Development of Stable Extractive Scintillating Materials for Quantification of Radiostrontium in Aqueous Solutions

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Seliman, Ayman F.
thesis / dissertation description
Strontium-90 is a fission product, and it is produced with a yield of about 6%. Strontium-90 is a major radionuclide in spent nuclear fuel, high-level radioactive wastes resulting from the processing of spent nuclear fuel, and radioactive wastes associated with the operation of reactors and fuel reprocessing plants. In addition to the four stable isotopes naturally present in soil, 90Sr is present in surface soil around the world as a result of fallout from past atmospheric nuclear weapons tests. Strontium-90 is relatively mobile and can move down with percolating water to underlying layers of soil and into groundwater. The maximum contaminant level (MCL) established by the Environmental Protection Agency (EPA) for 90Sr in public drinking water supplies is 0.33 Bq/L (8 pCi/L)(USEPA, 2008). In the human body, radiostrontium concentrates in bone surfaces and bone marrow, and its relatively long radioactive half-life combined with the long biological half-life make it one of the more hazardous products of radioactive fallout. Research is needed for the design of new selective monitoring systems to detect current and changing conditions of radiostrontium contamination in the subsurface. In-situ sensors, that respond to this criterion, avoid expensive sampling operations as well as laboratory analysis. They also facilitate real-time measurement, and decrease the risk to health and cost of long term monitoring. Simple and rapid procedures were developed and characterized using extractive scintillating sensors for online environmental radiation monitoring of 90Sr in aqueous samples. Three different approaches were applied to prepare highly selective structures for simultaneous extraction and detection of radiostrontium. The first approach involves the incorporation of SuperLig®620 solid phase extraction particles into porous scintillating polyvinyltoluene (PVT) beads formulated with the organic fluor monomer 2-(1-naphthyl)-4-vinyl-5-phenyloxazole (vNPO). The new extractive scintillating composite showed good selectivity and detection efficiency of 54.3±0.25% for 90Sr when offline detection mode was applied. However, when the online detection mode was used, the material showed regular decrease in the activity when a fresh background solution was passed through the sensor. The second approach was achieved by growing scintillating polymer chains on the surface of the silica particles of the SuperLig®620 extractant. The polymerization was performed within a thin layer of two high boiling point solvents after modifying the surface initially with 3-methacryloxypropyltrimethoxysilane (MPS) in ethanol to form vinyl- SuperLig®620. The polymerization conducted in dimethylformamide (DMF) or toluene showed high pulse height. The low detection efficiency of 27.4±1.7% in DMF was improved to 32.5±1.4% by conducting the polymerization in toluene. The final approach was achieved by simply mixing the raw SuperLig®620 extractant with inorganic scintillating beads. Three different materials were investigated, but the SuperLig®620/CaF2:Eu mixture (1:2) performed the best as an extractive scintillating sensor with detection efficiency of 54.3±1.3%. The sensor demonstrated a high sensitivity and a chemical stability throughout online detection mode. The setup meets all the requirements to be applied for the real-time measurement of 90Sr over a wide range of radioactivity levels including the MCL of 0.33 Bq/L for 90Sr in drinking water.