Decay properties of the fission isomer \(^{\text {236m}}\)U

The delayed \(\gamma \) decay and delayed fission of the shape isomer \(^{236m}\)U were measured using the Heidelberg-Darmstadt Crystal Ball spectrometer. It is demonstrated that the isomeric \(\gamma \) decay and the delayed fission represent two competing decay channels of the same isomeric state. This conclusion is supported by four independent observations: (i) the measured half-lives of the delayed \(\gamma \) decay and delayed fission are identical, (ii) the excitation energy of the isomer is the same for both decay pathways, (iii) the missing energy in the prompt energy balance is equal for both decays, and (iv) the prompt energy spectra populating the second minimum are in agreement. An improved statistical analysis of the \(\gamma \) decay indicates that the excitation energy of the isomeric ground state in the second minimum is \(2814 \pm 33\) keV. The \(\gamma \) back decay proceeds through several cascades involving known \(1^-\) states of the normally deformed \(^{236}\)U. A direct single-step \(\gamma \) transition is excluded within the experimental sensitivity.