Nuclear Science and Engineering / Volume 152 / Number 1 / January 2006 / Pages 1-14
The neutron capture cross section of thorium has been measured in the energy region between 4 and 140 keV at the GELINA time-of-flight facility of the Institute for Reference Materials and Measurements in Geel, Belgium. The gamma rays from capture events were detected by two C6D6 liquid scintillators, placed 14.37 m from the neutron source. The shape of the neutron flux was measured with a 10B-loaded ionization chamber. To obtain a detection efficiency independent of the gamma cascade and proportional to the total excitation energy, the pulse-height weighting technique was applied. The data have been normalized to the well-isolated and almost saturated 232Th resonance at 23.5 eV. The systematic uncertainties related to the normalization and weighting function, using an internal saturated resonance, are ~1.5%. An additional systematic uncertainty of 0.5% results from the self-shielding and multiple scattering corrections.
Between 4 and 140 keV, our data are ~9 and 6.5% higher than the data of Kobayashi et al. and Macklin et al., respectively, and in good agreement with the data of Poenitz and Smith. Below 15 keV our data deviate by up to 30% from the data reported by Wisshak et al. Our data have been analyzed in terms of average level parameters. The resulting parameters are consistent with the resolved resonance parameters deduced from the transmission measurements of Olsen et al.