Radon-in-water secondary standard preparation

Radon-in-water standard solutions are required for the calibration and quality control of instruments measuring Radon concentration in water samples. However, due to the short half-life of Radon, such standards are not widely available.This work presents a method developed at the NES-NTUA to generate Radon-in-water solutions of known concentration that can easily be applied in a laboratory with access to a Radon-in-air calibration facility. Air with the appropriate concentration of Radon is bubbled through deionized distilled water, giving a solution with the desired Radon concentration, as predicted by published Radon solubility data. Operating parameters have been suitably chosen so that volumes in the order of liters can be generated, with concentrations from less than 1 Bq/lt up to some tens of Bq/liter, depending on the Radon source used. The entire process takes less than 30 minutes, excluding the time for the original build-up of Radon in the reference Radon chamber.A series of experiments conducted to validate the method is presented, where the samples produced were measured using active instrumentation coupled to a Radon bubbler. The concentrations tested ranged from 3 to 17 Bq/lt. Performance in terms of precision and accuracy was found to be adequate. An example is also given, with this method applied to check the calibration of a Radon-in-water measuring apparatus based on a gas-transfer membrane.The method has been proven to be accurate enough for the solution produced to be usable as a secondary standard, traceable to within 10% of the calibration of the original Radon source. This fact, as well as other attractive features such as low running cost and ease of use, makes it appropriate for purposes such as quality control, intercalibration of instruments and laboratory intercomparison.