Observation of isoscalar and isovector dipole excitations in neutron-rich 20O

N Nakatsuka,B Million,P Schrock,M Shikata,A Bracco,T Murakami,N Imai,T Motobayashi,O Wieland,M Kurata-Nishimura,J Tscheuschner,V Derya,J Tsubota,F Camera,R Avigo,K Yoneda,C Caesar,M Matsushita,Satoshi Takeuchi ,F Schindler,Handing Wang ,S Koyama,K Wimmer,A Giaz,Y Togano,H Scheit,S Ceruti,H Sakuraï ,Takashi Nakamura ,I Syndikus,A Horvat,T Ozaki,R Taniuchi,Yoshinori Shiga ,S Michimasa,P Doornenbal,S Chen,Tsunenori Inakura ,T Kawabata,S Masuoka,S R Banerjee ,A Tamii,T Aumann,Y Kondo,N Kobayashi,Youhei Yamaguchi ,H Otsu,A Saito,H J Ong ,S Ota,T Ohnishi,Hiroyuki Takeda ,J Zenihiro,S Shimoura,K Ieki,D Steppenbeck,T Sumikama,H Baba

doi:10.1016/j.physletb.2017.03.017

Abstract

The isospin characters of low-energy dipole excitations in neutron-rich unstable nucleus 20O were investigated, for the first time in unstable nuclei. Two spectra obtained from a dominant isovector probe (O20+Au) and a dominant isoscalar probe (O20+α) were compared and analyzed by the distorted-wave Born approximation to extract independently the isovector and isoscalar dipole strengths. Two known 1− states with large isovector dipole strengths at energies of 5.36(5) MeV (11−) and 6.84(7) MeV (12−) were also excited by the isoscalar probe. These two states were found to have different isoscalar dipole strengths, 2.70(32)% (11−) and 0.67(12)% (12−), respectively, in exhaustion of the isoscalar dipole-energy-weighted sum rule. The difference in isoscalar strength indicated that they have different underlying structures.

Highlights

The electric dipole response, or E1 response, is one of the most interesting properties of atomic nuclei
These experiments compared the dipole excitations populated by two probes: the inelastic scattering of alpha particles, populated by a dominant isoscalar probe, and real photon scattering, populated by a dominant isovector probe. They revealed that some dipole excitations, mostly in the low-energy region, were populated by both probes. These results suggest that low-energy dipole excitations have underlying structures different from the widely observed isovector giant dipole resonance [1,2,3]
Comparison with N = Z nuclei, where pure isoscalar dipole excitations are observed, shows that the low-energy dipole excitations in N > Z nuclei differ from simple isoscalar dipole excitations [6]

Summary

Introduction

The electric dipole response, or E1 response, is one of the most interesting properties of atomic nuclei. Following an early study on a set of stable magic nuclei [4,5], recent experimental studies on 40,48Ca [6], 74Ge [7], 124Sn [8], 138Ba [9], and 140Ce [9,10] have demonstrated that low-energy dipole excitations exhibit a specific isospin character, sometimes referred to as “isospin splitting” These experiments compared the dipole excitations populated by two probes: the inelastic scattering of alpha particles, populated by a dominant isoscalar probe, and real photon scattering, populated by a dominant isovector probe.

Objectives

Results

Conclusion