Neutron capture of 232Th in the unresolved resonance region

The thorium cycle was mentioned in the potential needs of nuclear data for Accelerator Driven Systems (Massimo Salvatores, CEA-DEN/DDIN Cadarache, ND2001, Tsukuba, Japan, J.Nucl.Sci.Tech., S.2, p.4, 2002). A 10% uncertainty on the Th(n,γ) data can produce a 30% uncertainty on the proton current requirement to operate an ADS at the sub-critical level of eff keff ∼0.97. The strong impact of the 232Th(n,γ) data on the performance and safe operation of such systems is also discussed. An analysis of the existing experimental data reveals that the status of the 232Th capture data is far from the requested 2% uncertainty level. Recently 232Th average capture measurements, between 5-200 keV neutron energy, were performed at the FzK Karlsruhe (DE) using a total absorption detector consisting of a BaF crystal ball with 97% efficiency installed at the 3.75 MeV Van de Graaff. The experiments were carried out with two metallic discs of 232Th 0.5 and 1.0 mm thick respectively. The FzK average capture cross-section was measured between 5 - 200 keV neutron energy and the cross-section uncertainties estimated as 2% above 20 keV and 4% at 5 keV. A comparison of these measurements with the ENDF/BVI data files shows a reasonable agreement in the neutron energy range above 15 keV. However, discrepancies of up to 40% at lower neutron energies are observed. The same order of discrepancies is observed when comparing the FzK results with the results obtained by Macklin et al. at ORELA. To clarify these discrepancies we measured at IRMM the average capture cross-section at the GEel LINear Accelerator (GELINA)