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

Abstract

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 (\ensuremath{\beta}, ${\mathit{K}}_{\mathrm{\ensuremath{\alpha}}}$ x-ray) coincidences and \ensuremath{\beta}-delayed \ensuremath{\gamma} decay. The observed \ensuremath{\beta} half-lives range from 130 ms to approximately 1 s. All our recent \ensuremath{\beta} 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.