Unusual feature ofαspectra in theC12+Nb93reaction near Coulomb barrier energies

Proton and {alpha}-particle spectra have been measured at backward angles in the {sup 12}C + {sup 93}Nb reaction at E({sup 12}C)=37.5-60 MeV and in the {sup 16}O+{sup 93}Nb reaction at E({sup 16}O)=54-75 MeV. The inclusive particle spectra generally are in good agreement with the statistical model calculations except for the {alpha} spectra in the {sup 12}C-induced reaction at beam energies of 37.5-42.5 MeV which are close to the Coulomb barrier in the entrance channel. In these cases the evaporation peak is up to 2 MeV lower than its expected position from the statistical model predictions. Gamma-multiplicity-gated {alpha} spectra, measured in the same reaction, also show the aforementioned discrepancy. These observations reveal that the conventional statistical model cannot explain the {alpha}-particle spectra in the {sup 12}C-induced reactions near the Coulomb barrier.