Protactinium neutron-induced fission up to 200 MeV

The theoretical evaluation of 230-233Pa(n,F) cross sections is based on direct data, 230-234Pa fission probabilities and ratios of fission probabilities in first-chance and emissive fission domains, surrogate for neutron-induced fission. First chance fission cross sections trends of Pa are based on consistent description of 232Th(n,F), 232Th(n,2n) and 238U(n,F), 238U(n,xn) data, supported by the ratio surrogate data by Burke et al., 2006, for the 237U(n,F) reaction. Ratio surrogate data on fission probabilities of 232Th(6Li,4He)234Pa and 232Th(6Li,d)236U by Nayak et al., 2008, support the predicted 233Pa(n,F) cross section at En=11.5-16.5 MeV. The predicted trends of 230-232Pa(n,F) cross section up to En=20 MeV, are consistent with fissile properties of Pa nuclides, extracted by 232Th(p,F) (Isaev et al., 2008) and 232Th(p,3n) (Morgenstern et al., 2008) data analysis. The excitation energy and nucleon composition dependence of the transition from asymmetric to symmetric scission for fission observables of Pa nuclei is defined by analysis of p-induced fission of 232Th at Ep=1-200 MeV. Predominantly symmetric fission in 232Th(p,F) at En(p)=200 MeV as revealed by experimental branching ratios (Dujvestijn et al., 1999) is reproduced. Steep transition from asymmetric to symmetric fission with increase of nucleon incident energy is due to fission of neutron-deficient Pa (A≤229) nuclei. A structure of the potential energy surface (a drop of symmetric and asymmetric fission barriers difference (Ef(SYM) - Ef(ASYM)) from about 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 En(p) of the symmetric fission component contribution for 232Th(p,F) and 230-233Pa(n,F) reactions. That is a strong evidence of emissive fission nature of moderately excited Pa nuclides, reliably quantified only up to En(p) about 20(30) MeV. Predicted fission cross section of 232Pa(n,F) coincides with that of 232Th(p,F) at En(p)≥80 MeV, that means that entrance channel dependence of fission cross section with increase of nucleon incident energy diminishes. (authors)