Delayed neutron spectra decay group for fissioning systems from 227Th through 255Fm

Most applications of delayed neutrons use an approximate temporal group representation of measured aggregate data. Such data have been limited to the few fissioning nuclides that have aggregate measurements, and even these have inadequate or no spectral measurements. Improvements in the experimental techniques of isotope separation and neutron spectroscopy have made the study of delayed neutron emission from individual precursor nuclides more practical and productive over the past fifteen or so years. The quantity and quality of the delayed neutron emission probabilities and particularly the neutron emission spectra for the individual nuclides have been greatly improved. This paper will briefly review a recently completed evaluation of precursor data that comprises the largest single set of such data to date, and will also describe how the precursor data has been used to produced delayed neutron yields, halflives, and spectra in the classical six-group representation for 43 fissioning systems from 227Th to 225Fm. Comments and observations made concerning the use of more than six time groups will also be included. The application of the data in both its explicit and reduced (six temporal group representation) forms in the point reactor kinetics equations will also be discussed. Results from beta-effective calculations in a simple Godiva-type system will be presented, but the paper will concentrate on the data base and its few-group representations. 17 refs., 1 fig., 1 tab