Charged-particle evaporation from hot ${}^{164}$Yb compound nuclei and the role of ${}^5$He emission

A systematic study of the decay properties of ${}^{164}$Yb compound nuclei with excitation energies of 100–300 MeV was performed. The emission patterns of light charged particles detected in coincidence with evaporation residues produced in ${}^{64}$Ni+${}^{100}$Mo and ${}^{16}$O+${}^{148}$Sm reactions have been measured and compared to statistical model predictions. There is found to be significant disagreement between the experiment and the model calculations in the magnitude of the multiplicities and the peak position in the energy spectra. For deuterons and tritons, significant nonstatistical components were present at all excitation energies. The proton energy spectrum did not exhibit any entrance-channel dependence at an excitation energy of $\sim 170$ MeV. However for $\alpha$ particles, there is an enhancement in the spectrum at the lowest kinetic energies for the more symmetric reaction. It is suggested that the emission and subsequent decay of ${}^5$He fragments, can account for a large fraction of the apparent, and often reported, decrease in the Coulomb barrier for $\alpha$ particle emission.