Beryllium-10

It is formed in the Earth's atmosphere mainly by cosmic ray spallation of nitrogen and oxygen.

Light elements in the atmosphere react with high energy galactic cosmic ray particles.

The spallation of the reaction products is the source of 10Be (t, u particles like n or p): Because beryllium tends to exist in solutions below about pH 5.5 (and rainwater above many industrialized areas can have a pH less than 5), it will dissolve and be transported to the Earth's surface via rainwater.

[8] It is also formed in nuclear explosions by a reaction of fast neutrons with 13C in the carbon dioxide in air, and is one of the historical indicators of past activity at nuclear test sites.

10Be decay is a significant isotope used as a proxy data measure for cosmogenic nuclides to characterize solar and extra-solar attributes of the past from terrestrial samples.

Plot showing variations in solar activity, including variation in 10 Be concentration which varies inversely with solar activity. (Note that the beryllium scale is inverted, so increases on this scale indicate lower beryllium-10 levels).