 Radioactive cosmogenic 10Be is produced in the atmosphere of the Earth by the cosmic ray spallation of oxygen . 10Be accumulates at the soil surface , where its relatively long half @-@ life ( 1 @.@ 36 million years ) permits a long residence time before decaying to boron @-@ 10 . Thus , 10Be and its daughter products are used to examine natural soil erosion , soil formation and the development of lateritic soils , and as a proxy for measurement of the variations in solar activity and the age of ice cores . The production of 10Be is inversely proportional to solar activity , because increased solar wind during periods of high solar activity decreases the flux of galactic cosmic rays that reach the Earth . Nuclear explosions also form 10Be by the reaction of fast neutrons with 13C in the carbon dioxide in air . This is one of the indicators of past activity at nuclear weapon test sites . The isotope 7Be ( half @-@ life 53 days ) is also cosmogenic , and shows an atmospheric abundance linked to sunspots , much like 10Be . 
