Os-184 has been excluded as a promising candidate for the search of neutrinoless double-electron capture. High-precision mass measurements with the Penning-trap mass spectrometer TRIGA-TRAP resulted in a marginal resonant enhancement with = -8.89(58)
keV excess energy to the 1322.152(22) keV 0+ excited state in W-184. State-of-the-art energy density functional calculations are applied for the evaluation of the nuclear matrix elements to the excited states predicting a strong suppression due to the large deformation of mother and daughter states. The half-life of the transition in Os-184 exceeds T_{1/2} > 1.3 10^{29} years for an effective neutrino mass of 1 eV.
Structural evolution in neutron-rich Os and W isotopes is investigated in terms of the Interacting Boson Model (IBM) Hamiltonian determined by (constrained) Hartree-Fock-Bogoliubov (HFB) calculations with the Gogny-D1S Energy Density Functional (EDF)
. The interaction strengths of the IBM Hamiltonian are produced by mapping the potential energy surface (PES) of the Gogny-EDF with quadrupole degrees of freedom onto the corresponding PES of the IBM system. We examine the prolate-to-oblate shape/phase transition which is predicted to take place in this region as a function of neutron number $N$ within the considered Os and W isotopic chains. The onset of this transition is found to be more rapid compared to the neighboring Pt isotopes. The calculations also allow prediction of spectroscopic variables (excited state energies and reduced transition probabilities) which are presented for the neutron-rich $^{192,194,196}$W nuclei, for which there is only very limited experimental data available to date.
