The U-235 fission spectrum averaged cross section for the 199Hg(n, n')sup(199m)Hg reaction processed with covariances between the data uncertainties

The 235U fission spectrum averaged cross section for the 199Hg(n, n')sup(199m)Hg reaction has been measured relative to that for the 115In(n, n')sup(115m)In reaction. Fast neutrons were obtained with a fission plate in a large heavy water thermal neutron facility of KUR. The present value of the 235U fission spectrum averaged cross section for the 199Hg(n, n')sup(199m)Hg reaction is 277 +- 15.4 mb, while the average cross section calculated with the recent energy dependent cross section and a Watt-type fission neutron spectrum is about 14% lower than this value. In the present uncertainty analysis, it is demonstrated how to generate and combine the experimental uncertainties in the form of a covariance matrix in order to obtain the best value from the experimental data. (author)