Experimental-Series Parameters for the Decay of Multigroup Beta and Gamma Spectra from 0.1 to 1000 Seconds After a Fission Burst

Abstract

Abstract : Accurate predictions of beta and gamma spectra as a function of time following neutron-induced fission can be made by summation calculations from the fission-product data in the Evaluated Nuclear Data File, Part B(ENDF/B), but the calculations are expensive and the data base is very large. A multigroup spectrum can be represented sufficiently accurately over a limited time range by a few-term exponential series obtained by least-squares fitting to the summation calculations. This report summarizes the preparation of such a parameter set for the Air Force Weapons Laboratory (AFWL). The set describes decay of beta and gamma spectra from 0.1 to 1000 seconds after fission of U235, U238, or Pu239 induced by fission-spectrum or 14-MeV neutrons. Fits that are as accurate as the ENDF/B data warrant can be made with five or fewer terms per energy group. An alternative set of parameters that uses the same decay constants in each group for all six combinations of isotope and neutron energy, but different amplitudes for each combination, requires only one additional term for comparable accuracy.