Determination of the four natural Ra isotopes in thermal waters by gamma-ray spectrometry

Our method for the simultaneous determination of the four natural Ra isotopes (226Ra, 228Ra, 224Ra and 223Ra) in thermal waters involves a separation of Ra on a selective filter (3 M EMPORE™ Radium Rad disk), and a single counting using a broad-energy HPGe detector (BE Ge manufactured by CANBERRA™). The calculation of 223Ra and 228Ra activities requires interference and cascade summing corrections. The 226Ra activities in CO2-rich thermal waters of the Lodève Basin (South of France) range from 530 to 2240 mBq/l. The low (228Ra/226Ra) activity ratios (0.19–0.29) suggest that Ra is mostly derived from the aquifer carbonates. The short-lived 224Ra and 223Ra are probably added to the water through recoil or desorption processes from Th-enriched coatings on the fracture walls.