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[Xerxes314]

None, but you do get particle showers from energetic cosmic rays in the upper atmosphere.

Fly\'s Eye Cosmic Ray Experiment (http://www.cosmic-ray.org/reading/flyseye.html)

Xerxes

[Xerxes314]

It\'s easy enough to figure out on your own. Calculate the temperature at which each particle in a gas (say, the blast of an explosion) has an energy over 1MeV. It\'s much higher than you\'d think.

Hint: Boltzmann constant is 86 μeV/K

So unless the process is highly energetic on a per-particle basis (antimatter annihilation, maybe some nuclear processes, particle accelerator collisions), you don\'t get pair creation.

Xerxes

[Edited on 2004/10/28 by Xerxes314]