Saturday, July 21, 2012

Neon, primordial and nucleogenic

Mill Mountain Star Neon Lights
There is an excellent article on neon in wikipedia. Here are selected passages from it (rearranged and formatted by me):

Neon is a chemical element with symbol Ne and atomic number 10.

Neon is the first p-block noble gas. Neon is generally considered to be inert. No true neutral compounds of neon are known. It is commercially extracted from air, in which it is found in trace amounts.

Although neon is a very common element in the universe, it is rare on Earth; it is the fifth most abundant chemical element in the universe by mass, after hydrogen, helium, oxygen, and carbon.

Its relative rarity on Earth, like that of helium, is due to its relative lightness, high vapor pressure at very low temperatures, and chemical inertness, all properties which tend to keep it from being trapped in the condensing gas and dust clouds which resulted in the formation of smaller and warmer solid planets like Earth.

Stable forms of neon are produced in stars.

It is created in fusing helium and oxygen in the alpha process, which requires temperatures above 100 megakelvins and masses greater than 3 solar masses.

A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or in high-voltage discharge tubes or neon advertising signs. It is commercially extracted from air, in which it is found in trace amounts. Neon is monatomic, making it lighter than the molecules of diatomic nitrogen and oxygen which form the bulk of Earth's atmosphere; a balloon filled with neon will rise in air, albeit more slowly than a helium balloon

Neon isotopes
Neon has three stable isotopes: 20Ne (90.48%), 21Ne (0.27%) and 22Ne (9.25%).

21Ne and 22Ne are partly primordial and partly nucleogenic (i.e., made by nuclear reactions of other nuclides with neutrons or other particles in the environment) and their variations in natural abundance are well understood.

In contrast, 20Ne (the chief primordial isotope made in stellar nucleosynthesis) is not known to be nucleogenic or radiogenic (save for cluster decay production, which is thought to produce only a small amount). The causes of the variation of 20Ne in the Earth have thus been hotly debated.

The principal nuclear reactions which generate nucleogenic neon isotopes start with magnesium
  • start from 24Mg and 25Mg, 
  • which produce 21Ne and 22Ne, respectively, after neutron capture and immediate emission of an alpha particle. 
The neutrons that produce the reactions are mostly produced by secondary spallation reactions from alpha particles, in turn derived from uranium-series decay chains.

The net result yields a trend towards lower 20Ne/22Ne and higher 21Ne/22Ne ratios observed in uranium-rich rocks such as granites.

Neon-21 may also be produced in a nucleogenic reaction, when
  • 20Ne absorbs a neutron from various natural terrestrial neutron sources.
In addition, isotopic analysis of exposed terrestrial rocks has demonstrated the cosmogenic (cosmic ray) production of 21Ne.

This isotope is generated by spallation reactions on magnesium, sodium, silicon, and aluminium. By analyzing all three isotopes, the cosmogenic component can be resolved from magmatic neon and nucleogenic neon.

This suggests that neon will be a useful tool in determining cosmic exposure ages of surface rocks and meteorites.

Non-atmospheric source, Sun?
Similar to xenon, neon content observed in samples of volcanic gases is enriched in 20Ne, as well as nucleogenic 21Ne, relative to 22Ne content.

The neon isotopic content of these mantle-derived samples represents a non-atmospheric source of neon. The 20Ne-enriched components are attributed to exotic primordial rare gas components in the Earth, possibly representing solar neon.

Elevated 20Ne abundances are found in diamonds, further suggesting a solar neon reservoir in the Earth.

Read the entire article on Neon from wikipedia

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