An extended version of Strutinsky's macro~microscopic method is used to calculate effective potential energies for rotating, excited heavy compound nuclei undergoing fission.Nuclear deformation is parameterized in terms of Lawrence's family of shapes. A two~center single~particle potential corresponding to these shapes is employed, with BCS pairing added.Statistical excitation is introduced by temperature~ dependent occupation of (quasi-) particle energy levels. We calculate shell corrections to the energy, the free energy and the entropy as functions of deformation and temperature. The We have also calculated the effective potential for constant entropy rather than constant temperature. Although this isentropic process physically is more appropriate than the isothermal ocess, it has not been treated before. For the same amount of excitation energy in the spherical state of the compound nucleus, the isentropic barriers turn out higher than the isothermal ones. For both processes we have extracted the critical angular momentum (defined as the one for which the barrier approximately vanishes) as a function of excitation.
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