Comparison of two direct LS methods for measuring 222Rn in drinking water using α/β liquid scintillation spectrometry

Direct liquid scintillation (LS) methods are widely used for surveying ²²²Rn in drinking water. Two direct methods are used that differ in sample composition. In a two-phase sample, water lies below a water-immiscible cocktail, while in a homogeneous sample water is mixed with an emulsifying cocktail. Although these methods were developed in the late 1970s, their performances have not been simultaneously tested. Here, the methods were compared in two ways: by preparing both types of sample similarly from ²²²Rn-bearing groundwater in one emulsifying and in three organic cocktails, and by calibrating the methods with a ²²⁶Ra standard according to their respective procedures. The samples were measured using α/β LS spectrometry. The standard deviations of parallel samples and the repeatability of the measurements were excellent for both methods, except two-phase ²²⁶Ra samples, whose efficiencies decreased slightly over time. This instability was due to interference from ²¹⁰Pb, ²¹⁰Bi and ²¹⁰Po, which accumulated in the ²²⁶Ra standard, and possibly also to the migration of ²¹⁴Pb and ²¹⁴Bi into the aqueous phase and deficient transfer of ²²²Rn from the water to the cocktail.