4 - Nuclear Processes

Abstract

Nuclear reactions—those in which atomic nuclei participate—may take place spontaneously, as in radioactivity, or may be induced by bombardment with a particle or ray. Nuclear reactions are much more energetic than chemical reactions, but they obey the same physical laws: conservation of momentum, energy, number of particles, and charge. When any two particles approach each other, their mutual influence depends on the nature of the force between them. Final energies are found from mass differences and final speeds from conservation of momentum. The cross section for interaction of neutrons with nuclei is a measure of the chance of collision. Reaction rates depend mutually on neutron flows and macroscopic cross section. A stream of uncollided particles is reduced exponentially as it passes through a medium. Neutron absorption cross sections vary greatly with target isotope and with neutron energy, while scattering cross sections are relatively constant. Neutrons are slowed readily by collisions with light nuclei and migrate from their point of origin. On reaching thermal energy, they continue to disperse, with the net flow dependent on the spatial variation of flux.