Results of the BiPo-1 prototype for radiopurity measurements for the SuperNEMO double beta decay source foils

TL;DR

The BiPo-1 prototype has been used to measure ultra-low levels of radiopurity in detector materials, demonstrating the feasibility of future larger detectors to ensure the ultra-clean conditions needed for SuperNEMO double beta decay experiments.

Contribution

This paper reports the first measurement of surface radiopurity of scintillators using the BiPo-1 prototype, establishing a baseline for larger detectors in double beta decay experiments.

Findings

Measured surface activity of $^{208}$Tl in scintillators as 1.5 μBq/m².

Demonstrated that a 12 m² BiPo detector can achieve required sensitivity within six months.

Validated the BiPo-1 detector's capability for radiopurity assessment in underground laboratories.

Abstract

The development of BiPo detectors is dedicated to the measurement of extremely high radiopurity in $^{208}$Tl and $^{214}$Bi for the SuperNEMO double beta decay source foils. A modular prototype, called BiPo-1, with 0.8 $m^2$ of sensitive surface area, has been running in the Modane Underground Laboratory since February, 2008. The goal of BiPo-1 is to measure the different components of the background and in particular the surface radiopurity of the plastic scintillators that make up the detector. The first phase of data collection has been dedicated to the measurement of the radiopurity in $^{208}$Tl. After more than one year of background measurement, a surface activity of the scintillators of $\mathcal{A}$($^{208}$Tl) $=$ 1.5 $\mu$Bq/m$^2$ is reported here. Given this level of background, a larger BiPo detector having 12 m$^2$ of active surface area, is able to qualify the radiopurity of the SuperNEMO selenium double beta decay foils with the required sensitivity of $\mathcal{A}$($^{208}$Tl) $<$ 2 $\mu$Bq/kg (90% C.L.) with a six month measurement.