Discussion
The value of one barn in SI unit is
Answer: A
One barn is \(10^{-28}\ \text{m}^{2}\).
The barn is a unit of area used for nuclear cross-sections, the effective target area a nucleus presents to an incoming particle. It is defined as \(10^{-24}\ \text{cm}^{2}\). Converting to square metres, with \(1\ \text{cm}^{2} = 10^{-4}\ \text{m}^{2}\):
\[ 10^{-24} \times 10^{-4} = 10^{-28}\ \text{m}^{2} \]
The size makes sense: a nucleus has a radius of order \(10^{-14}\) m, so its cross-sectional area is of order \(10^{-28}\ \text{m}^{2}\). \(10^{-15}\) is the scale of a nuclear radius in metres (a femtometre), not an area, and the other powers do not match the definition.
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