The NEMO 3 detector, which has been operating in the Fréjus underground laboratory since February 2003, is devoted to the search for neutrinoless double-beta decay ($\beta \beta 0\nu$). The half-lives of the two neutrino double-beta decay ($\beta \beta 2\nu$) have been measured for ${}^{100}\mathrm{Mo}$ and ${}^{82}\mathrm{Se}$. After 389 effective days of data collection from February 2003 until September 2004 (phase I), no evidence for neutrinoless double-beta decay was found from $\sim 7\mathrm{kg}$ of ${}^{100}\mathrm{Mo}$ and $\sim 1\mathrm{kg}$ of ${}^{82}\mathrm{Se}$. The corresponding limits are $T_{1/2}(\beta \beta 0\nu )>4.6\times {10}^{23}\mathrm{yr}$ for ${}^{100}\mathrm{Mo}$ and $T_{1/2}(\beta \beta 0\nu )>1.0\times {10}^{23}\mathrm{yr}$ for ${}^{82}\mathrm{Se}$ (90% C.L.). Depending on the nuclear matrix element calculation, the limits for the effective Majorana neutrino mass are $⟨m_{\nu }⟩<0.7–2.8\mathrm{eV}$ for ${}^{100}\mathrm{Mo}$ and $⟨m_{\nu }⟩<1.7–4.9\mathrm{eV}$ for ${}^{82}\mathrm{Se}$.

• Received 19 July 2005

DOI:https://doi.org/10.1103/PhysRevLett.95.182302