Giant Resonances in the Doubly Magic Nucleus ⁴⁸Ca from the ( e,e^′n) Reaction

The ⁴⁸Ca(e,e^′n) reaction has been investigated for excitation energies 11–25 MeV and momentum transfers 0.22–0.43fm^-1 at the superconducting Darmstadt electron linear accelerator S-DALINAC. Electric dipole and quadrupole plus monopole strength distributions are extracted from a multipole decomposition of the spectra. Their fragmented structure is described by microscopic calculations allowing for coupling of the basic particle-hole excitations to more complex configurations. Comparison of the excitation spectrum of the residual nucleus ⁴⁷Ca with statistical model calculations reveals a 39(5)% contribution of direct decay to the damping of the giant dipole resonance.