Isovector Quadrupole Resonance in 13C(γ, n) Reaction

The photoneutron cross section for 13 C was measured in the tagged photon energy range E γ =17–70 MeV. Substantial forward to backward asymmetry was observed above the giant dipole resonance region. The observed asymmetries were analyzed by the direct and semi-direct model, assuming two isovector E2 resonances, which locate at E x =30.9 ±1.5 MeV with Γ=8 ±6 MeV and E x =43.2 ±1.8 MeV with Γ=13 ±6 MeV. They are identified as the T isospin components of the isovector quadrupole strengths, which exhaust most of the sum rule.