Radium activity can be converted to its concentration.

Mass of element (g) = (Activity * mass numb) / (decay constant * Avogadro’s numb)

Avogadro number - approximately 6.02*10^23 ;

Decay constant for 226Ra - 2.21*10-6

226Ra has a specific activity of 3.66×10^10 Bq/g (98.92×10^10 pCi/g):

1 g --> 98.92×10^10 pCi

Well,

Dear Mr. Zemour,

see my interpretation I gave to a similar question of yours: Radium (Ra-226) is a radioactive isotope of the parent radionuclide U-238. It is far less soluble than U-238 and less soluble than Th-234. Radium is chemical akin in the periodical chart of elements to barium (Ba) which forms the almost insoluble mineral BaSO4 (barite). Based upon its position in the 2nd group below Ba, Ra forms a short-lived equivalent sulfate under oxidizing conditions in the presence of sulfur called “radio-barite” (Ba,Ra)SO4. It is the least soluble mineral at all and also accountable for the disequilibria in the U-238 decay chain. Due to the short half-life it only comes up in Quaternary host environments, e.g., swamps or hydrothermalites/ thermal waters. This may contribute to variation of Ra in aquatic systems. H.G.Dill