predominant background. Radon is present in the air of the laboratory and originates from the rock surrounding. It can penetrate the detector through small leaks. A tent coupled to a radon-free air factory was installed around the detector in October 2004 in order to decrease the presence of radon inside the tracker. Since February 2003, after the final tuning of the experimental set-up, NEMO 3 has routinely been taking data devoted to double beta decay studies. The calibration with radioactive sources is carried out every six weeks. The stability of the calorimeter is checked daily with a laser based calibration system [17]. The advantage of the NEMO 3 detector rests in its capability to identify the two electrons from ββ decay and the de-excitation photons from the excited state of the daughter nucleus. The NEMO 3 calorimeter also measures the detection time of the particles. The use of appropriate time-of-flight (TOF) cuts, in addition to energy cuts, allows for an efficient reduction of all backgrounds. A full description of the detector and its characteristics can be found in [17]. The ββ events are selected by requiring two reconstructed electron tracks with a curvature corresponding to a negative charge and originating from a common vertex in the source foil. The energy of each electron measured in the calorimeter should be greater than 200 keV. Each track must hit a separate scintillator block. No trackless PMT signals are allowed. The event is recognized as internal using the measured time difference of the two PMT signals compared to the estimated time-of-flight difference of the electrons. The background can be classified into three groups: external γ -rays, radon inside the tracking volume and internal radioactive contamination of the source. All three were esti- mated from NEMO 3 data with events of various topologies [18]. In particular the radon and internal $^{214}$Bi is measured with e γα events. The e γ , e γγ and e γγγ events are used to measure the $^{208}$Tl activity requiring the detection of the 2.65 MeV γ -ray typical of the $^{208}$Tl β -decay. Single electron events are used to measure the foil contamination by β -emitters. The external background is measured with the events with the detected incoming γ -rays that produce an electron in the source foil. The control of the external background is done with two-electron events originating from the pure Copper and natural Tellurium foils. Mea- III. EXPERIMENTAL RESULTS 5