# Etude du bruit de fond de la double-désintegration beta sans émission de neutrino dans le détecteur Nemo 2: contribution du radon ambiant et mesure de la pollution interne de la source en $^{214}$Bi

Abstract : The NEMO experiment is designed to understand the nature of the neutrino by studing the double beta decay of Mo-100 which is related to the Majorana neutrino effective mass. In this kind of experiment a good understanding of the different sources of background is crucial as only few events are expected per year at the required level of sensitivity. In this thesis we present the main theoretical and experimental aspects of the measurement of the neutrinoless double beta decay of Mo-100 with the prototype detector NEMO2. The goal of this study is to obtain a realistic interpretation of the few events detected at high energy in the two-electron channel as a background to neutrinoless double beta decay. In particular, the contribution arising from Bi-214 has been investigated. These events have been selected and analysed by means of the beta-alpha decays of Bi-214 into Pb-210. The events are characterized by a delayed track in the wire chamber and the corresponding signal is rather clean. The study has demonstrated the diffusion of Rn-222 into the detector and its contribution to Bi-214 pollution has been estimated. A measurement of the Bi-214 internal contamination of the source has been made as well as an estimation of the Bi-214 deposit due to Rn-222. As a result of this study it appears that, under the conditions of the NEM02 experiment, the Bi and Rn contributions are of the same order of magnitude as the background induced at high energy by two-neutrino double beta decay. In conclusion, the backgrounds of the neutrinoless double beta decay of Mo-100 are well understood in the NEM02 experiment leading to an extrapolation for the NEM03 experiment.
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• HAL Id : in2p3-00018142, version 1

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F. Mauger. Etude du bruit de fond de la double-désintegration beta sans émission de neutrino dans le détecteur Nemo 2: contribution du radon ambiant et mesure de la pollution interne de la source en $^{214}$Bi. Physique Nucléaire Expérimentale [nucl-ex]. Université de Caen, 1995. Français. ⟨in2p3-00018142⟩

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