Spectroscopy of excited states in U-234 and search for a two-phonon state in U-234

Excited states of the isotope U-234 were studied employing the one-neutron transfer reaction U-235(d, t)U-234 at a beam energy of 11 MeV. The reaction channel was selected by an identification of the outgoing tritons in a Delta E-E measurement. In coincidence with the tritons, gamma rays were also detected with the highly efficient MINIBALL spectrometer. Based on the analysis of gamma gamma coincidence data, the level scheme of U-234 was extended. The 4(+) state at an energy of 1886.7 keV provides evidence for a possible two-phonon gamma gamma excitation, which is based on three gamma transitions from this state to the gamma vibrational band. However, this state is populated with a high cross section via the one-neutron transfer reaction, which is not expected for a highly collective excitation. The situation is compared to other known cases from the rare-earth region and to the neighboring isotone Th-232, which so far is the only known case for a two-phonon excitation in the actinide region.