Activity standardisation of 226Ra by 4πα liquid scintillation counting method

•A new primary method for 226Ra activity standardisation by LSC was developed.•The procedure for 226Ra daughter products extraction from measured sample was designed.•Result with total standard uncertainty of 0.34% was obtained.•Applicable for metrology of 226Ra and 222Rn.

Activity of 226Ra in radium daughter products free solution was determined by 4πα liquid scintillation counting (LSC) method, where the detection efficiency of radium decay is practically equal to 1. The sources were prepared from solution with known 226Ra mass concentration, from which, immediately before LS counting, 222Rn and its daughter nuclides were removed by solvent extraction. LS counting results were corrected practically only for a <0.6% loss of radium from measured sample and for the ingrowth of 222Rn and 218Po concentrations in the sample after the separation was completed. The combined relative standard uncertainty was estimated to be lower than 0.34%.