Locating Uranium in Single Particles

The main passive techniques for locating the particles are based on the natural radiation (alpha, beta, and gamma) emitted from the radioactive uranium nuclei and then-progeny. Devices that are sensitive to radiation, like old-fashioned photographic films or their modern electronic equivalent (Fuji plate), have the advantage of size (large samples with many particles can be measured simultaneously) but their sensitivity is low so that long exposure times are required. 

Therefore, several active methods were developed for locating uranium-containing particles. Scanning electron microscopy (SEM) is one of the most powerful techniques used for locating uranium-containing particles. The sample is placed on a stub that is inserted into the vacuum chamber of the SEM and is scanned by an electron beam. Typical peaks are obtained whenever uranium-bearing particles are encountered so that they can be mapped and their coordinates recorded. Characterization of elemental composition can also be carried out by SEM, but for isotopic measurements the particles must be transferred to a suitable device (usually TIMS or ICPMS). 

Another sensitive method is based on measurement of the secondary ions emanating from the sample when it is bombarded by a primary beam of energetic ions (SIMS). For particle location, only a rapid scan is carried out. A mass spectrometer 

Laser-ablation coupled to ICPMS can also be used for screening a sample and locating the uranium-containing particles. Here, too, a rapid scan is useful for mapping and imaging the distribution of relevant particles in the sample and a slower scan rate is required for characterization. As with SIMS analysis, the laser intensity should not be too strong in order to avoid destruction of the particle before it can be properly characterized. 

Some of the methods for locating uranium-containing particles, mentioned earlier, also provide morphological, elemental, or isotopic information. In principle, the whole sample or stub can be transferred from the screening apparatus to other analytical devices and the coordinates of the located particles can be used for further characterization or alternatively individual particles may be manipulated and placed on a TIMS filament, on a silicon planchet or dissolved for determination of the isotopic composition. 

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