Measurements of neutron-induced fission cross section of protactinium-231 from 0.1 eV to 10 keV with lead slowing-down spectrometer and at 0.0253 eV with thermal neutron facility

Protactinium(Pa)-231 is one of the most interesting nuclei which are related to the production of 232U in the 232Th-233U fuel cycle. The fission phenomena, fission energy and mass distributions for the neutron-induced fission of 231Pa have been investigated. Although several measurements of the 231Pa(n, f) cross section have been reported at higher energies, the fission cross section has rarely been measured in the lower/resonance energy region. One of the reasons may be due to sub-barrier fission which results in a low fission cross section. In addition, 231Pa is a radioactive actinide element. Then, a pure sample and an intense neutron source are required to overcome the severe experimental conditions for the fission cross section measurement. The thermal neutron-induced fission cross section of 231Pa was measured by Ghiorso and Van Winkle using a fission chamber and by Wagemans et al. with a surface barrier detector to detect the fission fragments at a neutron guide of the ILL. Mughabghab shows the thermal neutron cross section in the literature. Leonard and Odegaarden made the cross section measurement at 20 energy points between 0.37 eV and 0.52 eV by use of a crystal spectrometer. No experimental datum has been obtained at energies between 0.0253 eV and 100 keV except for those measured by Leonard and Odegaarden. The evaluated fission cross section data appear in ENDF/B-VI, JENDL-3.2 and JEF-2.2 (which is essentially same as ENDF/B-VI). The ENDF/B-VI data are markedly discrepant from the JENDL-3.2 data in the relevant energy region, especially above about 10 eV. In the present study, the neutron-induced fission cross section of 231Pa has been measured in the range of 0.1 eV to 10 keV relative to that of the 235U(n, f) reaction by making use of a back-to-back (BTB) type double fission chamber and a lead slowing-down spectrometer driven by a 46 MeV electron linear accelerator (linac) of the Research Reactor Institute, Kyoto University (KURRI). Below 1 keV, the relative measurement to the 10B(n, α) reaction has also been made with a BF3 counter, as we did before. In addition, we have employed a Maxwellian distribution neutron spectrum field of the Kyoto University Reactor (KUR) for the thermal neutron cross section measurement with the BTB chamber. The result obtained is compared with the measured and the evaluated data. (author)