Which statement correctly defines cross section?

Which statement correctly defines cross section?

• A. The energy of neutrons after scattering.
• B. The effective geometric area of a target nucleus for interaction with a neutron.
• C. The probability of absorption times speed.
• D. The number of neutrons produced per fission.

Answer: (B)

Cross section is a measure of how likely a neutron is to interact with a nucleus, expressed as the effective geometric area the nucleus presents to the incoming neutron. In other words, it quantifies the interaction probability for a given reaction channel and neutron energy, not a physical size of the nucleus itself. When you have a neutron flux passing through material, the rate of interactions with nuclei is proportional to this cross section: higher cross section means more interactions per unit neutron flux. The concept is widely used with reaction rates via the relation R = N σ Φ, where N is the number density and Φ is the neutron flux. Cross sections come in units called barns and vary with energy and with the type of interaction (such as absorption, scattering, or fission). The other statements describe energy after scattering, a speed-weighted probability, or yield per fission, which do not define cross section.

Cross section is a measure of how likely a neutron is to interact with a nucleus, expressed as the effective geometric area the nucleus presents to the incoming neutron. In other words, it quantifies the interaction probability for a given reaction channel and neutron energy, not a physical size of the nucleus itself. When you have a neutron flux passing through material, the rate of interactions with nuclei is proportional to this cross section: higher cross section means more interactions per unit neutron flux. The concept is widely used with reaction rates via the relation R = N σ Φ, where N is the number density and Φ is the neutron flux. Cross sections come in units called barns and vary with energy and with the type of interaction (such as absorption, scattering, or fission). The other statements describe energy after scattering, a speed-weighted probability, or yield per fission, which do not define cross section.