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24 May 2012

Answer to Question #266 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements

The following question was answered by an expert in the appropriate field:

How do I interpret the measurements from ²¹⁴Bi and ²¹⁴Pb in order to determine the concentration of ²²⁶Ra in a sample?

226The activity, or concentration, of ²²⁶Ra will be equal to the activity, or concentration, of ²¹⁴Bi and ²¹⁴Pb whenever ²²²Rn and its progeny are in secular equilibrium with ²²⁶Ra. Although dry soil sample preparation can cause radium progeny disequilibrium due to the loss of free radon, this factor can be quite small due to the small fraction of "free" radon in many solid radium-bearing media. For liquid samples or analyses involving dissolution of a solid, the process of preparation can cause the loss of a lot of radon and therefore must be considered during sample analysis. It generally takes about 20 to 30 days for ²²²Rn to return to equilibrium with ²²⁶Ra following any process which causes a substantial release of radon from the sample media and may require that the sample be sealed gas-tight--depending on the final form of the aliquot.

Robert Coleman
Oak Ridge National Laboratory

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