Neutron Capture Cross Section of 232Th

The neutron capture cross section of 232Th has been measured in the energy range from 5 to 225 keV at the Karlsruhe 3.7-MV Van de Graaff accelerator relative to the gold standard. Neutrons were produced via the 7Li(p,n)7Be reaction by bombarding metallic Li targets with a pulsed proton beam, and capture events were registered with the Karlsruhe 4(pi) barium fluoride detector. The main difficulty in this experiment is the detection of true capture events characterized by a comparably low binding energy of 4.78 MeV in the presence of the high-energy gamma background (up to 3.96 MeV) associated with the decay chain of the natural thorium sample. With the high efficiency and the good energy resolution of the 4(pi) detector, the sum energy peak of the capture cascades could be reliably separated from the background over the full range of the neutron spectrum, yielding cross-section uncertainties of ~2% above 20 keV and of 4% at 5 keV. The clear identification of the various background components represents a significant improvement compared to existing data for which sometimes high accuracy was claimed, but which were found to be severely discrepant. A comparison to the evaluated files shows reasonable agreement in the energy range above 15 keV, but also severe discrepancies of up to 40% at lower neutron energies.