Low-energy photon scattering experiments of ^151,153Eu, ¹⁶³Dy, and ¹⁶⁵Ho and the systematics of the M1 scissors mode in odd-mass rare-earth nuclei

Nuclear resonance fluorescence experiments were performed on the rare-earth nuclei ^151,153Eu, and with considerably increased sensitivity on ¹⁶³Dy and ¹⁶⁵Ho to study the fragmentation of the M1 scissors mode in odd-mass nuclei, and to clarify the puzzle of the missing total M1 strength observed for odd-mass nuclei so far. Using the bremsstrahlung photon beam of the Stuttgart Dynamitron (end point energy 4.05 MeV) and high-resolution Ge γ-ray spectrometers, detailed information was obtained on excitation energies, decay widths, transition probabilities, and branching ratios. Whereas in ¹⁵¹Eu only 11 weak excitations were observed, 161 and 138 excitations could be detected in the heavier nuclei ¹⁶³Dy and ¹⁶⁵Ho, respectively. The results are compared to those observed recently at the Stuttgart facility for the neighboring odd-mass nuclei ¹⁶¹Dy, ^155,157Gd, and ¹⁵⁹Tb. The measured total strengths increase with the mass number A. Ascribing the same portion of the dipole strength to M1 excitations as measured in the neighboring even-even nuclei, the total M1 strength deduced from the most sensitive experiment on ¹⁶³Dy is comparable to those found in the neighboring even-even nuclei. The results for ¹⁶³Dy and ¹⁶⁵Ho are compared with a fluctuation analysis of the photon scattering spectra to estimate the amount of still unresolved strength eventually hidden in the background due to the extreme fragmentation of the M1 scissors mode in odd-mass rare-earth nuclei. For ¹⁶⁵Ho, the total derived strength of B(M1)↑=2.9(5)μ_N² agrees within error bars with an earlier analysis of a different measurement of the ¹⁶⁵Ho(γ,γ^′) reaction. In ¹⁶³Dy the method leads to an unphysical background shape, underlining the experimental observation of a significantly reduced fragmentation pattern of the dipole modes in this nucleus, which must be traced back to structure features of the Dy isotopes.