Evaporation-residue-gated spin distribution measurements of the highly fissile compound nucleus *224 Th through 16 O+ 208 Pb and 18 O+ 206 Pb reactions

Measurements of cross sections of evaporation residue (ER) and spin distributions of heavy nuclei, produced via compound nuclear fusion-evaporation reactions, provide crucial information about the dynamics of nuclear fission and the survival probability of the system against fission. Such measurements help in probing the evolution of the compound system from equilibrium to the saddle and the underlying role of nuclear dissipation in hindering fission. The purpose of the present measurements is to understand the survival probability of the $^{224}\mathrm{Th}^{*}$ compound nucleus against fission and its dependence on angular momentum. Measurements of the ER cross sections and spin distributions have been carried out for the $^{16}\mathrm{O}+^{208}\mathrm{Pb}$ and $^{18}\mathrm{O}+^{206}\mathrm{Pb}$ reactions which form the same $^{224}\mathrm{Th}^{*}$ compound nucleus. The two reactions have been carried out at laboratory energies ranging from 87 MeV to 122.6 MeV and 85.7 MeV to 121.4 MeV, respectively. The measurements have been performed at Inter University Accelerator Centre, New Delhi using the Hybrid Recoil mass Analyzer in gas-filled mode, coupled with the $4\ensuremath{\pi}$ spin spectrometer of Tata Institute of Fundamental Research. The reduced ER cross sections (${\ensuremath{\sigma}}_{ER}/\ensuremath{\pi}{{R}_{B}}^{2}$) for both systems are comparable at low excitation energies while at higher excitation energies the $^{18}\mathrm{O}+^{206}\mathrm{Pb}$ system shows nearly 50% higher values. However, the $^{18}\mathrm{O}+^{206}\mathrm{Pb}$ system shows lower mean $\ensuremath{\gamma}$ ray multiplicity (and hence lower mean angular momentum) at all excitation energies which is a surprising result.