E2M1multipole mixing ratios of2′→2gamma transitions in even-even spherical nuclei

Abstract

A systematic survey is presented of $\frac{E2\mathrm{}}{M1\mathrm{}}$ mixing ratios of $\gamma$ transitions between the $2^{+}$′ and $2^{+}$ levels of even-even nuclei in the mass range $58\le A\le 152$. Particular attention is given to the variations in the phase of the mixing ratios, which are deduced from the literature in a systematic manner. It is shown that the systematics of both magnitudes and phases of the mixing ratios are explained quite well for a number of nuclei by a model proposed by Greiner, in which the magnitude of the mixing ratio is parametrized in terms of the deviation of the $g$ factor of the first $2^{+}$ state from the value $\frac{Z}{A}$. It is further shown that a semimicroscopic description, in terms of small admixtures of two-particle components to the phonon basis states, yields reasonable agreement with the observed phase variations and absolute magnitudes, even when only very few two-particle states are considered.