HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Limits on different Majoron decay modes of Mo and Se for neutrinoless double beta decays in the NEMO-3 experiment R. Arnold, C. Augier, J. Baker, A.S. Barabash, V. Brudanin, A.J. Caffrey, E. Caurier, V. Egorov, K. Errahmane, A.I. Etienvre, et al.

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 100Mo and 82Se 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 100Mo (T1/2>2.7×1022yr) and 82Se (T1/2>1.5×1022yr) have been obtained. Corresponding bounds on the majoron–neutrino coupling constant are 〈gee〉<(0.4–1.8)×10−4 and <(0.66–1.9)×10−4.

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 background induced by radioactive impurities of 208Tl and 214Bi in the source of the double beta experiment NEMO-3 has been investigated. New methods of data analysis which decrease the background from the above-mentioned contamination are identified. The techniques can also be applied to other double beta decay experiments capable of measuring independently the energies of the two electrons.

The γ-ray flux in the Fréjus underground laboratory has been studied using an NaI detector surrounded by different diagonostic shields. Below 4MeV, the spectrum is dominated by radioactivities in the surrounding materials and rocks. Between 4 and 6MeV, the shape of the spectrum is well explained by U, Th and daughters, which are internal contaminations in the NaI crystal. Between 6 and 10MeV, the γ-ray flux is strongly correlated with neutron captures in the surrounding materials. Finally, the γ-ray flux above 10MeV falls off and is related to the very weak cosmic muon flux via muon bremsstrahlung.

To be sensitive to an effective Majorana neutrino mass, 〈mν〉, on the order of 0.1 eV, the NEMO 3 double beta (ββ(0ν)) decay experiment requires precise knowledge and control of the backgrounds. The effect of neutrons and γ-rays from the Fréjus underground laboratory (LSM, Laboratoire Souterrain de Modane) has been studied during 10700 h of data collection. The data were taken with a NEMO 2 prototype detector using different shield configurations. Monte Carlo calculations with GEANT/MICAP code and a new library of γ-rays from neutron captures are presented. The implied consequences for the NEMO 3 detector which is under construction are discussed. The neutrinoless double beta decay background induced by neutrons and γ-rays within the LSM will be suppressed to the very suitable level of 0.1 event in a live time of five years given the appropriate shield and magnetic field.

Most currently, viable double beta decay experiments require highly enriched isotopic sources. These sources must be extraordinarily free of radioactive contamination. The double beta decay experiment NEMO 3 will study 100Mo, for which physical and chemical purification techniques have been investigated. The success of the chemical purification process is discussed in the context of ultra-low background, high-purity germanium spectrometer measurements.