A nuclear mass spectrometer for studying radon progeny clusters

A Nuclear Mass Spectrometer (NMS) system was developed and used to investigate the cluster formation of radon progeny (218Po) in gaseous H2O and H2O-SO2 environments. This NMS combines the mass-separation ability of a mass spectrometer and the low detection limit of a surface barrier detector for alpha particles and enables the detection of individual radon progeny cluster ions of defined mass at the molecular level. Clusters in the form of 218Po+(H2O)n were experimentally observed for 218Po at a relative humidity of 0.1%. The number n ranged from 1 to 7 with 4 being the most abundant peak under these conditions. No charged cluster of 218Po was detected when the relative humidity was 90%, possibly due to a neutralization process. On addition of SO2 at a low relative humidity, 218Po+(H2O)n (H2SO4)m clusters were experimentally detected for the first time by the NMS system in this study with n from 0-4 and m from 0-3. The maximum distribution occurs at a radius of about 3.65 angstrom or a mobility of about 2.00 cm2 V-1 s-1. These maximum clusters correspond to a composition of 218Po+(H2SO4)2 or 218Po+(H20)2 (H2SO4)1.