Collective Levels inSe72,74,76,78

Abstract

The even selenium isotopes ${}_{}{}^{72}{}_{}{}^{}\mathrm{Se}$, ${}_{}{}^{74}{}_{}{}^{}\mathrm{Se}$, ${}_{}{}^{76}{}_{}{}^{}\mathrm{Se}$, and ${}_{}{}^{78}{}_{}{}^{}\mathrm{Se}$ have been investigated by means of the $\mathrm{Ge}(\alpha ,2n\gamma )\mathrm{Se}$ and $\mathrm{Ge}(\alpha ,4n\gamma )\mathrm{Se}$ reactions. The $\gamma$ radiation from these nuclei was studied during the irradiation using techniques of in-beam spectroscopy. The energies of the $\gamma$ rays, their relative intensities, their angular distributions, and their time distributions relative to the 2-nsec beam bunch have been measured. Quasirotational ground-state bands were observed up to 10+ for ${}_{}{}^{76}{}_{}{}^{}\mathrm{Se}$, up to 8+ for ${}_{}{}^{74}{}_{}{}^{}\mathrm{Se}$ and ${}_{}{}^{78}{}_{}{}^{}\mathrm{Se}$, and up to 6+ for ${}_{}{}^{72}{}_{}{}^{}\mathrm{Se}$. The level energies are compared to a semiempirical formula.