NEMO3 Experiment: Study of stability of energy and time calorimeter calibrations: Measurement of neutrons contribution to neutrinoless double beta decay

The discovery of neutrinoless double beta decay (ββ0ν), forbidden in the Standard Theory, would permit to determine if neutrinos are massive Majorana particles (ν = ν-bar). With this aim, the NEMO collaboration has built the NEMO3 detector with a sensitivity of 0,2 eV on the neutrino effective mass. This detector is composed of a ββ emitter source, a wire chamber and a calorimeter made of 1940 scintillation counters in order to measure the energy and time of flight of the electrons. The ββ0ν signal expected is a few events per year. To measure it, it is essential to have a precise knowledge of the calorimeter energy and time calibration, and to master all the background sources. The first part of this work was dedicated to the study of the daily calibration survey with the help of a relative calibration system based on laser light. An automated program for laser spectra study and calculation of calibration corrections has been achieved. The behaviour of the whole calorimeter has been characterized for a 23 days period. The second part of this work was the study of the neutrons and γ ray contribution to the background of the experiment. The data taking with and without iron shielding,with and without neutron source were systematically compared with simulations. The very good agreement between experiment and simulations allows us to conclude that with γ and neutron shieldings together with a magnetic field, the objective of 0 external background event will be reached. (author)