Study on the method of ultra-low activity value transfer of ²³⁹Pu for accelerator mass spectrometry

Accurate measurement of ultra-low activity ²³⁹Pu is a critical technical requirement in the fields of environmental radioactivity monitoring and nuclear safeguards. Although accelerator mass spectrometry (AMS) offers extremely high sensitivity, as a relative measurement technique, its results must be calibrated against traceable reference materials. In this study, a²³⁹Pu/²⁴⁰Pu mixed solution (atomic ratio 49.599) was used as the target material. First, the absolute activity of ²³⁹Pu was determined using the TDCR liquid scintillation counting method, yielding a specific activity of 184.47 Bq/g (expanded uncertainty 1.4%, k = 2). Two-step gradient dilution was then performed using a precision gravimetric method (dilution factors of 434.6 and 201.2). LSC verification of the primary dilution solution showed excellent agreement between the measured and theoretical values (relative deviation≤2.4%). The activity was thus transferred to a theoretical level of 2.08×10^-3 Bq/g, with a combined relative standard uncertainty of 1.39% (expanded uncertainty 2.8%, k = 2) for the entire value transfer process. Using ²⁴²Pu as an internal standard, AMS measurement targets were prepared by ferric hydroxide coprecipitation, and independent measurements of the diluted sample gave an average ²³⁹Pu specific activity of 2.09×10^-3 Bq/g. The En value between the two methods was 0.054 (far less than 1), indicating excellent consistency within the uncertainty limits. The absolute determination-gradient dilution-AMS verification system established in this study provides a reliable technical approach for the preparation and metrological traceability of ultra-low activity ²³⁹Pu reference materials.