Short-Lived Isomers ofAs75andAs77

Sources of ${\mathrm{Se}}^{75}$ and ${\mathrm{Ge}}^{77}$ have been investigated by delayed-coincidence techniques for short-lived isomeric levels in the daughter nuclei ${\mathrm{As}}^{75}$ and ${\mathrm{As}}^{77}$, respectively. The 0.305-Mev level of ${\mathrm{As}}^{75}$ has a half-life of 17 milliseconds and is depopulated by a 0.025-Mev $M2\mathrm{}$ transition (90%) in competition with a 0.305-Mev $E3\mathrm{}$ transition (10%). The 0.475-Mev level of ${\mathrm{As}}^{77}$ has a half-life of 116 microseconds and is depopulated by a 0.210-Mev $M2\mathrm{}$ transition. An upper limit of 2% of the isomeric decays was placed on the intensity of any possible 0.475-Mev ground-state transition. In the beta decay of ${\mathrm{Ge}}^{77}$ to ${\mathrm{As}}^{77}$, gamma rays of the following energies were found to feed the metastable state: 0.153, 0.709, 1.08, and 1.50 Mev and probably also 0.92 and 1.96 Mev. The data obtained have been incorporated in a recently proposed decay scheme of ${\mathrm{Ge}}^{77}$. The previously discovered isomeric transition in ${\mathrm{As}}^{73}$, as well as those in ${\mathrm{As}}^{75}$ and ${\mathrm{As}}^{77}$, are probably all $M2\mathrm{}$ transitions between $\frac{9}{2}+$ and $\frac{5}{2}-$ levels, with transition probabilities of 0.9, 1, and 0.6%, respectively, of the shell-model single proton transition probability.