9
4Be + n → 2(4
2He) + 2n

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As a metal, beryllium is transparent to most wavelengths of X-rays and gamma rays, making it useful for the output windows of X-ray tubes and other such apparatus.

It is also a good source for the relatively-small numbers of free neutrons in the laboratory which are liberated when beryllium nuclei are struck by energetic alpha particles[3] producing the nuclear reaction
9
4Be + 4
2He → 12
6C + n , where 4
2He is an alpha particle and 12
6C is a carbon-12 nucleus.[4]
[edit]Isotopes and nucleosynthesis
Main articles: Isotopes of beryllium and beryllium-10

Plot showing variations in solar activity, including variation in 10Be concentration.

Note that the beryllium scale is inverted, so increases on this scale indicate lower 10Be levels

Beryllium contains only one stable isotope, 9Be, and therefore is a monoisotopic element.

Cosmogenic 10Be is produced in the atmosphere of the Earth by the cosmic ray spallation of oxygen and nitrogen.

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Cosmogenic 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.[5]

The production of 10Be is inversely proportional to the solar activity,

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because the increased solar wind during periods of high solar magnetic activity in turn 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.[6]

The isotope 7Be (half-life 53 days) is also cosmogenic, and shows an atmospheric abundance linked to sunspots much like 10Be.

8Be has a very short half-life of about 7×10−17 s that contributes to its significant cosmological role, as elements heavier than beryllium could not have been produced by nuclear fusion in the Big Bang.[7]