A barn is a unit of area used to express cross-sectional area of nuclei and equals which value?

• A. 10^-24 cm^2
• B. 10^-20 cm^2
• C. 10^-12 cm^2
• D. 10^-30 cm^2

Answer: (A)

The main idea is understanding what a barn measures in nuclear physics. A barn is a unit of area used to express nuclear cross sections, and it is defined as 10^-28 square meters. Converting to square centimeters, since 1 m^2 equals 10^4 cm^2, that becomes 10^-28 × 10^4 = 10^-24 cm^2. So a barn equals 10^-24 cm^2. This scale makes it convenient for talking about interaction probabilities in nuclear reactions, where cross sections are often on the order of a barn or fractions thereof. The other values correspond to areas that are either much larger or smaller than a barn: for example, 10^-20 cm^2 is much larger, 10^-12 cm^2 is vastly larger, and 10^-30 cm^2 is smaller by a factor of 100 compared to a barn.