Development of an on-line scintillation flow-cell detection system with pulse shape discrimination for quantification of actinides at environmental levels

Rapid screening of aqueous media for low concentrations of actinides is a critical need at DOE facilities. A laboratory technique that couples the effluent from a high performance liquid chromatography system to an on-line scintillation flow-cell detection system allowed for the determination of thorium, uranium, neptunium, plutonium, americium, and curium in less than 30 minutes. The technique was demonstrated on convenient activities of ~20-80 Bq/radionuclide. In order to apply this technique to environmental samples, sample preconcentration and a lower detection limit are a requisite. To accomplish the latter a flow-cell detection system was developed utilizing pulse shape discrimination as a means of reducing the background when quantifying alpha emitters. Flow-cells consisting of 63-90 μm particles of CaF₂:Eu, scintillating glass (GS-20), and BaF₂ were each packed into translucent Teflon tubing. CaF₂:Eu and GS-20 exhibited only marginal pulse shape resolution, achieving figures of merit, FOM=0.46 and 0.27, respectively. BaF₂ exhibited reasonable pulse resolution, FOM=0.75. Unfortunately, the presence of intrinsic alpha emitters in CaF₂ :Eu and BaF₂ resulted in an elevated background count rate in the alpha window. Coupled with a detection efficiency of ~50% for ²³³U (E_α=4.8 MeV) alpha particles, minimum detectable activities of 0.6 Bq for CaF₂:Eu and BaF ₂, and 1.1 Bq for GS-20 were obtained at the optimal pulse shape time discriminator setting