Lowestβ-Vibrational Phonon in the Second Minima ofU236,238

Conversion electrons from electromagnetic transitions preceding the fission decay of the shape isomers ${}_{}{}^{236,238}{}_{}{}^{}\mathrm{U}$ have been measured. For both isomers highly converted transitions are observed which are tentatively interpreted as the decay of the lowest $\beta$-vibrational band, via $E0\mathrm{}$ transitions, at excitation energies of 686.0±0.7 and 647.8±0.9 keV, respectively.