The BiPo-3 detector.

The BiPo-3 detector is a low radioactive detector dedicated to measuring ultra-low natural contaminations of Tl and Bi in thin materials, initially developed to measure the radiopurity of the double β decay source foils of the SuperNEMO experiment at the μBq/kg level. The BiPo-3 technique consists in installing the foil of interest between two thin ultra-radiopure scintillators coupled to low radioactive photomultipliers. The design and performances of the detector are presented. In this paper, the final results of the Tl and Bi activity measurements of the first enriched Se foils are reported for the first time, showing the capability of the detector to reach sensitivities in the range of some μBq/kg.