GADOLINIUM COMPOUNDS FOR PRODUCTION OF LOW-RADIOACTIVE CONSTRUCTION PLASTICS FOR NEUTRON ABSORPTION
Two stage purification process was developed for obtaining ultra-low-radioactive GdCl3 preparations. The process includes a chlorination annealing and a vacuum sublimation procedure. The concentrations of residual uranium and thorium in GdCl3 powder preparation were (0.06±0.04) ppb and (0.05±0.03) ppb, consequently. These values are close to the requirements that apply to structural composite material which is used as a neutron-absorbing additive in VETO detectors. A method has been developed for the determination of 65 impurity elements by ICP-MS technique in Gd-based preparations with limits of detection within the range of 100-0.01 ppb depending on the element. The limits of detection of U was 0.011 ppb, Th – 0.016 ppb. When analyzing Gd-based preparations by ICP-MS, it is recommended to use the following isotopes 82Se, 147Sm, 151Eu, 163Dy, 178Hf, 188Os, 194Pt for the quantitative measurement of concentrations of difficultly determined elements. It was found that the lowest content of U and Th was observed in commercial Gd-based preparations obtained from secondary raw materials (EPA - extraction phosphoric acid) or from mineral raw materials formed in sedimentary rocks (phosphogypsum) at the level of 1.2-0.2 ppb for both U and Th contents. Analysis of commercial polymers showed that due to low U/Th content and mechanical characteristics at low temperature polymethylmethacrylate (PMMA) is the best polymer matrix for Gd-doped structural composite material for neutron-absorption additive in VETO detectors.