Electric-Monopole Directional-Correlation Experiments

Angular-correlation experiments involving conversion electrons are sensitive to a small admixture of electric-monopole ($E0$) transitions. The coefficient of ${P}_{2}(cos\ensuremath{\theta})$ in the correlation function involving a mixed $E0+M1+E2$ transition contains an interference term between the $E0$ and $E2$ electrons, which is propertional to the ratio of the $E0$ and $E2$ matrix elements. Measurement of this coefficient provides a sensitive means for determining both the magnitude of the monopole matrix element and its phase relative to the $E2$ gamma-ray matrix element. The effect on the directional-correlation function between such $K$-conversion electrons and a coincident gamma ray is calculated explicitly. Specific numerical results are given for Pt ($Z=78$) over a range of energies, and the feasibility of the method is illustrated with reference to the particular case of the 2+\ensuremath{\rightarrow}2+\ensuremath{\rightarrow}0+ cascade in ${\mathrm{Pt}}^{196}$.