Electric-Monopole Directional-Correlation Experiments

Angular-correlation experiments involving conversion electrons are sensitive to a small admixture of electric-monopole ($E0\mathrm{}$) transitions. The coefficient of $P_2\left(cos\theta \right)$ in the correlation function involving a mixed $E0+M1+E2\mathrm{}$ transition contains an interference term between the $E0\mathrm{}$ and $E2\mathrm{}$ electrons, which is propertional to the ratio of the $E0\mathrm{}$ and $E2\mathrm{}$ 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 $E2\mathrm{}$ 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 ($Z=78\mathrm{}$) over a range of energies, and the feasibility of the method is illustrated with reference to the particular case of the 2+→2+→0+ cascade in ${\mathrm{Pt}}^{196}$.