Fission of light actinides: ${}^{232}\mathrm{Th}$($n$,$f$) and ${}^{231}\mathrm{Pa}$($n$,$f$) reactions

A model to describe fission on light actinides, which takes into account transmission through a triple-humped fission barrier with absorption, is proposed. The fission probability derived in the WKB approximation within an optical model for fission has been incorporated into the statistical model of nuclear reactions. The complex resonant structure in the first-chance neutron-induced fission cross sections of ${}^{232}\mathrm{Th}$ and ${}^{231}\mathrm{Pa}$ nuclei has been reproduced by the proposed model. Consistent sets of parameters describing the triple-humped fission barriers of ${}^{233}\mathrm{Th}$ and ${}^{232}\mathrm{Pa}$ have been obtained. The results confirm the attribution of the gross resonant structure in the fission probability of these light actinides to partially damped vibrational states in the second well and undamped vibrational states in the third well of the corresponding fission barriers.

Figure 1

Triple-humped fission barrier parametrization.

Figure 2

Mechanisms of transmission across the triple-humped fission barrier.

Figure 3

Total cross section for interaction of neutrons on ${}^{232}\mathrm{Th}$. Experimental data were taken from EXFOR [25, 26, 27, 29, 30, 31].

Figure 4

Nonelastic cross section of ${}^{232}\mathrm{Th}$. Experimental data were taken from EXFOR [32, 33, 34, 35].

Figure 5

Neutron capture cross section of ${}^{232}\mathrm{Th}$. Experimental data were taken from EXFOR [36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47].

Figure 6

Neutron emission cross sections at 30° for 2.03, 4.25, and 6.1 MeV incident neutron energy on ${}^{232}\mathrm{Th}$. Experimental data measured by Baba et al. [48]. 4.25 and 6.1 MeV plots are scaled by ${10}^3$ and ${10}^6$ factors, respectively. Fission neutrons' contribution is not shown.

Figure 7

Neutron-induced fission cross section of ${}^{232}\mathrm{Th}$ from 0.5 to 5.9 MeV. Experimental data were taken from EXFOR [55, 56, 57, 58, 59, 60, 61].

Figure 8

Neutron-induced fission cross section of ${}^{232}\mathrm{Th}$ near the fission threshold. Experimental data were taken from EXFOR [55, 56, 57, 58, 59, 60, 61].

Figure 9

Neutron-induced fission cross section of ${}^{231}\mathrm{Pa}$ from 0.1 to 5 MeV. Experimental data were taken from references [65, 66, 67, 68, 69, 70, 71].

Figure 10

Neutron-induced fission cross section of ${}^{231}\mathrm{Pa}$. Experimental data were taken from references [65, 66, 67, 68, 69, 70, 71, 72, 73].