Simultaneous measurement of 226Ra and 133Ba using liquid scintillation counting with pulse shape discrimination

The simultaneous counting of 226Ra from environmental samples and 133Ba added as a yield tracer is described. A liquid scintillation counter with pulse shape discrimination was used. The 226Ra decay emissions were recorded in the α-multi-channel analyser (α-MCA) and the 133Ba emissions simultaneously recorded in the β-multi-channel analyser (β-MCA) using experimentally determined discriminator settings. The 226Ra α-decay products were counted after four weeks ingrowth with an optimum counting efficiency of 250% (2.5 cps Bq−1) and an α-background of 0.002 cps within the channels corresponding to 300–900 keV. Emissions resulting from decay of 133Ba by electron capture, and the two emissions produced by the β-decay of the 214Pb and 214Bi daughters of 226Ra were counted in the β-MCA. The 133Ba decay products were counted with an efficiency of 7% (0.07 cps Bq−1) in the channels of the β-MCA corresponding to 1–50 keV. The ratio of counts from 226Ra in these channels to the counts from 226Ra in the 300–900 keV range of the α-MCA was found experimentally to be 0.148 over a broad range of 226Ra activities. In analyses the 133Ba yield tracer was added to each sample before radiochemical separation in preparing for liquid scintillation counting. Counts in the α-MCA were used as a measure of the 226Ra activity, then this was used to estimate the 226Ra β-contribution and hence the net count rate of 133Ba. Comparison of the 133Ba count rate with the standard addition was finally used to correct the 226Ra α-count for recovery. Addition of 10 Bq of 133Ba gave counting errors of about 1%. The simultaneous counting of 133Ba and 226Ra is simpler than separate counting of the 133Ba by γ-spectrometry using Nal or Ge detectors.