The BiPo detector for ultralow radioactivity measurements

The development of BiPo detectors is dedicated to the measurement of extremely high radiopurity in Tl-208 and Bi-214 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-I 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 Tl-208. After more than one year of background measurement, a surface activity of the scintillators of A(Tl-208) = 1.5 mu Bq/m(2) is reported here. Given this level of background, a larger BiPo3 detector having 3.25 m(2) of active surface area, will able to qualify the radiopurity of the SuperNEMO selenium double beta decay foils with the required sensitivity of A(Tl-208) < 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.