Neon is the second-lightest noble gas, glows reddish-orange in a vacuum discharge tube and has over 40 times the refrigerating capacity of liquid helium and three times that of liquid hydrogen (on a per unit volume basis). In most applications it is a less expensive refrigerant than helium. Neon has the most intense discharge at normal voltages and currents of all the rare gases.
Neon has three stable isotopes: Ne-20 (90.48%), Ne-21 (0.27%) and Ne-22 (9.25%). Ne-21 and Ne-22 are nucleogenic and their variations are well understood. In contrast, Ne-20 is not known to be nucleogenic and the causes of its variation in the Earth have been hotly debated. The principal nuclear reactions which generate neon isotopes are neutron emission, alpha decay reactions on Mg-24 and Mg-25, which produce Ne-21 and Ne-22, respectively. The alpha particles are derived from uranium-series decay chains, while the neutrons are mostly produced by secondary reactions from alpha particles. The net result yields a trend towards lower Ne-20/Ne-22 and higher Ne-21/Ne-22 ratios observed in uranium-rich rocks such as granites. Isotopic analysis of exposed terrestrial rocks has demonstrated the cosmogenic production of Ne-21. This isotope is generated by spallation reactions on Mg, Na, Si and Al. 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 surficial rocks and meteorites.
Similar to xenon, neon contents observed in samples of volcanic gases are enriched in Ne-20, as well as nucleogenic Ne-21, relative to Ne-22 contents. The neon isotopic contents of these mantle-derived samples represent a non-atmospheric source of neon. The Ne-20-enriched components were attributed to exotic primordial rare gas components in the Earth, possibly representing solar neon. Elevated Ne-20 abundances were also found in diamonds, further suggesting a solar neon reservoir in the Earth.