Investigation of neutron-rich rare-earth nuclei including the new isotopes ¹⁷⁷Tm and ¹⁸⁴Lu

Decays of neutron-rich isotopes in the rare-earth region were studied by means of on-line mass separation and β-γ spectroscopy using multinucleon-transfer reactions between beams of ¹³⁶Xe (9 and 11.7 MeV/u), ¹⁸⁶W (11.7 and 15 MeV/u) and ²³⁸U (11.4 MeV/u) and targets of ^natW and Ta. The higher beam energies appear to be advantageous for the production of such isotopes. Two new isotopes were identified: ¹⁷⁷Tm with a half-life T _{1 2} = 85_-15 ⁺¹⁰ s, and ¹⁸⁵Lu with T _{1 2} ≈ 18s. A new 47 s activity found at A = 171 is tentatively assigned to the decay of the new isotope ¹⁷¹Ho. The properties of the ground and excited states of neutron-rich lanthanide isotopes are interpreted within the shell model using the deformed Woods-Saxon potential. A change of the ground-state configuration for odd-mass neutron-rich lutetium isotopes from π 7 2⁺[404] to π 9 2^-[514] is suggested, this change being due to the influence of a large hexadecapole deformation. The role of the possible isomeric state in ¹⁸⁰Lu for the nucleosynthesis of ^180mTa is discussed.