Measurement of Neutron-Induced Fission Cross Sections of ²²⁹Th and ²³¹Pa Using Linac-Driven Lead Slowing-Down Spectrometer

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.