  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.
