Investigation of large angle structure in α-scattering from calcium isotopes betweenEα=36–61 MeV
Elasticα-scattering angular distributions have been measured atEα=36.2 MeV, 39.6 MeV, 42.6 MeV, 49.5 MeV, 61.0 MeV for40Ca and atEα=36.2 MeV, 42.6 MeV, 49.5 MeV and 61.0 MeV for44Ca, respectively. At backward angles the data display an oscillatory structure forEα<50 MeV and more smoothly decreasing slopes atEα =61.0 MeV resembling the data obtained at higher energies. The40Ca data belowEα =50 MeV show the well known backward enhancement, which at 42.6 MeV and 49.5 MeV can be fitted by aP142 and aP162 respectively. Together with previous data,PL2-structures have now been observed for allL-values betweenL=8 andL=16. The slope of the curveL(L+1) versus excitation energy is slightly smaller forL>12 than forL<12. Optical model analyses (within a Woods-Saxon-folding model) lead to large differences between the44Ca and40Ca parameters. Furthermore, in our parametrization, the40Ca real potential depth shows dramatic changes with energy. This feature seems outside the domain of the optical model and requires consideration of additional effects (e.g. antisymmetrization) not included in the standard optical model. Present microscopic calculations on the basis of the Resonating Group Method are discussed in connection with the characteristic features ofα-scattering from40Ca.