Decay Scheme ofMo91

Gamma-radiation energies were measured at 1540±20, 1210±30, and 658±3 kev, all of which decay with the known 65.5-second half-life. Three positron groups were found to be involved in the same decay. The end-point energies of these were found to be 3.99±0.05, 2.78±0.10, and 2.48±0.10 Mev. No gamma radiation was found decaying with the longer 15-minute half-life. A single positron group with this longer half-life was found to have an end point of 3.44±0.03 Mev. The long-lived positrons represent a transition between the ground states of ${\mathrm{Mo}}^{91}$ and ${\mathrm{Nb}}^{91}$, whereas the 65-second positrons proceed from a 658-kev excited state of ${\mathrm{Mo}}^{91}$ to three excited states in ${\mathrm{Nb}}^{91}$, the lowest of which is the well-known 104-kev excited state. The two higher excited states decay by gamma emission to this state.