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what is a barn? all i know is that it is a unit of area doing something with atoms. HELP!!!!!!!

The barn is a unit of capture cross section of an atomic nucleus. Cross section is a measure of the probability of something interacting with a nucleus, such as neutron capture. Cross section is expressed as a/(bc), where a is the number of processes occurring, b is the number of particles that could potentially interact with the nuclei, and c is the number of target nuclei per square centimeter. So, this will be in units of square centimeters. This can be expressed as an effective cross-sectional area per nucleus. A barn is 10^(-24) square centimeters per nucleus. It was given this name because it corresponds to a very large cross section; hitting a nucleus with a cross section of 1 barn is as easy as hitting the braod side of a barn.

Richard Barrans Jr., Ph.D.

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