Depth distribution

Chaimelling phenomena were studied before Rutherford backscattering was developed as a routine analytical tool. Chaimelling phenomena are also important in ion implantation, where the incident ions can be steered along the lattice planes and rows. Channelling leads to a deep penetration of the incident ions to deptlis below that found in the nonnal, near Gaussian, depth distributions characterized by non-chaimelled energetic ions. Even today, implanted chaimelled... 

Sorbed pesticides are not available for transport, but if water having lower pesticide concentration moves through the soil layer, pesticide is desorbed from the soil surface until a new equiUbrium is reached. Thus, the kinetics of sorption and desorption relative to the water conductivity rates determine the actual rate of pesticide transport. At high rates of water flow, chances are greater that sorption and desorption reactions may not reach equihbrium (64). NonequiUbrium models may describe sorption and desorption better under these circumstances. The prediction of herbicide concentration in the soil solution is further compHcated by hysteresis in the sorption—desorption isotherms. Both sorption and dispersion contribute to the substantial retention of herbicide found behind the initial front in typical breakthrough curves and to the depth distribution of residues. 

In another approach, which was previously mentioned, the mass thickness, or depth distribution of characteristic X-ray generation and the subsequent absorption are calculated using models developed from experimental data into a < )(p2) function. Secondary fluorescence is corrected using the same i flictors as in ZAP. The (pz) formulation is very flexible and allows for multiple boundary conditions to be included easily. It has been used successfully in the study of thin films on substrates and for multilayer thin films. 

Another basic approach of CL analysis methods is that of the CL spectroscopy system (having no electron-beam scanning capability), which essentially consists of a high-vacuum chamber with optical ports and a port for an electron gun. Such a system is a relatively simple but powerful tool for the analysis of ion implantation-induced damage, depth distribution of defects, and interfaces in semiconductors. ... 

Figure 3 Depth distribution of generation of Cu Ka X rays for an incident beam energy of 20 kaV, and the effect of absorption.

Since the composition of the unknown appears in each of the correction factors, it is necessary to make an initial estimate of the composition (taken as the measured lvalue normalized by the sum of all lvalues), predict new lvalues from the composition and the ZAF correction factors, and iterate, testing the measured lvalues and the calculated lvalues for convergence. A closely related procedure to the ZAF method is the so-called ())(pz) method, which uses an analytic description of the X-ray depth distribution function determined from experimental measurements to provide a basis for calculating matrix correction factors. 

Nuclear reaction analysis (NRA) is used to determine the concentration and depth distribution of light elements in the near sur ce (the first few lm) of solids. Because this method relies on nuclear reactions, it is insensitive to solid state matrix effects. Hence, it is easily made quantitative without reference to standard samples. NRA is isotope specific, making it ideal for isotopic tracer experiments. This characteristic also makes NRA less vulnerable than some other methods to interference effects that may overwhelm signals from low abundance elements. In addition, measurements are rapid and nondestructive. 

Fig. 16-8 Depth distribution of the concentration of redox couples in a partially anoxic f]ord, Saanich Inlet. (Redrawn from Emerson et al., 1979.)...

Modem Temperatum-Depth Distribution In Central Greenland... 

Fig. 18-8 Characteristic temperature-depth distributions at an ice divide. For a climatic temperature history as shown in (a) the temperature-depth distribution changes as shown in (b). Following the step increase in surface temperature, the initial steady temperature profile (fi in (b)) is altered by a warming wave (e.g., at time fa) but eventually reaches a new steady profile by time t. (c) Temperature data from Greenland measured by Gary Clow of the US Geological Survey, showing wiggles due to climate variations (Cuffey et ah, 1995).

Figure 8.5 shows the LEIS spectra of ZnAl204 and ZnO as a characteristic example of a multicomponent system analyzed by this technique [Brongersma and Jacobs, 1994]. Since only the surface peaks of A1 and O were detected for ZnAl204, the Zn atoms must be located in the subsurface layers. The onset of the tail agrees between the spectra, indicating that Zn is present in the second and deeper layers. This example illustrates the strength of the LEIS technique, in that characteristic peaks from different elements can be used to selectively analyze the atomic composition of the topmost surface. In addition, the shape of the tails could provide information on the in-depth distribution of the elements. 

FIG. 42. Simulated depth distributions of (a) hydrogen and (b) silicon ions incident ona-Si H with energies of 10 and 50 eV. Note the difference in depth scale. Simulations were performed with TRIM92. 

One of the limitations of the portable field survey instruments in the measurement of americium is that their quantitative accuracy depends on how well the lateral and vertical distribution of americium in the soil compares with the calibration parameters used. These methods can provide a rapid assessment of americium levels on or below surfaces in a particular environment however, laboratory-based analyses of samples procured from these environmental surfaces must be performed in order to ensure accurate quantification of americium (and other radionuclides). This is due, in part, to the strong self absorption of the 59.5 keV gamma-ray by environmental media, such as soil. Consequently, the uncertainty in the depth distribution of americium and the density of the environmental media may contribute to a >30% error in the field survey measurements. Currently, refinements in calibration strategies are being developed to improve both the precision and accuracy (10%) of gamma-ray spectroscopy measurements of americium within contaminated soils (Fong and Alvarez 1997). 

The depth distribution of light stabilisers in coatings has been studied in 1 - 3 mg microtomed slices, by means of SFE-GC with ToF-SIMS and nitrogen thermionic detection, as well as by direct ToF-SIMS analysis results were in good agreement [59]. As the SFE effluent... 

The applicability of alternative photothermal densitometric techniques, such as PAS, for characterisation of TLC plates with particular emphasis on the in-depth distribution of compounds in the sorbent, has been investigated [776], No specific applications for polymer/additive systems appear to have been reported so... 

As nuclear reactions are isotope specific, NRA may be used, for example, to distinguish the distribution of binary blends of polymers in a polymer film, where one of the polymers is labelled with deuterium. The depth distribution of the deuterium atoms can be established and hence that of the labelled polymers. 

In EPMA, X-rays are produced over a range of depths from the surface before their energy falls below Ec. In order to derive the effective absorption factor, an integration must be carried out which requires a knowledge of the shape of the depth distribution of X-ray production. 

The depth distribution function, 4>(p r) represents the X-ray intensity per unit mass depth (pz), relative to that in an isolated thin layer, and is of the form illustrated in Figure 5.12. 

The conversion factor varies much less when the mean dose to all epithelial cells is evaluated (Figure 2). This is especially marked for RaC decays which contribute most of the dose. In this case, very similar doses are calculated if the complex depth distributions of Table I are represented by a single epithelial thickness of 50 pm in the bronchi, i.e. generations 1-10, and 15 pm in bronchioles. 

Figure 2. Relative amounts of various iron species deduced from 57Fe Mossbauer spectra of the Fe-exchanged samples shown in relation to the progress of the hydrothermal crystallization process at 80°C (A), 57Fe Mossbauer spectra of the Fe-exchanged samples after 0 (a), 120 (b), 180 (c) and 240 min (d) of the hydrothermal crystallization process at 80°C (B) and RBS spectra collected on five different particles of the sample crystallized for 240 min (C). The position of surface Fe in Fig. 2C is marked by the vertical arrow. Depth scale (depth into each particle) is increasing toward left (marked with the horizontal arrow). Fit to experimental data with assumed homogeneous depth distribution of Fe is marked with the continuous line.

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