Comparison of radon and thoron exhalation and emanation in granites from Central Portugal

Thoron contribution to the inhaled dose is often neglected due to its shorter half-life and lack of strong gamma emissions that hinder its measurement. However, numerous studies report a significant contribution of thoron and/or its progeny to the dose received by the population. In the present work, radon and thoron exhalation rate and emanation coefficient were measured simultaneously with the accumulation method with an AlphaGuard DF2000 monitor in granite samples collected in high background radiation areas located in Central Portugal. The samples were also analyzed by gamma-ray spectrometry using an Ortec NaI(Tl) detector to determine 226Ra and 224Ra assuming secular equilibrium in the respective decay series. The relationship between radon and thoron exhalation rate and emanation coefficient, and the activity concentrations of their parent isotopes, are investigated. Radon exhalation and emanation are generally higher than thoron’s, however, thoron exhalation rate may exceed the radon exhalation rate in porphyritic granodiorites. Weak correlations are observed between radon and thoron, suggesting they must be estimated independently for the assessment of thoron’s contribution to the dose received by the population. Variations of the radon exhalation rate, radon and thoron emanation coefficient, and the activity concentration of radium (226Ra and 224Ra) are observed linked to the geologic time of emplacement of granitic intrusions during the Variscan orogeny.