Yields of Short-Lived Fission Products Produced by Thermal-Neutron Fission of Plutonium-239

Abstract

The absolute yields of 28 fission products representing 23 different mass chains produced by thermal-neutron fission of 239Pu and having half-lives between 30 and 1100 s have been determined using Ge(Li) spectroscopy methods. Spectra of 30 gamma rays emitted in the decay of the fission products between 35 and 1950 s after a 5-s irradiation were obtained. Gamma rays were assigned to the responsible fission products by matching gamma-ray energies and half-lives. Fission-product yields were then obtained from the data by first determining the appropriate gamma-ray activity as of the end of the irradiation, 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. Uncertainties assigned to the present results range between 6 and 45%, and are smaller than or comparable to uncertainties assigned to previous experimental or evaluated yields for six mass chains.