Determination of low-level Radium isotope activities in fresh waters by gamma spectrometry
Résumé
A new portable sampling system was developed to extract Radium isotopes from large volumes (up to 300 L) of fresh surface- and ground-waters of low Ra-activities (<5 mBq/L). Ra is quantitatively adsorbed on a small amount (6.5 g) of MnO2-coated acrylic fibers, which are then dried and burned at 600 °C in the laboratory. The resulting Mn-oxide powder (about 2 cm3 when compacted) is then analyzed through gamma-ray spectrometry which allows measurement of the whole Ra quartet (226Ra, 228Ra, 224Ra and 223Ra) in a single counting of a few days. The usual relative standard combined uncertainties (1σ) are 2–3% for 226Ra, 228Ra and 224Ra; and less than 10% for 223Ra. This method was applied to the analysis of Ra in karstic waters of the Lez aquifer, and surface- and ground-waters of the upper and middle Vidourle watershed (South of France). The analyzed waters have relatively low 226Ra activities (1–4 mBq/L) in both cases, regardless of the contrasted geology (Mesozoic limestone vs crystalline Variscan basement), but clearly distinct (228Ra/226Ra) ratios in agreement with the differences in Th/U ratios of the two drained areas. Short-lived Ra isotopes (224Ra and 223Ra) appear to be mainly influenced by near-surface desorption/recoil processes for most of the sampling sites.