Correlative morphological, elemental and chemical phase analyses at the micrometric scale of powdered materials: Application to nuclear forensics

To characterize a mixture of powders using several analytical techniques, it is necessary that successive analyses be carried out on well-identified and localized particles, so that each characterization corresponds to a given powder. In this publication, powdered nuclear materials are characterized at the morphological and elemental levels with a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) and at the chemical level with a micro-Raman spectrometer (MRS). However, to avoid a time-consuming and insufficiently accurate microparticle relocation process between SEM/EDS and MRS, micro-Raman analyses are carried out inside the SEM using a coupling device. In this way, all three pieces of information are obtained for exactly the same micrometric spot, without moving the sample or relocating the microparticles analyzed. The information can therefore be combined to characterize each component of the mixture. In this article, we describe in detail the methodology we have developed and optimized for morphological, elemental and chemical analysis of microparticles using a combined SEM/EDS and SRM. This methodology has been applied to powdered nuclear materials in two international nuclear forensics exercises. In the first exercise, named CMX-6, the combined use of the two instruments identified the presence of PuO₂ microparticles and several uranium compounds (UO₂, U₃O₈, UO₂F₂) in both materials. In the second exercise, called CMX-7, the methodology developed enabled us to distinguish two chemical phases of uranium, a uranyl oxy-hydroxide and a uranyl nitrate, each characterized by specific morphologies and the detection or non-detection of a minor elemental constituent (calcium).