Correcting for H2O interference using a RAD7 electrostatic collection-based silicon detector

The effect of water molecules on the electrostatic collection of (218)Po ions onto the surface of silicon detectors (neutralization) is evaluated through the comparison with a scintillation cell (ZnS), not affected by air humidity. A radon monitor (RAD7, Durridge Company) was connected to a stainless steel radon chamber, equipped with the scintillation cell. Radon gas, extracted from an acidified RaCl2 source, was injected into the chamber and the amount of water molecules in the system was alternatively lowered or increased (from 0.00075 to 0.014 g of water in RAD7) by connecting the chamber to a desiccant or to a bubbling water bottle. The relative efficiency of the silicon detector with respect to the scintillation cell decreases with the growth of water molecules inside RAD7. This dependence, with a fixed i) electrostatic chamber geometry and ii) nominal high voltage, diverges during the humidification or the drying phase because it is in turn influenced by the length of interaction of polonium atoms with water molecules, which impacts on the size of (218)Po clusters and thus on the neutralization process. For water contents higher that 0.01 g in RAD7, this effect is greatly enhanced. Temperature in the investigated range (18.5-35.6 °C) does not affect the efficiency of electrostatic collection-based silicon detectors. Based on these experiments, admitting a certain error on the efficiency (from 1.8 to 7.5%, depending on the water content), proper corrections were developed to adjust soil radon readings, when a desiccant is removed. This operation is necessary if recent Non-Aqueous Phase Liquids (NAPLs) leakage has occurred in the subsoil to avoid the sorption and possible later release of radon by Drierite, with related partition between the solid and liquid phases (water and NAPL).