DISCOVERY OF RARE NEUTRON-RICH ZR, NB, MO, TC, AND RU ISOTOPES IN FISSION - TEST OF BETA-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 U-238 by 20-MeV protons. The new isotopes 105Zr, 107Nb, 109Mo, 110Mo, Tc-113, and 115Ru were identified through (beta, K-alpha x-ray) coincidences and beta-delayed gamma-decay. The observed beta-half-lives range from 130 ms to approximately 1 s. All our recent 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.