The behavior of very high rotational states of the highly excited compound nuclear systems formed in fusion reactions is studied in the framework of the statistical theory. The very high spin states populated in these reactions results in the observation of phenomena like backbending and yrast traps. The experimental data on the angular momentum limitation on fusion probability are reproduced at higher entropy values. Shape transitions are observed for the systems ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$${+}^{12}$C, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$${+}^{16}$O, ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$${+}^{18}$O, and ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$${+}^{40}$Ca. We predict a shift in the yrast minima towards higher angular momentum states in the case of ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$${+}^{40}$Ca with the increase in the excitation energy and the entropy of the compound nuclear system. Lines of constant entropy or constant level density are found to be almost equally spaced.

• Received 25 June 1987

DOI:https://doi.org/10.1103/PhysRevC.36.1860