Status and covariance files of the scattering matrix of graphite

Calculations related to the temperature coefficient of reactivity of enriched gas-cooled reactors show the high sensitivity of this parameter to the proper description of thermalization effects in the moderator; additionally, calculation of the temperature dependence of the inelastic scattering cross section correlates the errors of the cross sections as functions of the temperature. The ENDF/B evaluation of S(α,β,T) is still based on the approximations (incoherent component only) and phonon spectras of the early 60's. Subsequent measurements showed that the structure observed in S(α,β,T) cannot be described using the incoherent approximation, and soon after the availability of highly intense neutron beams and large specimens of pyrolitic graphite better calculation of the phonon spectra were possible. Our calculations of the probability distributions of the momentum and energy transfer, α and β, allow one to define a range of α and β for which comparison of experimental and theoretical data is of most interest for reactor analysis, and to point out regions of deficient resolution or excessive details in the present α,β mesh used in the ENDF/B files. Because the phonon spectra defines S(α,β,T), we have found mathematical formulas that allow the calculation of the covariance matrix of S by propagating the errors of the phonon spectra