Influence of Single Particle Excitations on Barrier Distributions: $^{24}$Mg+$^{90,92}$zr

We present preliminary results of barrier height distribution measurements for Mg-24+Zr-90,Zr-92 systems. The experiment was performed at LNS INFN in Catania with the CHIMERA detector system and a Mg-24 beam accelerated by Tandem MP. The measurements were done at the near-barrier beam energies of 68-88.5 MeV. A discrepancy between experimental results and the predictions of Coupled Channels (CC) calculations was observed. We suggest that this discrepancy may be due to a cumulative effect of many individual weak channels such as non-collective excitations of the target, which cannot be fully implemented in practical CC calculations of a standard form.