Step 1: Understanding the unit "barn"
The barn (\( b \)) is a unit of area commonly used in nuclear physics to express the cross-sectional area of nuclear reactions. It is defined as:
\[
1 \text{ barn} = 10^{-28} \text{ m}^2
\]
Step 2: Application in Scattering Cross-Section
- The scattering cross-section represents the effective target area that a nucleus presents to an incoming particle in a nuclear reaction.
- It is crucial in understanding interactions such as neutron scattering and nuclear fission.
- Since the barn is used specifically in nuclear and particle physics, the correct physical quantity measured in barn is scattering cross-section. Final Answer: The correct option is Scattering cross-section (Option 3).