Elastic and inelastic α scattering onAr36atEα=40, 48, and 54 MeV

Differential cross sections for elastic and inelastic α- scattering on ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ar}$ have been measured at 40, 48, and 54 MeV. The data have been analyzed in the coupled-channel approach using double-folded optical α${\mathrm{-}}^{36}$Ar potentials considering the model of the symmetric rotator and the harmonic vibrator. The octupole transition probability deduced in the harmonic-vibrator model is almost identical to the value obtained from electromagnetic probes. In the symmetric-rotator model only the choice of negative ${\mathrm{\beta }}_2$ and ${\mathrm{\beta }}_4$ deformation parameters results in consistent fits to the data at all incident energies. This unambiguously proves the oblate shape of ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ar}$. The volume integrals and rms radii of the extracted optical potentials fit well into the systematics obtained for lighter sd-shell nuclei.