Anomalous E 1 and E 2 strengths in 40 Ca and 48 Ca at low excitation energy: A comparative study

We present the results of an experimental comparative study of the ${}^{40,48}\mathrm{Ca}$ resonance spectra between 6 and 12 MeV. The power of the heavy ion reaction ${}^{40,48}\mathrm{Ca}{(}^{86}\mathrm{Kr},{ }^{86}{\mathrm{Kr}}^{\ensuremath{'}}{)}^{40,48}{\mathrm{Ca}}^{*}$ at 60 MeV per nucleon was exploited to enhance greatly the low energy part of inclusive spectra in order to look for the possible presence of a low energy dipole mode in ${}^{48}\mathrm{Ca}$ due to a neutron skin. We did not observe any difference in the $l=1$ channel and therefore found no evidence of this mode. In the $l=2$ channel, an important excess of strength was observed in ${}^{40}\mathrm{Ca}$ compared to ${}^{48}\mathrm{Ca}$.