDarmstadt 5) E. Tryggestad et al., Phys. Rev. C 67, 064309 (2003). ∗ Department of Physics, Technische Universit¨atDarmstadt 4 5 2) T. Nakamura et al., Phys. Rev. Lett. 96, 252502 (2006). ∗ Institut f¨urKernphysik, Universit¨atzu K¨oln 6) V. Derya et al., Phys. Lett. B 768, 387 (2017). ∗ Department of Physics, Peking University 5 6 3) Y. Kikuchi et al., Phys. Rev. C 87, 034606 (2013). ∗ Istituto Nazionale di Fisica Nucleare Milan 7) D. Negi et al., Phys. Rev. C 94, 024332 (2016). ∗ IPN Orsay 6 7 ∗ Department of Physics, Tohoku University ∗ LPC Caen 4) Y. Kikuchi et al., Prog. Theor. Exp. Phys. 103D03 (2016). 7 8) J. Endres et al., Phys. Rev. Lett. 105, 212503 (2010). 8 ∗ Center for Nuclear Study, The University of Tokyo ∗ Department of Physics, Tohoku University 5) A. Obertelli et al., Eur. Phys. Jour. A 50, 8 (2014). 8 9) J. Endres et al., Phys. Rev. C 80, 034302 (2009). 9 ∗ Department of Physics, The University of Tokyo ∗ Department of Applied Physics, University of Miyazaki 6) T. Kobayashi et al., Nucl. Instrum. Methods Phys. Res. 9 10) D. Savran et al., Phys. Rev. Lett. 97, 172502 (2006). 10 ∗ Department of Physics, Tokyo Institute of Technology ∗ Department of Physics, Ehwa Womans University B 317, 294 (2013). 10 11 ∗ Research Center for Nuclear Physics, Osaka University 11) M. N. Harakeh et al., Phys. Rev. C 23, 2329 (1981). ∗ Department of Physics, University of Tokyo 11 12 7) J. Yasuda et al., Nucl. Instrum. Methods Phys. Res. B ∗ Department of Physics, Rikkyo University 12) J. Raynal et al., Phys. Rev. C 23, 2571 (1981). ∗ Department of Physics, Tokyo Institute of Technology 12 13 376, 393 (2016). ∗ Variable Energy Cyclotron Centre, The Indian Department 13) T. Furumoto et al., Phys. Rev. C 85, 044607 (2012). ∗ MTA Atomki 14 8) S. Takeuchi et al., Nucl. Instrum. Methods Phys. Res. of Atomic Energy 14) T. Furumoto et al., Phys. Rev. C 80, 044614 (2009). ∗ Department of Physics, Kyoto University 13 15 ∗ School of Physics, Peking University ∗ Department of Physics, Kyushu University A 763, 596 (2014). 14 15) T. Furumoto et al., Phys. Rev. C 78, 044610 (2008). 16 ∗ University of Milan ∗ Department of Physics, Technische Universit¨atM¨unchen 9) H. Simon et al., Phys. Rev. Lett. 83, 496 (1999).

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