Protactinium neutron-induced fission up to 200 MeV

Joint Institute for Nuclear and Energy Research, 220109, Minsk-Sosny, Belarus

^a e-mail: maslov@sosny.bas-net.by

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 E_n=11.5-16.5 MeV. The predicted trends of ^230-232Pa(n, F) cross section up to E_n=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 E_p=1-200 MeV. Predominantly symmetric fission in ²³²Th(p,F) at E_{n( 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 (E^f_SYM - E^f_ASYM) 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 E_{n( 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 E_{n( p)}~20(30) MeV. Predicted fission cross section of ²³²Pa(n,F) coincides with that of ²³²Th(p,F) at E_n(p)≥80 MeV, that means that entrance channel dependence of fission cross section with increase of nucleon incident energy diminishes.