Research on Absolute Activity Measurement Methods of 239 Pu

This study establishes a method for absolute ²³⁹Pu activity measurement without radioactive standards. Using a triple-PMT liquid scintillation counter, true alpha events were separated from instrumental background via Pulse Length Index (PLI). Combined with ICP-MS isotopic ratio, absolute quantification of ²³⁹ Pu in mixed plutonium solutions was achieved. Optimal sample acidity (0.6–0.8 mol/L HNO ₃ ) eliminated adsorption and chemiluminescence interferences. The method demonstrated excellent repeatability (RSD = 0.38%) and stability (0.94% over 7 days). The measured specific activity was 82.78 Bq/g, with combined uncertainty of 1.1% ( k  = 2). This work provides a traceable approach for nuclear material accounting.