222Rn detection at the microBq/m3 range in nitrogen gas and a new Rn purification technique for liquid nitrogen

For the Borexino solar neutrino experiment a concentration line for 222Rn from a large volume of nitrogen gas has been constructed. It is based on cryo-adsorption in a charcoal trap of very low intrinsic 226Ra contamination. Consequently, the blank for activity from the daughter nuclide 222Rn is very low. 222Rn is recorded with proportional counters. This allows the detection of Rn at concentrations below the microBq/m3 level in gaseous nitrogen. The removal of radon from liquid nitrogen is achieved by direct adsorption in the liquid phase. 222Rn-measurements on evaporated nitrogen with and without previous purification are reported.