Decay data evaluation for ENDF/B-VI

ENDF/B-VI decay files depart from earlier versions in that model data are now used as needed to provide a complete set of average decay energies and spectra. Delayed neutron spectra are also included for the first time. There are 979 radioactive nuclides in the file, and all but 8 have decay spectra. The use of model data has eliminated the ''Pandemonium'' problem in aggregate fission product calculations, and it greatly improves decay heat calculations. This report describes the data base with an emphasis on the need for models used in combining measurement and model data. Extraction of only the evaluated measurement data is simple, should it be desired by users. We present here an abbreviated chronology of the development of ENDF/B-VI