Neutron decay of the pygmy and giant resonances in the 13C(e,e′n)12C reaction

The cross sections and angular correlations for neutron emission to various states in the residual nucleus, following the C-13(e,e' n) reaction, have been measured over the excitation energy range 8-27 MeV at the effective momentum transfer of 0.35 fm(-1). In the pygmy resonance, neutron emission leads predominantly to the ground and first excited states of C-12, while in the giant resonance it leads to population of two higher excited states (12.71 MeV, T = 0; 15.11 MeV, T = 1). This is the first direct observation for neutron population of these states. The isospin assignments of the resonances are made as a result of the observed decays. The branching ratio of the transition to the T = 1 states compared to the total cross section for neutron emission is found to be 19 +/- 5%, which is inconsistent with the theoretical predictions of about 50%. Both isospin spitting and the relative T-< and T-> strengths of the giant resonance are obtained for the neutron decay. The angular correlation parameters for the decay of the giant dipole resonance are very similar for C-13 and C-12, which provides some support for a weak coupling model in which the giant resonance for C-13 proceeds via core excitations in C-12. [S0556-2813(99)02509-1].