Year
2006
Abstract
236U is an indicator for the source of uranium and is measured particularly in uranium in environmental samples where the isotopic abundance of this isotope can be very informative and show clearly to what extent the natural uranium has been mixed with recycled uranium from the nuclear industry. The natural abundance lies below or around 10-11 [1], whereas the abundance in recycled uranium can easily reach 10-3. The large dynamic range of this isotope means that methods of measurement are needed that are calibrated over many orders of magnitude. In particular, for the uranium samples that have 236U abundances < 10-5, electron multiplier collection systems are needed in the mass-spectrometry. A set of uranium isotopic reference materials with abundances of this isotope from 10-5 down to 10-9 at least are a general requirement for the calibration of massspectrometers over the complete range. To meet this challenge a small set of materials prepared by mixing two uranium solutions, certified for isotopic abundances, was prepared. This series showed that at isotopic abundances lower than 10-8 the specified uncertainties of the isotopic ratio 236/238 could not be achieved. However the materials prepared and certified were an excellent start for a further series of mixtures. The second series has been prepared using the same methods applied for the IRMM- 074 isotopic mixtures of uranium (233U, 235U, 238U). A natural material was carefully chosen for the low abundance of 236U. The other starting material was a uranium oxide enriched in 236U. Both oxides were carefully purified and sintered under identical conditions and from one solution of each of these materials and a series of careful dilutions of the 236U solution, a series of mixtures of 236U and natural uranium was made. The certified isotopic abundances were verified by measurements relative to the previous set of 236U isotopic reference materials [2].