The rare double gamma decay of the first excited $0^{+}$ state in ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ and ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$ has been measured with a segmented $4\pi$ NaI detector system, which allows suppression of the perturbing background due to positron annihilation in flight. In both cases the directional correlation of the two photons is found to be asymmetric around 90°, which is explained by an interference of $2E1$ and $2M1$ transitions. The deduced $M1\mathrm{}$ quenching factors agree with those from ($e$,$e^{\prime }$) and ($p$,$p^{\prime }$) measurements.

• Received 13 August 1984

DOI:https://doi.org/10.1103/PhysRevLett.53.1897