  The development of BiPo detectors is dedicated to the measurement of
extremely high radiopurity in 208Tl and 214Bi for the SuperNEMO double beta
decay source foils. A modular prototype, called BiPo-1, with 0.8 m2 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 208Tl. After more than
one year of background measurement, a surface activity of the scintillators of
A(208Tl) = 1.5 \muBq/m2 is reported here.
  Given this level of background, a larger BiPo3 detector having 3.25 m2 of
active surface area, will able to qualify the radiopurity of the SuperNEMO
selenium double beta decay foils with the required sensitivity of A(208Tl) <
3-4 \mu Bq/kg (90% C.L.) with a six month measurement. This detector is
actually under construction and will be installed in the Canfranc Underground
Laboratory mid 2011.
