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 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 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.