Decay of the Cerium-139 Isomers

Abstract

The decay of the 140-day ground state of ${\mathrm{Ce}}^{139}$ has been reinvestigated. A $4\pi$ scintillation spectrometer has been used to show that $K$-electron capture transitions between ground states is less than one percent of all $K$-electron captures. The same instrument has been used to measure the fraction of all transitions which terminate at the 166-kev level due to $K$-electron capture, and also has been used to determine the conversion coefficient of the 166-kev transition. The $K$-conversion coefficient was found to be 0.22±0.01. $K$-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 $K$-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 ${\mathrm{Ce}}^{139}$ has been found. The half-life of the transition is 55±3 sec and its $K$-conversion coefficient is 0.08±0.02. Mass and element assignments have been established and approximate cross sections for activation of each level in ${\mathrm{Ce}}^{139}$ by the ($n, \gamma$) reaction on ${\mathrm{Ce}}^{138}$ in the graphite reactor have been determined.