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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 0v). The half-lives of the two neutrino double-beta decay (beta beta 2v) have been measured for 100Mo and 82Se. 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 approximately 7 kg of 100Mo and approximately 1 kg of 82Se. The corresponding limits are T1/2(beta beta0v) > 4.6 x 10(23) yr for 100Mo and T1/2(beta beta 0v) > 1.0 x 10(23) yr for 82Se (90% C.L.). Depending on the nuclear matrix element calculation, the limits for the effective Majorana neutrino mass are < 0.7-2.8 e/v for 100Mo and < 1.7-4.9 eV for 82Se.

The development of the NEMO3 detector, which is now running in the Frejus Underground Laboratory (L.S.M. Laboratoire Souterrain de Modane), was begun more than ten years ago. The NEMO3 detector uses a tracking-calorimeter technique in order to investigate double beta decay processes for several isotopes. The technical description of the detector is followed by the presentation of its performance

The two-neutrino mode of double beta decay (2νββ) of 96 Zr is investigated both theoretically and experimentally. The calculations, using the quasiparticle random phase approximation approach, show that for 96 Zr the detection of 2νββ transitions to the two first excited states in the daughter nuclide will be possible in the very near future. On the experimental side, new, most stringent limits on half-lives for the double beta decay of 96 Zr to excited states in 96 Mo have been obtained with a 430 cm 3 'well-type' germanium detector. They range between 5 × 10 19 and 8 × 10 19 years (90% CL).

The NEMO-3 tracking detector is located in the Fréjus Underground Laboratory. It was designed to study double beta decay in a number of different isotopes. Presented here are the experimental half-life limits on the double beta decay process for the isotopes 100 Mo and 82 Se for different Majoron emission modes and limits on the effective neutrino-Majoron coupling constants. In particular, new limits on "ordinary" Majoron (spectral index 1) decay of 100 Mo (T 1/2 > 2.7 · 10 22 y) and 82 Se (T 1/2 > 1.5 · 10 22 y) have been obtained. Corresponding bounds on the Majoronneutrino coupling constant are g ee < (0.4 − 1.9) · 10 −4 and < (0.66 − 1.7) · 10 −4 .

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23 pages, 7 figures, 4 tables, submitted to Nucl. PhysThe double beta decay of 100Mo to the 0+1 and 2+1 excited states of 100Ru is studied using the NEMO 3 data. After the analysis of 8024 h of data the half-life for the two-neutrino double beta decay of 100Mo to the excited 0+1 state is measured to be T(2nu)_1/2 = [5.{+1.3-0.9}(stat)+/-0.8(syst)]x 10 20 y. The signal-to-background ratio is equal to 3. Information about energy and angular distributions of emitted electrons is also obtained. No evidence for neutrinoless double beta decay to the excited 0+_1 state has been found. The corresponding half-life limit is T^(0nu)_1/2(0+ --> 0+_1) > 8.9 x 10 22 y (at 90% C.L.). The search for the double beta decay to the 2+_1 excited state has allowed the determination of limits on the half-life for the two neutrino mode T(2nu)_1/2(0+ --> 2 +_1) > 1.1 x 10 21 y (at 90% C.L.) and for the neutrinoless mode T(0nu)_1/2(0 + --> 2+_1) > 1.6 x 10 23 y (at 90% C.L.)

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