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.
A square meter (symbol: m²) is a unit of area in the International System of Units (SI), which is the metric system commonly used in many countries worldwide. It is defined as the area of a square with sides that each measure one meter in length.
To give you a visual sense of the size of a square meter, imagine a square that is one meter long on each side. It is equal to 10,000 square centimeters (cm²) or 0.0001 hectares (ha). In square feet, one square meter is approximately 10.7639 square feet (sq ft).
The square meter is the primary unit for measuring area in the metric system and is widely used in various scientific, engineering, and everyday applications for describing land area, room dimensions, and many other surface measurements.