Unexpected Entrance-Channel Effects in the Decay of the Compound NucleusEr156

Angular momentum distributions for the dominant decay channels of ${}_{}{}^{156}{}_{}{}^{}\mathrm{Er}$ compound nuclei have been studied with the Darmstadt-Heidelberg "crystal ball" detector in nearly mass-symmetric (${}_{}{}^{64}{}_{}{}^{}\mathrm{Ni}$+${}_{}{}^{92}{}_{}{}^{}\mathrm{Zr}$) and asymmetric (${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$+${}_{}{}^{144}{}_{}{}^{}\mathrm{Sm}$) entrance channels. Strong differences in the $\alpha n$ yield and $\frac{2_n}{3_n}$ cross-section ratios are observed at the same excitation energy (47 MeV) and spin in ${}_{}{}^{156}{}_{}{}^{}\mathrm{Er}$. This effect indicates that there is memory of the entrance channel during the particle-evaporation stage of the compound-nucleus decay. The dominant exit channels do not exhibit high-angular-momentum components.