The BiPo-3 detector, running in the Canfranc Underground Laboratory (Laboratorio Subterr\'aneo de Canfranc, LSC, Spain) since 2013, is a low-radioactivity detector dedicated to measuring ultra low natural radionuclide contaminations of $^{ 208}$Tl ($^{ 232}$Th chain) and $^{ 214}$Bi ($^{ 238}$U chain) in thin materials. The total sensitive surface area of the detector is 3.6 m$^2$. The detector has been developed to measure radiopurity of the selenium double $\beta$-decay source foils of the SuperNEMO experiment. In this paper the design and performance of the detector, and results of the background measurements in $^{ 208}$Tl and $^{ 214}$Bi, are presented, and validation of the BiPo-3 measurement with a calibrated aluminium foil is discussed. Results of the $^{ 208}$Tl and $^{ 214}$Bi activity measurements of the first enriched $^{ 82}$Se foils of the double $\beta$-decay SuperNEMO experiment are reported. The sensitivity of the BiPo-3 detector for the measurement of the SuperNEMO $^{ 82}$Se foils is $\mathcal{ A}$($^{ 208}$Tl) $<2$ $\mu$Bq/kg (90\% C.L.) and $\mathcal{ A}$($^{ 214}$Bi) $<140$ $\mu$Bq/kg (90\% C.L.) after 6 months of measurement.
Categories: physics.ins-det
