barn(redirected from Bärn)
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a unit of effective cross-sectional area of nuclear processes. One barn = 10–24 cm2 = 10–28 m2. Selection of this unit is related to the fact that the geometric cross sections of atomic nuclei are on the order of 10–24 cm2 in size. The magnitudes of the majority of cross sections of nuclear reactions usually lie in the range between 10–22 cm2 and 10–27cm2. For this reason fractional and multiple barn units are used, such as millibarn (10–27 cm2), microbarn (10–30 cm2), and megabarn (10–18 cm2).