Use is made of a back-to-back type of double fission chamber and an electron linear accelerator-driven lead slowing-down spectrometer to measure the neutron-induced fission cross sections of 229Th and 231Pa below 10 keV relative to that of 235U. A measurement relative to the 10B(n, ) reaction is also made using a BF3 counter at energies below 1 keV and normalized to the absolute value obtained by using the cross section of the 235U(n,f) reaction between 200 eV and 1 keV.

The experimental data of the 229Th(n,f) reaction, which was measured by Konakhovich et al., show higher cross-section values, especially at energies of 0.1 to 0.4 eV. The data by Gokhberg et al. seem to be lower than the current measurement above 6 keV. Although the evaluated data in JENDL-3.2 are in general agreement with the measurement, the evaluation is higher from 0.25 to 5 eV and lower above 10 eV. The ENDF/B-VI data evaluated above 10 eV are also lower. The current thermal neutron-induced fission cross section at 0.0253 eV is 32.4 ± 10.7 b, which is in good agreement with results of Gindler et al., Mughabghab, and JENDL-3.2.

The mean value of the 231Pa(n,f) cross sections between 0.37 and 0.52 eV, which were measured by Leonard and Odegaarden, is close to the current measurement. The evaluated data in ENDF/B-VI are lower below 0.15 eV and higher above ~30 eV. The ENDF/B-VI and the JEF-2.2 are extremely higher above 1 keV. The JENDL-3.2 data are in general agreement with the measurement, although they are lower above ~100 eV.