Evaluations of Fission Chain Yields for ²³⁹Pu from Fission-Spectrum Neutrons

Because of the importance of accurate data for fission chain yields (FCYs) for many applications, we present a rigorous “clean sheet” evaluation of all available data to provide an accurate set of pertinent FCYs. Because some nuclear data (e.g., half-lives, branching ratios, etc.) have been refined since the original analyses, where possible we update the data and their associated uncertainties. This evaluation is particularly topical since there are differences in the nuclear data used by radiochemists at different laboratories internationally and since some experiments from the 1970s have been recently reexamined with details published for the first time.

The focus of this work is the production of a small set of fission products (⁹⁵Zr, ⁹⁹Mo, ¹⁴⁴Ce, ¹⁴⁷Nd) from plutonium irradiated by fission spectrum neutrons. Because ¹⁴⁷Nd is a common isotope used at several laboratories, its production rate is examined critically. We find that most of the interlaboratory discrepancies can be explained by a dependence of its yield on the energy of the neutron causing fission, so we consider in detail the statistical significance of this claim. The potential for neutron energy dependence of ¹⁴⁷Nd production from plutonium was first recognized in 1977 by Maeck and recently raised again as a possibility by Chadwick. The data for ⁹⁵Zr, by contrast, demonstrate no statistically significant energy-dependence trends, but the data at the higher energies demonstrate significant scatter.

With the relatively small number of data points, and recognizing that measurement methods and technologies have likely significantly improved in the nearly 30 years since the last measurement, additional measurements to refine the assessment and improve the uncertainties may be warranted.