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The excitation energy and nucleon composition dependence of the transition from asymmetric to symmetric scission of fission observables of Th(Pa) and U(Np) nuclei is interpreted for nucleon-induced fission cross sections of 232Th(p,F)(232Th(n,F)) and 238U(p,F) (238U(n,F)) reactions at En(p) = 1–200 MeV. Predominantly symmetric fission in 232Th(p,F) and roughly equal contributions of symmetric and asymmetric modes in 238U(n,F) at En(p) = 200 MeV as revealed by experimental branching ratios is reproduced. Steep transition from asymmetric to symmetric fission with increase of nucleon incident energy is due to fission of neutron-deficient Th(Pa) (A<229) nuclei, since about 20 neutron-deficient nuclides might contribute to the fission reaction at En(p)∼200 MeV. A structure of the potential energy surface (a drop of symmetric and asymmetric fission barriers difference (ESYMf-EASYMf) from ∼3.5 MeV to ∼1 MeV) of N-deficient Pa nuclides (A<226) and available phase space at outer fission saddles, are shown to be responsible for the sharp increase with Bn(p) of the symmetric fission component contribution for Th(p,F) and 232Th(n,F) reactions. That is a strong evidence of emissive fission nature of higly excited actinides, reliably quantified only up to En(p∼20(30) MeV. In case of U nucleon-induced fission the much lower yield of the symmetric fission mode at En(p)<=200 MeV is due to much lower contribution of neutron-deficient U(Np)nuclides, which may split symmetrically. [ABSTRACT FROM AUTHOR]

Nuclear level densities have been extensively studied on or near the valley of stability. Because the data base is so sparse away from the valley of stability, the systematics found for stable nuclei are assumed to apply off the line of stability as well. A model which predicts different systematics off of the stability line is examined. A pair of recent measurements provide tentative support for the new hypothesis, though it is clear additional data are needed. [ABSTRACT FROM AUTHOR]

New energy-, spin- and parity-dependent nuclear level densities based on the microscopic combinatorial model are proposed for practical applications. The combinatorial model includes a detailed microscopic calculation of the intrinsic state density and of the rotational enhancement factor, but a phenomenological treatment of the vibrational effect. The vibrational description has been improved using a boson partition function to construct phonons' state densities which are then folded in with the incoherent particle-hole densities. The present model is shown to predict the experimental s-wave neutron resonance spacings with a degree of accuracy comparable to that of the best global models available. [ABSTRACT FROM AUTHOR]