Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 48 (2015)

Search of E1 strength around threshold in 70Ni†
R. Avigo,*1,*2 O. Wieland,*1 A. Bracco,*1,*2 F. Camera,*1,*2 H. Baba,*3 N. Nakatsuka,*4 P. Doornenbal,*3 Y. Togano,*5J. Tscheuschner,*6 T. Aumann,*6 G. Benzoni,*1 N. Blasi,*1 K. Boretzky,*7 S. Brambilla,*1 S. Ceruti,*1,*2 S. Chen,*8F.C.L. Crespi,*1,*2 N. Fukuda,*3 A. Giaz,*1,*2 K. Ieki,*9 N. Kobayashi,*10 Y. Kondo,*5 S. Koyama,*10 T. Kubo,*3 S. Leoni,*1,*2M. Matsushita,*11 B. Million,*1 A.I. Morales,*1,*2 T. Motobayashi,*3 T. Nakamura,*5 M. Nishimura,*3 S. Nishimura,*3 H. Otsu,*3T. Ozaki,*5 L. Pellegri,*1,*2 A. Saito,*5 H. Sakurai,*3 H. Scheit,*6 P. Schrock,*6 Y. Shiga,*3 M. Shikata,*5 D. Steppenbeck,*3T. Sumikama,*12 S. Takeuchi,*3 R. Taniuchi,*10 J. Tsubota,*5 H. Wang,*3 and K. Yoneda*3The structure and nature of the E1 strength distribution
around the separation energy depends mainly on the neutronexcess. Some information in stable nuclei has been obtainedin the past but only very few data are available for exotic
neutron rich nuclei using mainly Coulomb excitation
techniques1) and references therein.
There is an ongoing discussion on the possibility of
extracting information on the neutron skin from the pygmy
resonance strength2). This quantity can be related to theisospin-dependent part of the nuclear equation of state(EOS) which in turn has relevant implications for thedescription of neutron stars. Furthermore, the presence of astrength with E1 character close to the particle threshold has
important astrophysical implications in explosive
nucleosynthesis scenarios.
In order to understand better the characteristics of thispygmy dipole strength it is important to study an isotopic
chain of a nucleus with increasing neutron number. As thepygmy dipole strength distribution in 68Ni around thethreshold has recently been studied1,2,3), a new high intensity
and high resolution experiment was performed on 70Ni atRIKEN Radioactive Isotope Beam Factory (RIBF) inNovember 2014.
A 238U primary beam was accelerated up to an energy of345 AMeV and made to impinge on a thick rotating Be
production target. In BigRIPS4) the B-E- B method wasapplied to select and purify a secondary beam of 70Ni (30kcps with 40% purity at a beam energy of 260 AMeV). The
70
Ni isotope was incident on a 2 g/cm2 thick gold secondary
target. Reaction residues from the secondary target wereidentified using the ZeroDegree Spectrometer. Scattering
angles were determined using parallel plate avalanche
counters.

To detect gamma rays from the decay of different nuclear
levels the reaction target was surrounded by a combinationof the DALI2 array5) (consisting of 96 NaI(TI) crystals) and8 large volume 3.5’’ x 8’’ LaBr3:Ce detectors6) mounted at
30° in the forward direction. These latter detectors have ahigh efficiency, excellent energy resolution for high energy
gamma rays and very good time resolution.

Fig. 1. Particle identification plot (B-E- B TOF of thesecondary beam in front of the Au reaction target. The 70Ni
isotopes are labeled.
Fig. 1 shows the particle identification plot taken during
the experiment can be seen. The data are under analysis.
The first preliminary outcome will be the 2+ state strengthof 70Ni. This known 2+0+gs E2 transition strength will betaken as a benchmark for the determination of the unknown
E1 transitions strengths below and above the threshold.
Presently high energy gamma ray events in the LaBr3:Ce
detectors and add back spectra in DALI2 are being analyzed.
The determination of the E1 strength distribution of themeasured 70Ni and the subsequent analysis of the relationbetween strength and neutron skin together with the data ofthe measured 68Ni will give a better and important
contribution to the understanding of the features of thepygmy dipole strength.

†Experiment NP1306 -RIBF51R1, November 2014
INFN sezione di Milano*2
University of Milano*3
RIKEN Nishina Center*4
Kyoto University*5
Department of Physics, Tokyo Institute of Technology*6
TU Darmstadt*7
GSI Darmstadt*8Peking University*9
Rikkyo University*10
University of Tokyo*11CNS, University of Tokyo*12Tohoku University*1

References

1) O. Wieland et al. PRL 102, 092502 (2009).
2) A. Carbone et al. Phys. Rev. C 81 041301(R) (2010).
3) D. Rossi et al. PRL 111, 242503 (2013).
4) T. Kubo et al. Prog. Theor. Exp. Phys. 03C003 (2012).
5) S. Takeuchi et al. NIM A763, 596 (2014).
6) A. Giaz et al. NIM A729, 910 (2013).

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