The double-beta $\beta \beta \left(2\nu \right)$-decay rate of ${}^{100}\mathrm{Mo}$ to the first excited $0^{+}$ state of ${}^{100}\mathrm{Ru}$ has been measured by a $\gamma -\gamma$ coincidence technique that uses two HPGe detectors to observe the two $\gamma$ rays ( $E_{\gamma 1}=590.76\mathrm{keV}$; $E_{\gamma 2}=539.53\mathrm{keV}$) from the ${}^{100}\mathrm{Ru}$ nucleus as it deexcites to the ground state via the $0^{+}\to 2^{+}\to 0^{+}$ sequence. Unlike all previous $\beta \beta$-decay experiments, this technique provides data which have a large signal-to-background ratio. After a 440-day measurement of a 1.05-kg isotopically enriched $\left(98.4\%\right)$ disk of ${}^{100}\mathrm{Mo}$, 22 detected coincidence events (with an estimated background of 2.5 events) yield a half-life of $\left[{5.9}_{-1.1\mathrm{}}^{+1.7\mathrm{}}\right(\mathrm{stat})\pm 0.6(\mathrm{syst}\left)\right]\times {10}^{20}$ years.

• Received 7 December 2000

DOI:https://doi.org/10.1103/PhysRevLett.86.3510