Determination of low-level Radium isotope activities in fresh waters by gamma spectrometry

Abstract

A new portable sampling system was developed to extract Radium isotopes from large volumes (up to 300L) of fresh surface- and ground-waters of low Ra-activities (<5mBq/L). Ra is quantitatively adsorbed on a small amount (6.5g) of MnO₂-coated acrylic fibers, which are then dried and burned at 600°C in the laboratory. The resulting Mn-oxide powder (about 2cm³ 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 (1-4mBq/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.

Keywords: Fresh-waters; Gamma spectrometry; Groundwaters; Mn-fibers; Radium isotopes.