# The BiPo-3 detector for the measurement of ultra low natural radioactivities of thin materials

## TL;DR

The BiPo-3 detector, operational since 2013 at LSC, measures ultra low natural radioactivity in thin materials, crucial for the SuperNEMO experiment's radiopurity assessment, demonstrating high sensitivity and validated performance.

## Contribution

This paper presents the design, performance, and validation of the BiPo-3 detector for measuring ultra low radioactivity in thin materials, specifically for SuperNEMO's selenium foils.

## Key findings

- Achieved sensitivity of <2 μBq/kg for $^{208}$Tl
- Achieved sensitivity of <140 μBq/kg for $^{214}$Bi
- Validated measurement accuracy with calibrated aluminium foil

## Abstract

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.