Branching and Fragmentation of Dipole Strength in ^{181}Ta in the Region of the Scissors Mode.

Abstract

In recent measurements of the scissors mode in radiative decay experiments, transition strengths were observed that were double that expected from theory and systematics well established from measurements on the radiative excitation channel, that is, using nuclear resonance fluorescence (NRF). Additional strength as measured with NRF can only be present as heretofore unobserved branching or fragmentation of the scissors mode. Such possibilities were investigated in a transmission NRF measurement on the deformed, odd-mass ^{181}Ta, using a quasimonoenergetic γ-ray beam at two beam energies. This measurement further influences applications using transmission NRF to assay or detect odd-mass fissile isotopes. A large branching, ≈75%, of small resonances to excited states was discovered. In contrast, previous studies using NRF of the scissors-mode strength in odd-mass nuclei assumed no branching existed. The presently observed branching, combined with the observed highly fragmented elastic strength, could reconcile the scissors-mode strength observed in NRF measurements with the expectations for enhanced scissors-mode strength from radiative decay experiments.