In physics, particularly in the field of nuclear and particle physics, a "barn" is a unit of area used to describe the likelihood of nuclear reactions or scattering events. The name "barn" comes from the idea that it's a very large target area, as if hitting the broad side of a barn were an easy task.
The barn is not part of the International System of Units (SI), but it's a widely recognized unit in nuclear physics. One barn is equal to 1 square femtometer (fm²), which is an incredibly small area. Specifically:
1 barn (b) = 1 square femtometer (fm²) = 10^(-28) square meters (m²)
To put it in perspective, a barn is used to describe the effective target area for nuclear reactions. Nuclear cross-sections, which describe the likelihood of nuclear interactions occurring, are often measured in barns. It's a unit that allows scientists to quantify the probability of certain atomic and subatomic processes taking place.
The "square mile (US survey)" is a unit of area used in the United States primarily for land surveying and mapping purposes. It is different from the standard square mile, as it is based on the U.S. survey foot, which is a historical unit of length used in land surveying in the United States. The U.S. survey foot is slightly different from the international foot.
One square mile (US survey) is equal to approximately 3,097,600 square square meters (m²) or about 2.589988110336 square kilometers (km²).
This unit is used in various land-related applications, particularly in the United States, where the U.S. survey foot and the square mile (US survey) are still employed for legal and historical reasons in certain land surveys and property descriptions. It is important to be aware of the specific units being used when dealing with land measurements to avoid any discrepancies or errors.