What is a barn in nuclear physics?

• A. A unit of energy
• B. A unit of time
• C. A unit of volume
• D. A unit of area equal to 10^-24 cm^2

Answer: (D)

In nuclear physics, a barn is a unit of area used to express the cross section, which reflects the effective target area presented by a nucleus to an incoming particle and thus the probability of interaction. A barn equals 10^-28 square meters, or 10^-24 square centimeters, making it a convenient scale for nuclear reaction probabilities that are typically tiny. The term comes from a historical joke about how these cross sections are “big enough to hit,” so the unit is named barn. This is an area unit, not a unit of energy, time, or volume.