Precise validation of neutron cross section data bases using a lead slowing down spectrometer and simulation from 0.1 eV to 40 keV. Methodology and data on thin and thick targets and data bases adjustment

Research on accelerator driven systems (ADS), related new fuels and their ability for nuclear waste incineration has led to a revival of interest in nuclear cross sections of many nuclides over a large energy range. Discrepancies observed between nuclear data bases require new measurements in several cases. A complete measurement of such cross sections including resonance resolution consists of an extensive beam time experiment associated to a long process of analysis and validation. With a slowing down spectrometer associated to a pulsed neutron source, it is possible to determine good cross section profile in an energy range from 0.1 eV to 40 keV. These measurements performed at ISN (Grenoble) with neutron source GENEPI requires only small quantities of matter (as small as 0.1 g) and about one day of beam per target. We present measured cross section profiles and an experimental study of self-shielding effect. A CeF3 scintillator coupled with a photomultiplier detects gamma rays from neutron capture in the studied target. The neutron flux is also measured with a 233U fission detector and a 3He detector placed symmetrically to the PM in relation to the neutron source. Absolute flux values are given by activation of Au and W foils. Test of slowing down properties is done with a precision of 1% The cross section profiles can then be deduced from the target capture rate and are compared with very detailed MCNP code simulations, which reproduce the experimental set-up and provide also capture rates and neutron flux. A good agreement between experimental and simulated profiles for well-known cross sections like Au for different thicknesses is found in our energy range, and therefore validates the method and the taking into account of self-shielding. The method is then applied to 232Th which is of main interest for new fuel cycle studies, and is complementary to higher energy measurements made by Karamanis et al. Results obtained for three target thicknesses will be compared with the simulations based on different data bases. Special attention will be paid to unresolved resonances region (E> 100 eV). (authors)