Coulomb Excitation of Second 2+ States in Ge74, Ge76, Se76, Se78, and Se80

Abstract

The location of a second 2+ state has been established for five even-even nuclei by means of Coulomb excitation produced by 6-, 7-, and 8-Mev $\alpha$ particles. The relatively weak excitation of these states is detected by a coincident measurement of the cascade gamma rays. The $B\left(E2\mathrm{}\right)\text{'}\mathrm{s}$ for decay of the second 2+ state to the ground state by the crossover transition are rather small, being about single-particle value or a little less. For ${\mathrm{Ge}}^{74}$, ${\mathrm{Se}}^{76}$, and ${\mathrm{Se}}^{78}$, the cascade/crossover ratio for the decay of the second 2+ state is known from radioactive decay measurements. The upper cascade $B(E2, 2^{\prime }\to 2)\text{'}\mathrm{s}$ exhibit enhancements comparable to those for the lower cascade $B(E2, 2\to 0)\text{'}\mathrm{s}$. The ratios of the $B\left(E2\mathrm{}\right)\text{'}\mathrm{s}$ for the decay of the first and second 2+ states are compared to the predictions of several collective models. The $B(M1, 2^{\prime }\to 2)$ values for ${\mathrm{Ge}}^{74}$ and ${\mathrm{Se}}^{76}$ are small, being about ${10}^{-3\mathrm{}}$ times the single-particle estimate. This result is in qualitative agreement with the collective models for vibrational excitations.