Abstract
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 neutroninduced 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(6 Li,4 He)234Pa and 232 Th(6 Li,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 fissilities 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 f f symmetric and asymmetric fission barriers difierence (EfSYM - EfASYM) 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 En( p) of the symmetric fission component contribution for 232Th(p,F) and 230-233 Pa(n, F) reactions. That is a strong evidence of emissive fission nature of moderately excited Pa nuclides, reliably quantified only up to En( p)~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.
Highlights
Neutron-induced cross sections of 231Pa(n,F) and 233Pa(n,F) data [1,2,3] when complemented with surrogate fission data, measured in reactions 232Th(3He,d)233Pa, 231Pa(d,p)232Pa, 230Th(3He,d) 231Pa and 230Th(3He,t)230Pa at excitation energies 6-11.5 MeV [4] and fission probabilities measured in [5] via reactions 232Th(3He,p)234Pa, 232Th(3He,d)233Pa and 232Th(3He,t)232Pa at excitation energies 6-15 MeV pose a number of severe problems for consistent theoretical description
Symmetric/asymmetric (p(n),xnf) contributions to observed fission cross sections are largely defined by the level density parameters a f and an for fissioning and residual nuclides and damping of the rotational modes contributions to the level densities and saddle asymmetries [14, 23, 33]
That means in case of p+232Th interaction the fissilities of Pa nuclei are relatively higher than those of respective Th nuclei for the n+232Th interaction, which influences the observed fission cross section at En(p)¡100 MeV
Summary
Neutron-induced cross sections of 231Pa(n,F) and 233Pa(n,F) data [1,2,3] when complemented with surrogate fission data, measured in reactions 232Th(3He,d)233Pa, 231Pa(d,p)232Pa, 230Th(3He,d) 231Pa and 230Th(3He,t)230Pa at excitation energies 6-11.5 MeV [4] and fission probabilities measured in [5] via reactions 232Th(3He,p)234Pa, 232Th(3He,d)233Pa and 232Th(3He,t)232Pa at excitation energies 6-15 MeV pose a number of severe problems for consistent theoretical description. In an emissive fission domain data by Petit et al [5], as well as older indirect data by Birgul et al [6] provoke assumption of steep decrease of the first-chance fission cross sections of 233Pa(n,F) and 231Pa(n,F) and systematically lowered fission probabilities of relevant Pa nuclides [7]. Emissive fission threshold the sensitivity to the angular momentum may again increase. Developed surrogate ratio method [10] largely removes the uncertainty, imposed by preequilibrium effects and different angular momentum spectra of excited and fissioning states in (n,F) and transfer reactions.
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