Discovery of rare neutron-rich Zr, Nb, Mo, Tc, and Ru isotopes in fission: Test of β half-life predictions very far from stability

In experiments employing fast ion-guide-based on-line mass separation we have identified six new, rare nuclides of highly refractory elements produced in the fission of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ by 20-MeV protons. The new isotopes ${}_{}{}^{105}{}_{}{}^{}\mathrm{Zr}$, ${}_{}{}^{107}{}_{}{}^{}\mathrm{Nb}$, ${}_{}{}^{109}{}_{}{}^{}\mathrm{Mo}$, ${}_{}{}^{110}{}_{}{}^{}\mathrm{Mo}$, ${}_{}{}^{113}{}_{}{}^{}\mathrm{Tc}$, and ${}_{}{}^{115}{}_{}{}^{}\mathrm{Ru}$ were identified through (β, ${\mathit{K}}_{\mathrm{\alpha }}$ x-ray) coincidences and β-delayed γ decay. The observed β half-lives range from 130 ms to approximately 1 s. All our recent β half-life data are compared with predictions of the deformed quasirandom-phase-approximation model. The main uncertainty is found to be the inaccuracy of the decay energy predictions.