Evaluations of the fast neutron cross sections of sup 2 sup 8 Si including complete covariance information

A new evaluation of all important neutron cross sections of sup 2 sup 8 Si was performed in the neutron energy range 1.75 - 20 MeV, that is for the whole energy range above the resonance region. The evaluation combines the results of nuclear model calculations and the complete existing experimental data base in order to obtain the most accurate description of the cross sections possible within our present knowledge. The evaluation was performed in the following way: The cross sections from the EFF - 2 file (results of model calculations) and their estimated covariances are used as prior information which is successively improved by adding experimental data and by applying Bayes' theorem to obtain the posterior information. For this process the code GLUCS was used. For some cross sections, not covered in EFF - 2, priors were taken from BROND and recent nuclear model calculations. As results we obtained evaluated cross sections and their covariances for a chosen set of 15 independent cross sections in a group structure of 31 neutron energy groups 0.2 - 1 MeV wide. The evaluated cross sections thus are the average cross sections over the respective energy group. Thus before use as evaluated data file, fine structure of the various cross sections has to be added whenever such fine structure has been established with sufficient accuracy. A final coupled set of evaluated cross sections and covariances was obtained by a last GLUCS run including the experimental data for 'redundant' cross sections, that is all cross sections which can be expressed as sum or difference of the basic cross sections chosen for the evaluation. The results of our new evaluation agree with ENDF/B-VI and EFF - 2 within the uncertainties of these evaluations. Most of the uncertainties of our evaluated cross sections, however, are considerably smaller than those of ENDF/B-VI and the estimated uncertainties of EFF - 2.