What specific term describes the measurement of the probability or 'chance' that a single neutron will interact with a specific target nucleus?

The specific term that describes the measurement of the probability or 'chance' that a single neutron will interact with a specific target nucleus is the microscopic cross section. This fundamental quantity in nuclear physics represents the effective area that a target nucleus presents to an incident neutron for a particular type of interaction to occur. It is not the physical geometric area of the nucleus, but rather a conceptual area directly proportional to the likelihood of an interaction. A larger microscopic cross section indicates a higher probability that the neutron will interact with that nucleus. The standard unit for microscopic cross section is the barn (b), where one barn equals 10⁻²⁴ cm². This value is highly dependent on both the specific target nucleus (its isotope, meaning its unique combination of protons and neutrons) and the kinetic energy of the incident neutron. Different nuclei exhibit different cross sections for neutrons of the same energy, and a single nucleus will show varying cross sections depending on the neutron's energy. 'Interaction' encompasses various nuclear processes, each having its own specific microscopic cross section. These processes include scattering, where the neutron changes direction and possibly energy (elastic or inelastic scattering), and absorption, where the neutron is taken into the nucleus, potentially leading to radiative capture (gamma emission), fission (nucleus splitting), or other transmutations. It is called 'microscopic' because it quantifies the interaction probability for a single nucleus, distinguishing it from the 'macroscopic cross section' which describes interactions in a bulk material.