The SuperNemo ββ 0 ν enriched ⁸²Se source foils and their radiopurity measurement with the BiPo-3 detector

The SuperNemo collaboration is currently building the SuperNemo demonstrator at the Modane Underground Laboratory, as the proof of concept for the full SuperNemo program. The enriched ββ 0 ν source consisting of thin foils containing 7 kg of ⁸²Se is enclosed by the gas tracker and the plastic scintillator calorimeter. The full reconstruction of the ββ 0 ν event topology ensures an excellent background rejection and points at a true zero-background search. One of the most critical sources of background is a contamination in the source foils. The required radiopurity is ²⁰⁸Tl < 2 µ Bq/kg and ²¹⁴Bi < 10 µ Bq/kg to achieve the sensitivity T_1/2( ββ 0 ν ) > 10²⁶ years. The collaboration has developed a dedicated detector to measure the ultra high natural radiopurities requested, the BiPo-3 detector, installed in the Canfranc Underground Laboratory. The experimental design and performances of BiPo-3 are presented. Dedicated background measurements have been performed. After an exposure of about 2 years.m² the surface activities of the scintillators of A(²⁰⁸Tl) = 1.0 ± 0.2 µ Bq/m² and A(²¹⁴Bi) = 1.0 ± 0.3 µ Bq/m² are measured. Results of the ²⁰⁸Tl and ²¹⁴Bi activity measurements of the first enriched ⁸²Se foils of SuperNemo are also presented. (paper)