A sensitive effluent method for measuring radon gas emanation from low-emanating materials

Abstract

The fraction of radon ( 222Rn) gas emanated from soils and building materials is critical in estimating radiation source strengths for human dosimetry. A sensitive method has been developed to measure emanation fractions ( E) from sample radon effluents. The method equilibrates samples with their emanated radon in sealed cans for 24–30 days. Samples are then assayed for radium ( 226Ra) by high-efficiency gamma scintillation spectrometry that eliminates thorium ( 232Th) interferences. Radon in the can head space is then measured, and E is calculated as the ratio of emanated radon activity to sample radium activity. Values of E measured by the effluent method were compared with corresponding measurements by the differential gamma assay method. Precisions evaluated on 259 Florida soil samples indicated 1000-s detection limits for emanating radium of Ra · E = 0.02 pCi g −1 by the effluent method and Ra · E = 0.5 pCi g −1 by the differential gamma method. The effluent method has superior precision by a factor of eight for background soils containing Ra · E = 0.25 pCi g −1. The effluent method averaged about 8% lower than the differential gamma method on 21 comparison samples. Twenty were within a ±20% data accuracy objective.