Beryllium-10
Beryllium-10 (Be) is a radioactive isotope of beryllium. It is formed in the Earth's atmosphere mainly by cosmic ray spallation of nitrogen and oxygen. Beryllium-10 has a half-life of 1.39 × 10 years, and decays by beta decay to stable boron-10 with a maximum energy of 556.2 keV. It decays through the reaction Be→B + e. Light elements in the atmosphere react with high energy galactic cosmic ray particles. The spallation of the reaction products is the source of Be (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. As the precipitation quickly becomes more alkaline, beryllium drops out of solution. Cosmogenic Be thereby accumulates at the soil surface, where its relatively long half-life (1.387 million years) permits a long residence time before decaying to B. Be and its daughter product have been used to examine soil erosion, soil formation from regolith, the development of lateritic soils and the age of ice cores. It is also formed in nuclear explosions by  a reaction of fast neutrons with C in the carbon dioxide in air, and is one of the historical indicators of past activity at nuclear test sites. Be 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.