GADOLINIUM COMPOUNDS FOR PRODUCTION OF LOW-RADIOACTIVE CONSTRUCTION PLASTICS FOR NEUTRON ABSORPTION

Abstract

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.