Helium migration

ShelbyJ.E. (1972a) Helium migration in natural and synthetic vitreous silica. /. Am. Ceram. Soc. 55, 61-64. 

Even more significant is the suggestion (Fig. 6) that helium migrates vertically from its progenerator uranium accumulation... 

The column is swept continuously by a carrier gas such as helium, hydrogen, nitrogen or argon. The sample is injected into the head of the column where it is vaporized and picked up by the carrier gas. In packed columns, the injected volume is on the order of a microliter, whereas in a capillary column a flow divider (split) is installed at the head of the column and only a tiny fraction of the volume injected, about one per cent, is carried into the column. The different components migrate through the length of the column by a continuous succession of equilibria between the stationary and mobile phases. The components are held up by their attraction for the stationary phase and their vaporization temperatures. 

Cathodoluminescence microscopy and spectroscopy techniques are powerful tools for analyzing the spatial uniformity of stresses in mismatched heterostructures, such as GaAs/Si and GaAs/InP. The stresses in such systems are due to the difference in thermal expansion coefficients between the epitaxial layer and the substrate. The presence of stress in the epitaxial layer leads to the modification of the band structure, and thus affects its electronic properties it also can cause the migration of dislocations, which may lead to the degradation of optoelectronic devices based on such mismatched heterostructures. This application employs low-temperature (preferably liquid-helium) CL microscopy and spectroscopy in conjunction with the known behavior of the optical transitions in the presence of stress to analyze the spatial uniformity of stress in GaAs epitaxial layers. This analysis can reveal,... 

Thus selective adsorption occurs with certain directions of approach of the molecules to the surface. Further calculations indicate, however, that the helium atoms are mobile on the surface, migrating along it freely. After travelling a distance of the order 10 5 cm., they will probably evaporate in the same direction as if they had been simply reflected in the first instance. If this had occurred everywhere in the experiments of Frisch and Stern, no dips in the curves would have been noticed, and the phenomenon of selective adsorption would not have been discovered experimentally. But the re-evaporation in specified directions will not occur unless the crystal surface is both perfect and clean if the migrating helium atoms hit, while on the surface, another adsorbed atom, or an imperfection in the crystal, they will change direction and energy, and the reflected or diffracted beams will be absent, or nearly absent, from those directions in which the selective adsorption has occurred. 

During migration and subsurface storage groundwater comes into contact with crustal rocks that continuously release helium from the decay of uranium and thorium. The basic assumption is made that the water acts as a sink for the helium evolved from the local rocks. The age of groundwater, t, is calculable from the equation given in section 14.2. 

The tritium diffuses to the grain boundaries, migrates through the grain boundaries to the particle surface, and then percolates through the packed bed to low pressure helium purge channels. Heat is removed by pressurized water flowing in a stainless steel tube network in the packed bed. 

Gerling, E.K., 1957. The migration of helium from minerals and rocks. Trudy Radievogo Inst. Im. V.G. Khlopina 6 64-87 (AEC-tr-4208). 

This suggests that double-bond migration and cis-trans isomerization do not proceed by reversion from monoadsorbed alkane. This conclusion is consistent with the fact that isomerization is at least as fast in the presence of helium as in that of hydrogen (Table V). 

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