• Energy Research

The (n,γ ) cross section of 96Zr has been investigated at the CERN n TOF spallation neutron source. Highresolution time-of-flight measurements using an enriched ZrO2 sample allowed us to analyze 15 resonances below 40 keV with improved accuracy. On average, the capture widths were found to be 25% smaller than reported in earlier experiments. If complemented with the contribution by direct radiative capture, the derived Maxwellian averaged cross sections are consistent with activation data at kT = 25 keV. The present results confirm the astrophysical implications for the s-process branching at 95Zr.

The evaluated $^{16}\mathrm{O}(n,\ensuremath{\alpha})$ cross sections in the ENDF/B-VIII.0 nuclear data library remain uncertain because of systematic discrepancies in the measured data. In the energy region below the first excited state, $R$-matrix analyses rely heavily on the inverse reaction, and in particular, they rely on the measured $^{13}\mathrm{C}(\ensuremath{\alpha},n)^{16}\mathrm{O}$ cross sections reported by Bair et al. in 1973 and Harissopulos et al. in 2005. The Harissopulos cross section values are systematically lower than those previously reported by Bair. Drawing on the available experimental information, this paper briefly describes and demonstrates that the two sets of measured cross sections are consistent.

A method for a reliable estimate of uncertainties and associated covariances of integral cross sections obtained via model calculations is proposed. The method accounts for uncertainties of model parameters, the numerical implementation errors and the model deficiencies, where emphasis is given on the latter one. Covariance matrices for phenomenological models as well as for almost parameter‐free microscopic calculations are obtained on the same footing. In a schematic example the method is applied for the determination of the contribution of model deficiencies associated with the optical model and multistep reaction models to the covariance matrices of 56Fe.

The neutron-induced fission cross-section of 233U has been measured at the CERN n TOF facility relative to the standard fission cross-section of 235U between 0.5 and 20MeV. The experiment was performed with a fast ionization chamber for the detection of the fission fragments and to discriminate against α-particles from the natural radioactivity of the samples. The high instantaneous flux and the low background of the n TOF facility result in data with uncertainties of ≈ 3%, which were found in good agreement with previous experiments. The high quality of the present results allows to improve the evaluation of the 233U(n,f) cross-section and, consequently, the design of energy systems based on the Th/U cycle.

The neutron-induced fission cross section of 245Cm was measured at n_TOF in a wide energy range and with high resolution. The energy dependence, measured in a single measurement from 30 meV to 1 MeV neutron energy, has been determined with 5% accuracy relative to the 235U(n,f) cross section. In order to reduce the uncertainty on the absolute value, the data have been normalized at thermal energy to recent measurements performed at ILL and BR1. In the energy range of overlap, the results are in fair agreement with some previous measurements and confirm, on average, the evaluated cross section in the ENDF/B-VII.0 database, although sizable differences are observed for some important resonances below 20 eV. A similar behavior is observed relative to JENDL/AC-2008, a reactor-oriented database for actinides. The new results contribute to the overall improvement of the databases needed for the design of advanced reactor systems and may lead to refinements of fission models for the actinides.