SIMULTANEOUS MEASUREMENT OF RA-226 AND BA-133 USING LIQUID SCINTILLATION-COUNTING WITH PULSE-SHAPE DISCRIMINATION

The simultaneous counting of Ra-226 from environmental samples and Ba- 133 added as a yield tracer is described. A liquid scintillation count er with pulse shape discrimination was used. The Ra-226 decay emission s were recorded in the alpha-multichannel analyser (alpha-MCA) and the Ba-133 emissions simultaneously recorded in the beta-multi-channel an alyser (beta-MCA) using experimentally determined discriminator settin gs. The Ra-226 alpha-decay products were counted after four weeks ingr owth with an optimum counting efficiency of 250% (2.5 cps Bq(-1)) and an alpha-background of 0.002 cps within the channels corresponding to 300-900 keV. Emissions resulting from decay of Ba-133 by electron capt ure, and the two emissions produced by the beta-decay of the Pb-214 an d Bi-214 daughters of Ra-226 were counted in the beta-MCA. The Ba-133 decay products were counted with an efficiency of 7% (0.07 cpa Bq(-1)) in the channels of the beta-MCA corresponding to 1-50 keV. The ratio of counts from Ra-226 in these channels to the counts from Ra-226 in t he 300-900 keV range of the alpha-MCA was found experimentally to be 0 .148 over a broad range of Ra-226 activities. In analyses the Ba-133 y ield tracer was added to each sample before radiochemical separation i n preparing for liquid scintillation counting. Counts in the alpha-MCA were used as a measure of the Ra-226 activity, then this was used to estimate the Ra-226 beta-contribution and hence the net count rate of Ba-133. Comparison of the Ba-133 count rate with the standard addition was finally used to correct the Ra-226 alpha-count for recovery. Addi tion of 10 Bq of Ba-133 gave counting errors of about 1%. The simultan eous counting of Ba-133 and Ra-226 is Simpler than separate counting o f the Ba-133 by gamma-spectrometry using NaI or Ge detectors.