Research on the Absolute Activity Measurement Method for ²³⁹Pu

To achieve the absolute measurement of ²³⁹Pu activity, a method combining inductively coupled plasma mass spectrometry (ICP-MS) with liquid scintillation counting (LSC) was proposed. The total α activity of the sample was measured using the triple-to-double coincidence ratio (TDCR) mode of a liquid scintillation counter, and the atomic ratio of ²³⁹Pu to ²⁴⁰Pu was determined by ICP-MS. The activity of ²³⁹Pu was then calculated using the decay constants. The nitric acid concentration for sample dilution was optimized to be 0.6-0.8 mol·L⁻¹ to suppress quenching and adsorption. The pulse level index (PLI) was set to 14. A plutonium solution was measured, and the repeatability of the total α specific activity measurement was 0.49%, remaining stable over a seven-day observation period. The specific activity of ²³⁹Pu was 184.47 Bq/g, accounting for 93.11% of the total α activity. The relative expanded uncertainty of the measurement was 1.4% (k=2), with major contributions from the sample weighing, coincidence resolving time, and isotopic ratio measurement. This method does not require standard solutions for efficiency calibration and is essentially an absolute measurement. The applicability of this method to simple plutonium solutions has been validated, providing metrological technical support for nuclear material accounting and nuclear safeguards.