YIELDS OF SHORT-LIVED FISSION-PRODUCTS PRODUCED FOLLOWING U-235(N(TH),F)

Measurements of gamma-ray spectra, following the thermal neutron fissi on of U-235 have been made using a high purity germanium detector at t he University of Massachusetts Lowell (UML) Van de Graaff facility. Th e gamma spectra were measured at delay times ranging from 0.2 s to nea rly 10000 s following the rapid transfer of the fission fragments with a helium-jet system. On the basis of the known gamma transitions, for ty isotopes have been identified and studied. By measuring the relativ e intensities of these transitions, the relative yields of the various precursor nuclides have been calculated. The results are compared wit h the recommended values listed in the ENDF/B-VI fission product data base (for the lifetimes and the relative yields) and those published i n the Nuclear Data Sheets (for the beta branching ratios). This inform ation is particularly useful for the cases of short-lived fission prod ucts with lifetimes of the order of fractions of a second or a few sec onds. Independent yields of many of these isotopes have rather large u ncertainties, some of which have been reduced by the present study.