By bombardment of zirconium, niobium, yttrium, and silver targets in the 184-inch cyclotron it has been possible to produce the isomer pair ${\mathrm{Nb}}^{89m}$ and ${\mathrm{Nb}}^{89}$. The isomerism arises from the odd 41st proton below the 50-proton closed shell of the single-particle nuclear model. ${\mathrm{Nb}}^{89}\left(g_{\frac{9}{2}}\right)$ is a 1.9-hour activity emitting 2.85-Mev positrons to produce ${\mathrm{Zr}}^{89}$. The yield of the ${\mathrm{Nb}}^{89m}\left(p_{\frac{1}{2}}\right)$ is about 30-100 fold less and its radiations were not directly observed. By scintillation spectrometer measurements on zirconium daughter activity it is shown that ${\mathrm{Nb}}^{89m}$ decays with about a 2-hour half-life to 4.4-minute ${\mathrm{Zr}}^{89m}\left(p_{\frac{1}{2}}\right)$. The $M4\mathrm{}$ gamma transition between the two niobium levels is not observed, indicating that the level spacing is small and the upper level deactivates primarily by positron emission. The $log\mathrm{ft}$ values are larger than expected and are discussed in terms of the even-even core rearrangement considerations of A. De-Shalit and M. Goldhaber [Phys. Rev. 92, 1211 (1953)]. ${\mathrm{Nb}}^{89}$ was also prepared by bombardment of NaBr with accelerated carbon ions in the 60-inch cyclotron.

• Received 18 August 1954

DOI:https://doi.org/10.1103/PhysRev.97.117