Decay ofY85Isomers

Two isomers of ${\mathrm{Y}}^{85}$ have been identified and their half-lives determined as 2.68±0.05 h and 5.0±0.2 h. Measurements were made by means of successive timed daughter separations and by direct observation of the decay of radiations from yttrium. Both isomers decay independently by positron emission. The 2.68-h isomer decays to the 70-min ${\mathrm{Sr}}^{85m}$ while the 5.0-h isomer decays to the 64-d ${\mathrm{Sr}}^{85}$. No isomeric transition was observed. Positron spectra were measured with a $\beta$-ray spectrometer. The spectrum for the 2.68-h isomer has an end-point energy of 1.54±0.02 MeV, and the components of the 5-h isomer have end points of 2.24±0.01 MeV, 2.01±0.01 MeV, and 1.1±0.1 MeV. Conversion electron measurements disclosed a 503-keV transition, with a half-life of 2.68 h, $K$ conversion coefficient of 1.9×${10}^{-3\mathrm{}}$, and $\frac{K}{L}$ ratio of ∼10. The $\gamma$ spectrum for the ${\mathrm{Y}}^{85}$ isomeric mixture is complex. The observed transitions are: 503 and 925 keV from the 2.68-h isomer, and 231, 700, 772, (1030), (1130), 1230, 1390, 1590, 1870, 2160, 2340, and 2750 keV from the 5.0-h isomer. Sum-coincidence spectra showed summation of the following radiations: (231-511), (503-511), (231-1390), and (772-1390) keV. Beta-gamma coincidence measurements showed that the 2.01-MeV positron group is coincident with the 231-keV $\gamma$ ray and that the 1.54-MeV positron from the 2.68-h isomer is in coincidence with the 503-keV $\gamma$ ray. The 5.0-h isomer of ${\mathrm{Y}}^{85}$ is 3.26 MeV above the ground level of ${\mathrm{Sr}}^{85}$ while the 2.68-h isomer is at 3.30 MeV, thus being 40±30 keV higher. Spins of $\frac{9}{2}+$ and ½- have been assigned to the 5.0-h and 2.68-h isomers, respectively. A partial decay scheme is presented.