Neutron-rich ${}_{39}$Y to ${}_{43}$Tc isotopes have been produced by fission of uranium with a 50 MeV ${\mathrm{H}}_2^{+}$ beam. Beta-decay half-lives, delayed neutron-emission probabilities, and production yields have been measured and compared with theory. Beta decay of 4 new isotopes is reported, and the $\beta$-delayed neutron-emission mode has been discovered for 12 isotopes of the elements niobium and technetium. The results compared to quasiparticle random phase approximation predictions indicate the increasing importance of fast $\beta$ transitions to high-lying states of nuclei with large neutron excess.

• Received 26 February 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.458