Search for Double-Photon Decay inZr90

A search was made for double-photon decay occurring in the $0^{+}$→$0^{+}$ transition in ${\mathrm{Zr}}^{90}$, with the aid of a "fast-slow" coincidence spectrometer which permitted two-dimensional pulse-height analysis, and which was modified to suppress higher-order accidental coincidences and to accumulate prompt and delayed coincidences sumultaneously. Coincident $\gamma$ rays were observed; however, neither the energy spectra nor the angular correlation corresponds to two-photon decay. Analysis of the observed spectrum yields an upper limit to the probability of double-photon decay of $\frac{W_{\gamma \gamma }}{(W_{\mathrm{pair}}+W_e)}\le 1.2\times {10}^{-4\mathrm{}}$. The significance of the negative result is discussed in this paper, while the process that accounts for the observed twofold coincidence spectrum is the subject of the following two articles.