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

Abstract The isospin characters of low-energy dipole excitations in neutron-rich unstable nucleus 20 O were investigated, for the first time in unstable nuclei. Two spectra obtained from a dominant isovector probe ( O 20 + Au ) and a dominant isoscalar probe ( O 20 + 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 ( 1 1 − ) and 6.84(7) MeV ( 1 2 − ) were also excited by the isoscalar probe. These two states were found to have different isoscalar dipole strengths, 2.70(32)% ( 1 1 − ) and 0.67(12)% ( 1 2 − ), respectively, in exhaustion of the isoscalar dipole-energy-weighted sum rule. The difference in isoscalar strength indicated that they have different underlying structures.