Geometry and Composition

In Chapter 6, I discussed the effect of sample geometry on count rate. Once again, it would be useful to consider a practical example. Table 8.4 lists the peak areas measured when the same amount of Eu was counted as a point source and when distributed in water and in sand. The distributed sources were 13 mm in diameter and 20 mm high and measured on the cap of a 45 % p-type HPGe detector. As one would expect, there is an obvious overall loss of count rate due to the lower effective solid angle of the distributed sources and a more pronounced loss of count rate in the low energy peaks. 

The inescapable conclusion is that, unless sample and standard (or calibration) sources have identical shape and density, are in identical containers and are measured at the same distance, there will be differences in summing which will not be accounted for by the routine calibration process. 

It would seem that, if we must measure our samples close to the detector, our results would be in error due to TCS. It is important to remember that the degree of summing will be different for each nuclide and, if we are measuring more than one gamma-ray from a nuclide, different for each gamma-ray. Is there any way in which we can correct 

As an example, take the measurement of the NORM nuclides (Chapter 16). Most of the nuclides to be measured, the U and Th decay series, have complicated decay schemes and suffer from TCS, seriously in some cases. If reference materials containing the relevant nuclides (IAEA RGU-1 and RGTh-1, come to mind) were to be used as calibration standards, then effective efficiency data could be acquired. The efficiency curve would not be a pretty sight, because of the TCS, but as long as the interpolative mode was used, the correct, TCS-accounted-for, efficiency data would be used when analysing the sample spectra. It would not be acceptable, though, to use that calibration for measurements of any nuclides other than those represented in the calibration data. 

3 Using correction factors derived from efficiency calibration curves 

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The materials described in the preceding section may be combined in an OLED de vice in a variety of different geometries and compositions. The simplest of these is a single oiganic layer sandwiched between two electrodes. In contrast to the convention used in surface science, it is customary to list the layers in the order of deposition. Thus, anode/organic/cathodc (for example, ITO/PPV/A1) implies that the anode (ITO) is deposited first on the (presumably transparent) substrate. 

Earlier studies demonstrated a rich variety of oxidation states, geometries and compositions of the intermediates and products formed in the autoxidation reactions of cysteine (RSH). Owing to the complexity of these systems, only a limited number of detailed kinetic papers were published on this subject and, not surprisingly, some of the results are... 

Wool and Cole (6) described a simulation model based on percolation theory for predicting accessibility of starch in LDPE to microbial attack and acid hydrolysis. This model predicted a percolation threshold at 30% (v/v) starch irrespective of component geometry, but the predicted values are not in accordance with results of enzymatic or microbial attack on these materials (Cole, M.A., unpublished data). Since a model that incorporates component geometry provides a better fit to experimental data than a geometry-independent model does, development of advanced models should be based on material geometry and composition, rather than on composition alone. 

Calibration of the intensities of the radiation flelds is traceable to the NIST. The ionization chambers and electrometers used by the service laboratories to quantify the intensity of the radiation fields must be calibrated by the NIST or an accredited secondary standards laboratory. The intensity of the field is assessed in terms of air kerma or exposure (free-in-air), with the field collimated to minimize unwanted scatter. Conversion coefficients relate the air kerma or exposure (free-in-air) to the dose equivalent at a specified depth in a material of specified geometry and composition when the material is placed in the radiation field. The conversion coefficients vary as a function of photon energy, angle of incidence, and size and shape of backscatter mediiun. 

The photolyses of 2-aziadamantane (16) within CyD hosts give product distributions that largely depend on the type of CyD and thus the supramolecular structure. The formation of 2,4-didehydroadamantane (22) is essentially precluded when other inter- and innermolecular reaction channels are available. However, the formation of 22 is greatly enhanced within supramolecular coniines. The amount of reduction to hydrocarbon 2, the share of azine 19 and 2-adamantanone, the formation of CyD ethers 25a and 25b, and the insertion into solvent molecules yielding 25c-e can widely be controlled by geometry and composition of the ICs. This very interesting influence of the host as mediator can be observed especially well for the photolyses of 16, which primarily yield carbene 21—a species that has no intramolecular pathways to stabilization. 

The additivity of tba individual resistances is dependant on tbe linearity of the flux expressions and of the equilibrium relationship. For nonlinear equilibrium relationships Eq. (2,4-IOa) and (2,4-lOb) can still ha used provided m is recognized to he a function of the interfacia] composition. Overall coefficients are often employed in the analysis of fluid-fluid mass transfer operations despite their complex dependence on the hydrodynamics, geometry and compositions of the two phases, In some instances the overall coefficients cen bs predicted from correlations for the individual coefficients for each phase provided the conditions in the apparatus are comparable to those for which the correlation was developed. 

Phospholipase C activity is not directly influenced by the formation of non-bilayer structures. However, the presence of lipids (e.g., PE) with a tendency to form such structures stimulates the enzyme even under conditions at which purely bilayer phases exist. Conversely, sphingomyelin, a stabilizer of the bilayer phase, inhibits the enzyme. Thus, phospholipase C appears to be regulated by the overall geometry and composition of the bilayer (M.B. Ruiz-Arguello, 1998) supporting the hypothesis that the collective physical properties of the lipid bilayer can modulate the activities of membrane-associated proteins. 

It is known [1] that when electromagnetic radiation with wavelength, comparable to the array s periodicity passes through such an array, the dispersion relation is modified according to the array geometry and composition. Thus, such artificially engineered periodic structures can be used as a photonic crystal. 

In this respect, colloidal particles are ideal candidates serving as templates for the formation of amorphous and highly ordered porous networks, particularly for macroporous networks, owing also to the maturing synthesis routes of various colloidal particles with controlled size, geometry, and composition [14-18]. Due to their small sizes and the forces exerted upon... 

Ceramics are usually used in a polycrystalline form. GBs in ionic and covalent materials must be better understood to improve the science of processing of many modem ceramic materials the properties of polycrystalline ceramics depend directly on the geometry and composition of GBs. The types of GBs commonly found in ceramic materials range from situations in which the distance between the grains is >0.1 pm and such grains are separated by a second phase (glass), to the basal twin boundary in AI2O3, which is atomically abrupt and potentially very clean. 

The main reactivity coefficients are shown in Table II compared to the previous design (7). The differences reflect core geometry and composition effects, as well as different cross section input and, in some instances, different calculational methods. 

The properties, size, geometry, and compositions of materials are the key factors in modifying the material activity to improve H2 production (Murphy et al., 2006). Figure 7.3 shows the schematic of photoelectrochemical H2O decomposition. 

B. K. Juluri et al.. Effects of geometry and composition on charge-induced plasmonic shifts in gold nanoparticles. Journal of Physical Chemistry C, 112(19), 7309-7317 (2008). 

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