A New Technique to Measure the Mobility Size of Ultrafine Radioactive Particles

The 218pO clusters formed during radon decay in well controlled composition 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 mobilite electriques circulaire) and made it able to detect such particles/ clusters. The geometry of this radial flow electric mobility analyzer is well adapted for ultrafine clusters measurement because of the short particle residence time and hence lower effects of diffusion. The major modification of this device was the detection system. At the exit of the SMEC, a filter and an alpha particle detector holder were mounted. The airflow containing the particles is drawn through the system when the particles exiting the SMEC are trapped on the filter. An appropriate radioactivity detector mounted directly opposite the filter can separate and count the 218pO particles/clusters. We have used this electrostatic size discrimination technique to investigate the formation of 218pO clusters in well controlled atmospheres. We measured clusters in the size range [0.7-2.0 nm} formed in N2, clean dry air, and gas mixtures containing N2 and short chain alcohols.