Fission Product Yields for Thermal-Neutron Fission of Plutonium-239

Abstract

Absolute cumulative yields have been determined for 49 fission products representing 36 mass chains created during thermal-neutron fission of 239Pu, including 3 mass chains for which no prior data exist. Using Ge(Li) spectroscopy, spectra were obtained of gamma rays from decay of fission products between 1550 s and 31 days after a 100-s irradiation. Data were obtained for all fission products simultaneously. Gamma rays were assigned to the responsible fission products by matching gamma-ray energies and half-lives. Gamma-ray data associated with decay of 135I and 140Ba-140La, in particular, were thoroughly studied; uncertainties were obtained for the two largest intensity gamma rays from decay of 135I that are smaller than previously evaluated uncertainties. Fission product yields were obtained from the data by first determining the appropriate gamma-ray activity as of the end of the irradiation, then correcting for detector efficiency and gamma-ray branching ratio, and, finally, dividing by the number of fissions created in the sample. The number of fissions was determined by direct comparison of gamma rays emanating from fission products created during a careful irradiation of a well-calibrated 239Pu-loaded fission chamber. The resulting fission product yields are compared with previous measurements and with recommended yields given in two recent (and independent) evaluations. The present results are significantly larger for mass chains 101 and 105, somewhat smaller for mass chains 87 and 151, and in reasonable agreement with the remaining mass chains. Uncertainties assigned to the present results range between 2.5 and 25%, and are smaller than or comparable to uncertainties assigned to previous experimental (or evaluated) yields for 14 mass chains.