Abstract
The nuclear non-proliferation program is based among others on analysis of uranium microparticles. This thesis work consisted in developing and studying performances, limitations and application to various supports of a methodology for direct analysis of the isotopic compositions of uranium particles by means of the coupling between a nanosecond UV (213 nm) laser ablation (LA) device and an inductively coupled plasma mass spectrometer (ICP-MS). After the optimization of the experimental set-up and of the analytical methodology, several batches of uranium particles with different sizes and isotopic composition have been analyzed, especially spherical sub-micrometer uranium particles (415 ± 60 nm) produced in collaboration with the Institute de Chimie Séparative de Marcoule. For these ones, the measured isotopic ratios have been in perfect consistency with the expected values. In addition, thanks to an excellent signal to noise ratio, limits of detection are particularly low, in the attogram range for the 234U isotope. The combined standard uncertainties, around 1 % for the 235U/238U ratios and of a few percent for the 234U/238U ratios, are comparable to those obtained by other particle analysis techniques used in the laboratory. However, the ions separation technology (magnetic field) and the different detectors technologies of the ICP-MS used in this study (Faraday cups and ion counters) are not fully suited for the rapid variations (a few hundreds of µs) of the signal intensity produced by the laser ablation. This represents of major error source which limit the measurement repeatability. Therefore, we have evaluated the potential of the coupling of laser ablation device (ns, UV 193 nm) with a time of flight ICP-MS (icpTOF, Tofwerk, Thun, Switzerland) which allows the acquisition of the entire mass spectrum in 30 µs, for the analysis of micrometer-sized uranium particles. Combined standard uncertainties of approximately 0.3 % and an excellent trueness have been obtained for the measurement of the 235U/238U ratios in micrometer-sized enriched uranium particles. However, performances are still inadequate for the measurement of the minor isotopic ratios (234U/238U, 236U/238U) because of the relatively low sensibility of the instrument and of a high peak tailing effect. Finally, the developed analytical methodology has been applied, for the first time, to the direct analysis of uranium particles deposited onto a pine needle. A significant loss of sensibility compared to a deposition onto graphite disk and moderate increases of the uncertainties have been observed. However, the trueness is still satisfactory.