Complete scissors mode strength in heavy deformed odd-mass nuclei: a case study of 165Ho and 169Tm

The low-energy dipole strength distributions in ¹⁶⁵Ho and ¹⁶⁹Tm have been studied in nuclear resonance fluorescence experiments at the S-DALINAC utilizing a Euroball Cluster detector. In the energy interval between 2.5 and 4.0 MeV, where the scissors mode is expected, 35 ground state transitions could be observed in ¹⁶⁵Ho and 53 in ¹⁶⁹Tm. Assuming M1 character for all these transitions corresponds to ΣB(M1) ↑= 1.54(23), μ_N² and ΣB(M1) ↑= 2.15(63), μ_N² for ¹⁶⁵Ho and ¹⁶⁹Tm, respectively. A statistical analysis is applied to the measured spectra which is capable of reconstructing the complete strength, including contributions of transitions below the observation limit, with the ratio between M1 and E1 excitations and their total strengths taken as an average over the even-mass neighbours. This results in summed M1 transition strengths of ΣB(M1) ↑= 3.54_−0.95^+0.75, μ_N² (¹⁶⁵Ho) and ΣB(M1) ↑= 3.36_−0.57^+1.00μ_N² (¹⁶⁹Tm). The large variations of the summed dipole strength and the fragmentation in other deformed odd-mass nuclei can be reproduced by the same statistical ansatz. The total low-lying M1 strength in heavy deformed odd-mass nuclei is in agreement with the findings in the neighbouring even-mass nuclides as well as with sum-rule predictions.