Absolute Measurement of Alpha Radioactivity and Elemental Amount of Substance Using Liquid Scintillation Method

The liquid scintillation counting method is a simple technique for the absolute activity measurement of α particles. However, it is less common in analytical applications than might be expected. Following a literature analysis, verification experiments using a ²⁴¹Am standard solution were performed and comparative experiments using three liquid scintillation counters were conducted to measure α-nuclides, including ²³⁷Np, ^238-240Pu, ²⁴²Pu, ²⁴¹Am, ²⁴⁴Cm, and ²⁵²Cf. Generally, the counting efficiency of α-nuclides measured by this method is essentially 100%, as confirmed by the triple-to-double coincidence ratio (TDCR) method. TDCR values for all α-nuclides were consistently determined to be 1, indicating 100% counting efficiency. Therefore, this method enables the absolute activity measurement of α-nuclides. For the verification experiments using the ²⁴¹Am standard solution, the relative error between the measured results and the nominal value is less than 1%, and for the comparative experiments across the three counters, the relative standard deviation of the results is also less than 1%. Determining the optimal parameters for α/β discrimination is crucial for accurately measuring α-nuclides. For liquid scintillation counters incapable of providing a two-dimensional spectrum, such as the PerkinElmer 1220 Quantulus and TriCarb 2910, the crossover plot method can be used in the absence of β- and γ-ray interference. Conversely, the inflection point method is recommended when such interference is present. The use of liquid scintillation counters capable of providing a two-dimensional spectrum, such as the Hidex 300 SL, is particularly recommended, as it facilitates the acquisition of the optimal discrimination parameters from the spectrum. The liquid scintillation counting method has poor energy resolution for α-nuclides, limiting its application to the measurement of the total alpha activity in the sample. For U, Np, Pu, and Am, when limited interference conditions are met, the amount of substance can be calculated from the total alpha activity by integrating liquid scintillation, alpha spectroscopy, and gamma spectroscopy. These limited interference conditions are follows. 1) The activity originates exclusively from nuclides of the same element and their daughters, where the element has no stable nuclide. 2) The sample has been aged sufficiently, so that only nuclides with half-lives exceeding 1 year and their daughters need to be considered. 3) The half-life of the primary radioactive nuclide is sufficiently long, and isotopes with longer half-lives are absent or negligible in the sample. 4) All radioactive isotopes are identified; if they cannot be identified, the uncertainties introduced are sufficiently small to be negligible.