The decay of the 140-day ground state of Ce 139 has been reinvestigated. A 47r scintillation spectrometer has been used to show that iC-electron capture transitions between ground states is less than one percent of all iC-electron captures. The same instrument has been used to measure the fraction of all transitions which terminate at the 166-kev level due to ^-electron capture, and also has been used to determine the conversion coefficient of the 166-kev transition. The iT-conversion coefficient was found to be 0.22±0.01. iT-electron capture accounts for 73% of all transitions to the 166-kev level. The energy available for decay to the excited level is 104±6 kev as computed from the experimental capture ratio. A value of 0.88±0.01 was obtained for the ^-fluorescence yield at Z of 57 from beta-ray spectrometer measurements and coincidence counting. An isomeric level of 740±5 kev above the ground state of Ce 139 has been found. The half-life of the transition is 55±3 sec and its .^-conversion coefficient is 0.08±0.02. Mass and element assignments have been established and approximate cross sections for activation of each level in Ce 139 by the (n,y) reaction on Ce 138 in the graphite reactor have been determined.
FIG. 1. Allowed Kurie plot of Kr 85 beta-spectrum. FIG. 2. Kr 85 beta-spectrum plotted as a first-forbidden transition with A J =2.straight line yields a maximum energy for the beta-transition of 0.695d=0.005 Mev. The value of log (ft) is 9 which is in agreement with //-values computed by Nordheim 4 for other first-forbidden transitions with a spin change of two. Scintillation spectrometer and lead absorption measurements of the gamma-radiation of long-lived fission krypton both indicate that the energy is 0.54±:0.02 Mev. Coincidence absorption data show that the gamma-ray is in coincidence with a 0.15=fc0.02 Mev maximum energy beta-ray. Since extensive purification precludes the presence of another element and since the sum of the energies of the coincident beta-and gamma-rays is the same within experimental error as the energy of the more abundant beta-group of Kr 85 , these radiations are almost certainly associated with Kr 85 .The branching ratio in the decay of Kr 86 was calculated from the coincidence data and the calibrated detection efficiency of the coincidence gamma-counter. It was found that Kr 85 decays by emission of a 0.15-Mev beta-ray in coincidence with a 0.54-Mev gamma-ray in 0.65±0.15 percent of the disintegrations.We would like to thank G. W. Parker for the sample of fission product gases used in this work.
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