Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 49 (2016)

E1 strength
strength aroundaround thresholdE1thresholdin
in7070Ni
Ni††
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 electric dipole response of atomic nuclei is presently
attracting increasing attention from the nuclear physics
research community. In particular, the E1 strength in neutronrich nuclei, located at around one particle separation energy(6-12 MeV energy range) is the object of a large effort1,2) (andreferences therein). In this energy region structures andaccumulations of the E1 strength were measured in a varietyof nuclei along along the entire valley of stability, but very
scarce data for exotic nuclei are available. These structures
are commonly called pygmy dipole resonance (PDR) as theylie at energies below the giant dipole resonance (GDR) andhave lesser strength. They are at the center of the scientificdebate as the strength is connected to the neutron skinthickness3)., the symmetry energy term of the nuclear
equation of state and has important astrophysicalimplications in explosive nucleosynthesis scenarios.
In order to understand better the characteristics of thisPDR strength it is important to study an isotopic chain of a
nucleus with increasing neutron number. As the pygmy
dipole strength distribution in 68Ni around the threshold has
recently been observed,1,2,3) a high-intensity and highresolution experiment was performed on 70Ni at RIKENRadioactive Isotope Beam Factory (RIBF).
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 a secondary beam of 70Ni (30 kcps with 40%
purity at a beam energy of 260 AMeV). The 70Ni isotope wasincident on a 2 g/cm2 gold secondary target. Reaction
products from the secondary target were identified using theZeroDegree Spectrometer in the large acceptance mode,
while the scattering angles were determined using parallel
plate avalanche counters.
†Experiment NP1306 -RIBF51R1, November 2014

*1

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*8

Peking University*9Rikkyo University*10

University of Tokyo*11CNS, University of Tokyo*12Tohoku University

In order to detect gamma rays from the decay of different
nuclear levels, the reaction target was surrounded by acombination of eight large-volume 3.5’’ x 8’’ LaBr3:Cedetectors (providing high efficiency and resolution5))
mounted at 30° in the forward direction and of the DALI2array6) (consisting of 96 NaI(TI) crystals) at mid andbackward angles.

Fig. 1. 2+0+gs E2 transition in 70Ni. In the inset, high-energy
 ray spectra are shown till 8 MeV. The shaded area is thecontribution from the thick gold target, deduced from themeasured spectra at backward angles and the projectile
GDR–tail calculated with a statistical model.
Figure 1 shows the first outcome of the experiment, whichis the observation of the first 2+ state of 70Ni. It is planned
that the 2+0+gs B(E2) transition strength will be determined.
In the inset an important unresolved E1 contribution between
5 and 8 MeV is observed. The E1 character is deduced fromangular distribution measurements. This issue will be underinvestigation in near future. In order to determine the E1
strength distribution in 70Ni an absolute efficiency
measurement of the detectors at NewSUBARU is planned in2016. The analysis of the relation between the strength andthe neutron skin in 70Ni together with the data of themeasured 68Ni will make an important contribution to theunderstanding of the features of the pygmy strength.
References

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

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