Disintegration ofY91

The radiations of the fission product ${\mathrm{Y}}^{91}$ have been investigated with beta- and gamma-scintillation spectrometers. In addition to the well-known 1.55-Mev ground-state beta transition, there is a weak (0.3 percent) beta transition of energy 0.330 Mev which is followed by a gamma ray of energy 1.22±0.01 Mev. Definite assignment of these radiations to ${\mathrm{Y}}^{91}$ was established through exacting chemical procedures, measurement of the half-life of the gamma ray, and energy balance considerations. There is reason to believe that the 1.22-Mev level of ${}_{40}{}^{}{}_{}{}^{}\mathrm{Zr}_{}^{91}{}_{}{}^{}$ is not a simple single-particle state.