The lifetimes in the decay of compound nuclei formed in the fusion reactions ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ → ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ and ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ + ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ → ${}_{}{}^{47}{}_{}{}^{}\mathrm{V}$ are measured to be (5–10)×${10}^{\mathrm{-}18}$ s using the blocking of evaporation residues in Si 〈100〉 single crystal. Similar measurements using low energy alpha particles evaporated in the decay of ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ yield lifetimes in the same range. However, the shape of the blocking pattern for the high energy α particles in the same decay is similar to that for the elastically scattered particles indicating a lifetime shorter than ${10}^{\mathrm{-}18}$ s. These results, along with our earlier measurements in ${}_{}{}^{44}{}_{}{}^{}\mathrm{Ti}$ and those reported by others, show that the lifetimes for the later stages of the compound nuclear decay are nearly the same for nuclei in the mass range 40–160 and are relatively insensitive to the initial excitation energy.

• Received 8 December 1994

DOI:https://doi.org/10.1103/PhysRevC.51.2439