M2Transitions in Nuclei

A survey of experimental data about $M2\mathrm{}$ transitions between low-lying nuclear states shows that the great majority of transitions in nuclei with $A>\mathrm{}30\mathrm{}$ is severely inhibited. For heavy nuclei with a permanent deformation, the inhibition can be ascribed either to $K$ forbiddenness or to an asymptotic selection rule. Even when a permanent deformation is not present, the effect of this selection rule may persist, and it can lead to $M2\mathrm{}$ transitions whose lifetimes are several hundred times the single-particle estimate. In light nuclei, some transitions are inhibited by an isobaric-spin effect. Theroetical values are obtained for comparison with some recently measured $M2\mathrm{}$ lifetimes in the region around $A=40\mathrm{}$. Both quadrupole deformation and the isobaric-spin effect can contribute to inhibitions in this region. The isobaric-spin effect is sufficient to account for the lifetimes in ${\mathrm{K}}^{39}$ and ${\mathrm{Ca}}^{41}$. For nuclei further removed from ${\mathrm{Ca}}^{40}$, the effect of deformation is also necessary. In some simple examples we show that such a trend results from the nature of the particle-hole interaction.