Measuring Radioactivity and the Mass of Radionuclides

Natural radionuclide concentrations are usually reported in units of radioactivity (e.g., Bq/g or pCi/g). However, to study their geochemical reactions, it is necessary to know radionuclide concentrations in mass-based units, such as moles per liter. 

Properly, radionuclide concentrations are reported in units consistent with the method used for their determination. A radioanalytical technique such as liquid scintillation or alpha spectrometry yields a result expressible in terms of radioactivity. A mass-based analytical technique, such as laser phosphorimetric determination of U(aq) yields mass-based concentrations such as micromoles per liter. Detection limits using radioactivity measuring methods are often many orders of magnitude lower than those possible with mass-based methods (cf. Krieger and Whittaker 1980). For example, 100 pCi/L of Rn, a concentration easily determined by liquid scintillation, equals 2.93 x 10 M, or approximately 1.76 x 10 atoms/L. At standard temperature and pressure, this amount of radon has a minuscule partial pressure of 6.6 x 10 bar, assuming ideal gas behavior. 

If a conversion is to be made between mass-based and radioactivity-based concentrations (or vice versa), the specific activity (SpA) is used. Specific activity is the mass per unit radioactivity of an individual radioisotope and may be calculated with one of the following equations  

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