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

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 ${}^{181}\mathrm{Ta}$, using a quasimonoenergetic $\gamma$-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, $\approx 75\%$, 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.

Figure 1

The background-subtraction sequence in order of subtraction for the spectrum taken with no upstream absorber. The peaks at energy less than the beam energy (congruent with coherent scattering) are transitions to the excited state at 136 keV.

Figure 2

Background-subtracted spectrum for having no upstream absorber showing statistical uncertainty (blue points with error bars) and a fit to the NRF peaks (solid red line) with individual peaks displayed (dotted magenta lines). The beam profile is overlaid (dashed black line). Solid triangles mark previously observed transitions, and empty triangles mark newly observed transitions. Apparent peaks at the low-energy tail of the beam profile (below about 2.22 and 2.70 MeV, top and bottom panel, respectively) were not reproduced with an absorber in place, so they were not included in the fit. The peak of the 2.298 MeV resonance is at around 960 counts per keV.

Figure 3

The elastic cross section ${\sigma }_{\gamma \gamma }$ rebinned to 2 keV with statistical uncertainty. Cyan (blue circles) represents the no upstream absorber (or Pb absorber) spectrum. Magenta (orange squares) represents the Ta absorber spectrum. Resonant absorption (cyan above blue; blue indicates overlap of cyan and magenta) is evident at each beam energy.