The on-line determination of low and medium radon activity concentrations within the range from 1 to 10 3 Bq m -3 is not possible with the detectors available so far. A large-volume multiwire pulse ionization chamber with an optimized electrode structure has therefore been developed at the German radon reference chamber of the PTB. The position of the parallel electrode wires follows the shape of two archimedian spirals lying one inside the other. This ensures optimization of the electric field distribution, resulting in an improvement in the energy resolution. It is thus possible to separate the events of the radon decay (E α( 222Rn)=5489,5(3) keV) from those of its short-lived progenies, especially from Eα( 218Po)=6002,35(9) keV. This is achieved by α-spectrometry in air, under atmospheric pressure, without counting gas, in volumes ranging from 5 to 13 l  up to now.