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

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 (²⁰O + Au) and a dominant isoscalar probe (²⁰O + 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.