The NEMO3 detector was designed for the study of double beta decay and in particular to search for the neutrinoless double beta decay process (0νββ). The intended sensitivity in terms of a half-life limit for the 0νββ decay is of the order of 10²⁵ yr which corresponds to an effective neutrino mass 〈m_ν〉 on the level of (0.3 – 0.1) eV. The 0νββ process is today the most promising test of the Majorana nature of the neutrino. The detector was constructed in the Modane Underground Laboratory (LSM) in France by an international collaboration including France, Russia, the Czech Republic, the USA, the UK, Finland, and Japan. The experiment has been taking data since May 2002. The quantity of ¹⁰⁰Mo in the detector (7 kg) allows an efficient measurement of the two-neutrino double beta decay (2νββ) of ¹⁰⁰Mo to the excited 0₁⁺ state (eeNγ channel). Monte-Carlo simulations of the effect and of all the relative sources of background have been produced in order to define a set of appropriate selection criteria. Both Monte-Carlo simulations and special runs with sources of ²⁰⁸Tl and ²¹⁴Bi showed that the only significant background in the eeNγ channel comes from radon that penetrated inside the wire chamber of NEMO3. The experimental data acquired from May 2002 to May 2003 have been analysed in order to determine the signal from the 2νββ decay of ¹⁰⁰Mo to the excited 0₁⁺ state and the corresponding background level. The physical result, which was obtained at the level of four standard deviations, is given in the form of an interval of half-life values at 95% confidence level.
The Ph.D. theses has been supported with a scholarship of the French government (thèse en cotutelle) and has been realized at both LAL Orsay/IN2P3/CNRS and the FNSPE CTU Prague.