A NEW TECHNIQUE TO MEASURE THE MOBILITY SIZE OF ULTRAFINE RADIOACTIVEPARTICLES

The Po-218 clnsters formed during radon decay in well controlled compo sition atmospheres are too small to be detected by a classical aerosol detection device such as a condensation nuclei counter. In this work we modified the first order version of the SMEC (spectrometre de mobil ite electriques circulaire) and made it able to detect such particles/ clusters. The geometry of this radial how electric mobility analyzer i s well adapted for ultrafine clusters measurement because of the short particle residence time and hence lower effects of diffusion. The maj or modification of this device was the detection system. At the exit o f the SMEC, a filter and an alpha particle detector holder were mounte d. The airflow containing the particles is drawn through the system wh en the particles exiting the SMEC are trapped on the filter. An approp riate radioactivity detector mounted directly opposite the filter can separate and count the Po-218 particles/clusters. We have used this el ectrostatic size discrimination technique to investigate the formation of Po-218 clusters in well controlled atmospheres. We measured cluste rs in the size range [0.7-2.0 nm] formed in N-2, clean dry air, and ga s mixtures containing N-2 and short chain alcohols. (C) 1997 American Association for Aerosol Research.