Radon emanation coefficients of coal fly ash and soil: Measurement, wall flux geometry correction, and indoor dose implications.

Radon (222Rn) is a radioactive noble gas released from terrestrial materials via the alpha recoil of its parent radium-226 (226Ra), and is the second leading cause of lung cancer worldwide. The fraction of radon atoms that escape from the solid grain matrix into connected pore spaces - the emanation coefficient (η) - governs the radon source term and is essential for indoor radiological hazard assessment. This study estimates η for one coal fly ash and one agricultural soil using γ-spectrometry for 226Ra quantification and an expanded 16-can sealed-can method with LR-115 type II solid-state nuclear track detectors (SSNTDs). The approximately 2.3-fold difference in η (fly ash: 0.0056 ± 0.0015; soil: 0.0127 ± 0.0035, expanded uncertainty k = 2), despite the higher 226Ra activity of fly ash (102.4 vs. 77.6 Bq kg-1), is consistent with the hypothesis that radium is less accessible for recoil release in the glassy aluminosilicate matrix of fly ash; no microstructural measurements were made to test this. The emanation data are combined with the semi-empirical Sahoo et al. wall exhalation model, using estimated diffusion lengths and assumed block and wall geometries, to assess the 3-D to 1-D diffusion geometry transition from sample to constructed wall. The resulting model-derived wall-to-sample flux enhancement factors are 2.35 (fly ash) and 4.1 (soil). Under the assumptions of this first-order model, the estimated dose for the investigated fly ash sample remains below the EU reference level of 1 mSv y-1; this has not been validated by direct wall-flux measurements and should not be generalized to fly ash as a material class. The results indicate that geometry correction may be important when extrapolating laboratory-scale exhalation measurements to building-scale exposure.