Gamma-gamma coincidence measurements of the radiations of 5.7-hour ${\mathrm{Mo}}^{90}$ showed that the 120- and 250-kev gamma rays previously reported by Diamond were delayed. Further experiments showed the existence of a 24-second isomer of ${\mathrm{Nb}}^{90}$ emitting 120-kev gamma rays and a 10 to 20 millisecond isomer emitting 250-kev gamma rays. The transition types were not assigned unambiguously but the 24-second isomer is probably $E2\mathrm{}$ and the 10 to 20 millisecond isomer $E3\mathrm{}$ or $M3\mathrm{}$ in nature.

The radiations of the 14.6-hour ground state of ${\mathrm{Nb}}^{90}$ were also studied. The 140-kev and 1.14-Mev gamma rays are in coincidence with each other and with annihilation radiation. A prominent 2.2-Mev gamma ray is not in coincidence with annihilation radiation or x-rays. Hence, it must be a delayed transition with $\Delta I\ge 3$. This is of interest because of the rarity of such transitions in even-even nuclei.

• Received 15 October 1954

DOI:https://doi.org/10.1103/PhysRev.98.79