4 - Nuclear processes

Abstract

This chapter discusses the process of nuclear reactions. Chemical and nuclear equations have similarities in the form of equations and in the requirements on conservation of particles and charge. 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. The bombardment of nuclei by charged particles or neutrons produces new nuclei and particles. 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 is 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.