Decay of ²⁰²Pb* formed in ⁴⁸Ca+¹⁵⁴Sm reaction using the dynamical cluster-decay model

The decay of compound nucleus ²⁰²Pb*, formed in entrance channel reaction ⁴⁸Ca+¹⁵⁴Sm at different incident energies, is studied by using the dynamical cluster-decay model (DCM) where all decay products are calculated as emissions of preformed clusters through the interaction barriers. The calculated results show an excellent agreement with experimental data for the fusion-evaporation residue cross-section σ_ER together with the fusion-fission cross-section σ_FF (taken as a sum of the energetically favored symmetric A_CN^*/2 ± 20 and near symmetric A=65–75 plus complementary fragments), and the competing, non-compound-nucleus quasi-fission cross-section σ_QF where the entrance channel is considered not to lose its identity (and hence with preformation factor P0=1). The interesting feature of this study is that the three decay processes (ER, FF and QF) are quite comparable at low energies, ER being the most dominant, whereas at higher energies FF becomes most probable followed by ER and QF. The prediction of two fission windows, the symmetric fission (SF) and the near symmetric fission (nSF) whose contribution is more at lower incident energies, suggests the presence of a fine structure effect in the fusion-fission of ²⁰²Pb*. This result is attributed to the shell effects (magic shells) playing effective role in the fragment preformation yields for ⁴⁸Ca+¹⁵⁴Sm reaction at lower excitation energies, giving rise to ^shoulders^, to an otherwise Gaussian FF mass distribution, responsible for the QF process. As a further verification of this result, absence of "shoulders" (hence, the QF component) in the decay of ¹⁹²Pb* due to ⁴⁸Ca+¹⁴⁴Sm reaction is also shown to be given by the calculations, in agreement with experiments. The only parameter of the model is the neck-length ΔR which shows that the ER occurs first, having the largest values of ΔR, and the FF and QF processes occur almost simultaneously at lower incident energies but the FF takes over QF at higher incident energies. In other words, the three processes occur in different time scales, QF competing with FF at lower incident energies. (author)