Scanning particle microprobe

As NRA has grown from accelerator-based nuclear physics and expanded after the invention of solid-state detectors (the surface barrier Si detector for the detection of particles and Ge(Li) detectors for the detection of y rays), its instrumentation is very much similar to those used in particle and y-ray nuclear spectroscopy. The PIXE method also started to use an existing instrumentation, the Si(Li) X-ray detectors, nearly a decade later. Consequently, this review will refer to the previous O Sects. 33.1 and O 33.2 on PIXE and RBS concerning the acceleration and the formation of energetic ion beams, the internal and external sample chambers, scanning particle microprobe facilities, particle detection, and data acquisition. It will only deal with the characteristic features of the detection of ions and y rays produced in nuclear reactions. Neutrons are also produced in these reactions, but in practice they are rarely used for NRA. Because of space limitations, that technique (Bird and Williams 1989) will not be discussed. 

Two-phase particles ranging from 10 to 20 microns in size, supported on a graphite substrate, were observed in-situ in the UHV chamber of a scanning Auger microprobe. Both surface composition analysis and imaging of the particles could be undertaken. The preparation of the samples has been described in detail elsewhere. ... 

K IxlO 7 torr of oxygen was added to the vacuum system to remove carbon contamination and keep the particle clean. Sample cleanliness was examined by rcannealing the sample in a PHI 660 Scanning Auger Microprobe (SAM). The panicle starts out with a rounded shape and no distinct structure. When the sample is annealed in vacuum, the particle is convened to a nearly spherical shape with a series of flat facets. Electron channeling patterns taken in the scanning electron microscope indicate that the larger facets are oriented in the (100) direction while the smaller facets... 

FIGURE 21-17 Scanning eleclron microprobe oulpul across the surface of an a-cohenite particle in a lunar rock. 

A wide range of analytical techniques is necessary to provide an unambiguous identification of pigments in a sample. Elemental techniques are often used, such as scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), X-ray fluorescence (XRE) spectrometry, scanning electron microprobe analysis (EPMA), X-ray photoelectron spectroscopy (XPS), particle-induced X-ray emission (PDCE), neutron activation analysis (NAA), atomic absorption spectrometry (AAS), inductively coupled... 

Scanning electron microscopy and replication techniques provide information concerning the outer surfaces of the sample only. Accurate electron microprobe analyses require smooth surfaces. To use these techniques profitably, it is therefore necessary to incorporate these requirements into the experimental design, since the interfaces of interest are often below the external particle boundary. To investigate the zones of interest, two general approaches to sample preparation have been used. 

Ion beam spectrochemical analysis Auger emission spectroscopy Scanning electron microscopy (SEM) Electron microprobe (EMPA) Particle-induced X-ray emission spectroscopy (PIXE)... 

Elemental maps obtained using an ion microprobe will be highly surface specific as in SAM. However, since ion sputtering is destructive, repeated scans over the field of particles will penetrate deeper and deeper into the particle interiors. McHugh and Stevens have demonstrated the utility of IMP elemental maps in the identification and chemical characterization of oil soot particles in the atmosphere (38). 

Figure 6. Photographs of CRT output from characteristic x-ray scans for the various elements listed using electron microprobe flOj. Particle scanned was a graphite nodule. Scans verify presence of a heterogeneous inclusion in the nodule. Iron was treated with magnesium and a rare earth silicide. The element distribution pictures were taken at ISOOX-...

Mullite is almost twice as abundant in low-Ca fly ash when compared to high-Ca fly ash, mainly due to differences in the Al content of the clay minerals associated with the coal (McCarthy et al. 1990). Using atomic absorption spectroscopy (AAS) and scanning electron microscopy /electron microprobe analyses (SEM/ EMPA) Stevenson Huber (1987) found a correlation between the elemental composition of ash particles and the clay mineral species in the raw coal. They concluded that the geologic origin of the coal had a significant impact on the microchemical composition of the ash. 

Although a number of secondary minerals have been predicted to form in weathered CCB materials, few have been positively identified by physical characterization methods. Secondary phases in CCB materials may be difficult or impossible to characterize due to their low abundance and small particle size. Conventional mineral identification methods such as X-ray diffraction (XRD) analysis fail to identify secondary phases that are less than 1-5% by weight of the CCB or are X-ray amorphous. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM), coupled with energy dispersive spectroscopy (EDS), can often identify phases not seen by XRD. Additional analytical methods used to characterize trace secondary phases include infrared (IR) spectroscopy, electron microprobe (EMP) analysis, differential thermal analysis (DTA), and various synchrotron radiation techniques (e.g., micro-XRD, X-ray absorption near-eidge spectroscopy [XANES], X-ray absorption fine-structure [XAFSJ). 

Aerosol Heterogeneity. The variation of the chemical composition from particle to particle within an aerosol size class has been probed in a number of ways. Single-particle chemical analysis has been achieved by using the laser Raman microprobe (25) and analytical scanning electron microscopy (26). With the electron microscope techniques, the particle can be sized as well as analyzed chemically, so the need for classification prior to sample collection is reduced. Analyzing hundreds to thousands of particles provides the information necessary to track the particles back to their different sources but is extremely time consuming. 

Forslind, B. etal., Elemental analysis on freeze dried sections of human skin studies by electron microprobe and particle induced x-ray emission analysis, Scanning Electron Microsc., 2, 755, 1984. 

Some of these methods (x-ray fluorescence spectrometry, neutron activation analysis) can directly be applied to the analysis of rocks and soils. No dissolution is needed, and therefore they are called dry methods. The chemical composition of rock and soil samples can directly be analyzed by microprobe techniques (e.g., scanning electron microscopy). The resolution of these methods is about 1 pm, so the composition of individual particles can be investigated separately. 

Size and size distribution can be studied by classical sedimentation techniques. The classification of soils is based on particle sizes (Chapter 1, Section 1.1.3, Table 1.6). The size and shape of the particles can be observed by different microscopes, from the traditional light microscopes to scanning and transmission electron microscopes. The nanometer-sized particles can be observed by the atomic force microscope. This microscope, equipped with a microprobe (scanning and transmission microscope), is suitable for the chemical analysis of the sample. 

Since the last edition (1978), technological developments, such as improved electron microscopy (since Ernst Ruska, 1931), chemical analysis by microprobe (since Raymond Castaing, 1951), scanning electron microscopy (since Oatley McMullan, 1952), automatic computer-controlled instrumentation and software for structure determination, have made it possible to carry out the chemical, structural, morphological and physical characterization of tiny particles of new minerals (on the scale of micrograms) within a few days or weeks computerized structural and morphological drawings can be produced within minutes. 

In one experiment, a few small ring-shaped paiti-cles were placed on top of the reactor. Some of the rings were sealed in one end, others were open in both ends. After the experiment, where Ni and V were allowed to deposit, the particles were soxhlet extracted and prepared for microprobe analysis of V-distribution. The V-distribution was determined at two distances, 2 and 4 mm from the pellet end. Pellet dimensions and positioning of microprobe scans are given in Figure 1. 

Interparticle mobility is proven by electron microprobe scans of cyclic metal impregnated (CMI)[6] Residcat 767Z4+ which incorporates RV4+ technology. Since the catalyst and the RV4+ were simultaneously exposed to the metals during the CMI procedure, the rate of deposition of vanadium on the catalyst and trap surfaces should be similar. However, the catalyst particles, contain virtually no detectable vanadium. In contrast, the RV4+ particles containing the Active Trap Component are high in vanadium. This is another indication of particle to particle vanadium mobility[6]. ... 

The particle size distribution of powders in the range 0.2-0.5 pm can be determined by automated electron probe microanalysis, as developed for particle characterization work at the University of Antwerp (see e.g. Ref. [202]). Here the exciting electron beam of a microprobe scans a deposit of the aerosol particles collected on a Nuclepore filter under computer control, and from the detection of element specific x-ray fluorescence signals, the diameters of a large number of particles are determined automatically. As shown by results for AI2O3, the particle size distributions determined by automated electron probe microanalysis agree to a first approximation with those of stray laser radiation (Fig. 62) [203], Deviations, however,... 

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