Noble gases nuclear components

Some noble gas components are referred to as in situ, meaning that they were produced within a given meteorite sample by nuclear transmutations. These are the nuclear... 

In noble-gas geo-/cosmochemistry the term m situ component is conventionally used to designate a component which is produced in situ, i.e., within any given (solid) sample, by nuclear transmutation, and whose constituent atoms are still identifiably in the places in which they were made. For the noble gases, there are many in situ components of interest in one context or another. As stressed above, this is because noble gases are very scarce in most solid materials. The small quantities of atoms produced in most nuclear transmutation are essentially lost and overwhelmed in the background sea of most elements, but they can be quite prominent when the product atoms are noble gases. 

Meteorites contain a variety of trapped noble gas components other than the solar wind. In many cases their compositions have not been measured in pure form, but have been determined from measured (i.e., not pure) compositions based correlation / mixing lines involving assumptions about one isotopic ratio in the end-member. Some of the components established in this way are of nucleosynthetic origin and carried by presolar grains (diamond, SiC, graphite). Their isotopic compositions are testament to the nuclear processes by which elements are made in the interior of the stars around which those carrier grains formed. 

Nuclear reactions produce not only stable, but also some radioactive noble gas isotopes. Due to their radioactivity the natural background of these isotopes is small and the nuclei are easily detected by their radioactive decay. Although radioactive, these isotopes are noble gases and thus behave chemically inert. As a result, any change in concentration is controlled solely by physical processes, such as radioactive production / decay and mixing of different components. The concentrations of radioactive noble gas isotopes therefore can most directly be employed to calculate the time elapsed since the system was isotopically closed, i.e., the time since radioactive decay alone determined the change of the concentrations of the radioactive isotopes. Some radioactive noble gas isotopes have half-lives similar to renewal times of natural water resources and hence can be used to determine water residence times. 

There has been considerable interest in the possibility of bound excited states arising from the interaction of unMe noble gas atoms. Jortner and co-workers have searched for second continue in radiolysis studies of noble gas mixtures in the solid and liquid phase and at pressures of the order of 1000 mmHg (1 mm Hg = 133 Nm" ) in the gas phase. In general, emission from the homo-nuclear molecules is favoured even for the minor component, but weak continua have been ascribed to ArKr and KrXe. These states are postulated to correlate with the excited state of the heavier atom and their weaker bonding relative to homonuclear molecules is clear from comparison of the wavelengths corresponding to peak intensity of the continua ... 

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