LSC-BASED APPROACH FOR RADON IN SOIL GAS MEASUREMENTS

Estimation of radon risk in soil gas is a preventive practice when building safe dwellings. We propose a method for measuring of 222Rn in soil gas based on liquid scintillation counting (LSC). A small-diameter hollow steel probe with a sharpened lower-end tip combined with a large-volume syringe (RADON v.o.s., Czech Republic) is used as a soil gas sampling system. One measurement approach includes direct soil gas counting in a special LS vial coated inside with plastic scintillation material, MeltilexTM. Another approach is about 10 times more sensitive. It includes a radon trap-glass bubbler filled with a 20-mL portion of a toluene-based LS cocktail. After sampling, the LS solution is transferred into conventional vials for LS counting. The air inside the 150-mL syringe in both cases is blown through a vial or a bubbler. In the bubbler method, 3 portions of the air syringe volume are blown through the bubbler to make the method more reproducible. Rn sorption in the trap depends on the toluene and bubbler volumes, temperature, and humidity. In both measurement approaches, modern LSC measurement equipment with alpha/beta separation is used. The method was tested on Quantulus 1220TM and Triathler LS spectrometers. The described measurement approaches yielded count rates in the alpha window of 2.4 cpm for direct counting in Meltilex-coated vials and ~24 cpm when Rn is trapped in toluene, both corresponding to a concentration of 1 kBq m−3 for Rn in soil gas.