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barn,abbr. b, in physics, unit of nuclear cross section, i.e., the effective target presented by a nucleusnucleus,
in physics, the extremely dense central core of an atom. The Nature of the Nucleus
Atomic nuclei are composed of two types of particles, protons and neutrons, which are collectively known as nucleons.
..... Click the link for more information. for collisions leading to nuclear reactions; it is equal to 10−24 square centimeters. The barn is approximately the size of the geometric cross section of an atomic nucleus; the term was coined because an effective cross section that large would present a target "as big as a barn," i.e., an easy target for nuclear bombardment. In practice, effective cross sections of nuclei for many reactions are measured in millibarns (10−3 barn) because, for most interactions, only a small fraction of collisions cause reactions.
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).