Dipole and electric quadrupole excitations in40,48Ca

Photon scattering experiments have been performed to investigate the structure of the two doubly magic nuclei ${}^{40,48}\mathrm{Ca}.$ The method is highly selective to induce low-order multipole transitions, i.e., $E1,$ $M1,$ and $E2\mathrm{}$ from the ground state. We determined the energies and spins of excited states and the absolute strengths of the $\gamma$ decays in a model independent way. We find the summed electric dipole strengths below 10 MeV to exhaust the energy weighted sum rule by 0.023 and 0.27 %, respectively. The summed electric quadrupole strengths are $\sum B\left(E2\mathrm{}\right)\mathrm{}\uparrow =332\mathrm{}{e}^2{\mathrm{fm}}^4$ and $407e^2{\mathrm{fm}}^4$ for ${}^{40}\mathrm{Ca}$ and ${}^{48}\mathrm{Ca},$ respectively. In order to explain the difference in the $E1\mathrm{}$ strengths of the two isotopes several theoretical models are discussed.