What does the term effective cross section describe?

What does the term effective cross section describe?

• A. The size of the nucleus.
• B. The speed of incident neutrons.
• C. The probability of a reaction between a neutron and a nucleus.
• D. The energy of the neutron.

Answer: (C)

The key idea is that the effective cross section represents how likely it is for a neutron to interact with a nucleus, treated as an area that the neutron “sees” as a target. A larger cross section means a higher probability of reaction per encounter, so it directly quantifies the chance of absorption, fission, or scattering for a given neutron energy and reaction channel. It’s not the physical size of the nucleus, nor the neutron’s speed, nor simply the energy itself—though the cross section can change with energy and reaction type. In practice, the probability per unit path length is given by the macroscopic cross section, Σ = Nσ, where N is the number density of nuclei and σ is the microscopic cross section, linking the concept to actual reaction rates and mean free path.

The key idea is that the effective cross section represents how likely it is for a neutron to interact with a nucleus, treated as an area that the neutron “sees” as a target. A larger cross section means a higher probability of reaction per encounter, so it directly quantifies the chance of absorption, fission, or scattering for a given neutron energy and reaction channel. It’s not the physical size of the nucleus, nor the neutron’s speed, nor simply the energy itself—though the cross section can change with energy and reaction type. In practice, the probability per unit path length is given by the macroscopic cross section, Σ = Nσ, where N is the number density of nuclei and σ is the microscopic cross section, linking the concept to actual reaction rates and mean free path.