Search for Double Gamma Emission from the First Excited States ofCa40andZr90

A search for double $\gamma$ emission from the first excited ${\mathrm{O}}^{+}$ states in ${\mathrm{Ca}}^{40}$ and ${\mathrm{Zr}}^{90}$ has been carried out using a NaI(Tl) scintillation spectrometer and a sum-coincidence technique. No definite evidence for double $\gamma$ emission is found in either case. In the 3.35-MeV, ${\mathrm{O}}^{+}$-to-ground-state transition in ${\mathrm{Ca}}^{40}$, an upper limit of 4 × ${10}^{-4\mathrm{}}$ has been established for the branching ratio of double $\gamma$ emission with respect to internal pair emission. For the 1.75-MeV ${\mathrm{O}}^{+}$-to-ground-state transition in ${\mathrm{Zr}}^{90}$, an upper limit of 1.8 × ${10}^{-4\mathrm{}}$ has been obtained for the branching ratio of double $\gamma$ emission with respect to the sum of internal-conversion electrons and internal pairs. The significance of these results in the light of existing theories is discussed.