Branching and fragmentation of dipole strength in Ta in the region of the scissors mode

Angell, C.; Hajima, Ryoichi; Shizuma, Toshiyuki; Ludewigt, B.*; Quiter, B. J.*

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 present itself as heretofore unobserved branching or fragmentation of the scissors mode. Such a possibility was investigated in a transmission NRF measurement on the deformed, odd-mass Ta, using a quasi-monoenergetic -ray beam at two beam energies, and additionally informing applications using transmission NRF to assay or detect odd-mass fissile isotopes. An anomalously large branching, 70%, 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. This branching, if present at all energies, combined with the observed highly-fragmented elastic strength would reconcile the scissors-mode strength observed in NRF measurements with the expectations for enhanced scissors-mode strength from radiative decay experiments.