Inelastic interactions, charged particle

Charged particles can interact with matter and lose energy in two ways (i) emission of electromagnetic radiation and (ii) inelastic collisions. A brief review of both processes is now presented. 

A fast neutron loses its energy by interaction with an atomic nucleus and ejection of a proton. A slow neutron is absorbed by the nucleus giving rise to a radioactive atom which may decompose by the emission of a /1-particle or y-ray. Charged particles in general lose their energy by inelastic collisions with the electrons within an atom leading to the ejection of an electron. 

When a beam of charged particles passes through a thin specimen, the beam transmitted in the forward direction includes some particles that scattered elastically off atomic nuclei or lost energy due to interaction with electrons (inelastically scattered) as well as those particles that were left unscattered. An image formed with this forward-transmitted beam is referred to as a bright field image. 

The neutron interacts primarily with the nucleus of the absorbing atom, by elastic and inelastic scattering. In elastic collisions, the target nucleus is not itself ionized. However, it may produce ionization in other atoms. In inelastic scattering, including nuclear transformation, the newly formed nuclide and the emitted photon or charged particle may produce ions. 

However, for the real interaction scattering also occurs outside the region of geometric contact. This is especially substantial, for example, for the Coulomb interaction of charged particles. Formulas for the cross section of inelastic processes substantially depend on the fact which form of the energy (electron, vibrational or rotational) changes. 

The electron excitation process is caused by the interaction with the incoming atom or molecule. Since we have seen that some charge transfer takes place from the metal to the incoming atom/molecule the simplest model assumes that this charged particle can induce the e-h excitation through its interaction with the electrons. In this approach the e-h processes are viewed as additional inelastic (nonadiabatic) processes taking place on top of the adiabatic electronic adjustment connected with the charge transfer and the Bom-Oppenheimer construction of the adiabatic surfaces used for the nuclear motion. In order to facilitate the theoretical treatment of these inelastic processes, it is convenient to introduce the concept of second quantization. 

The inelastic Coulomb interaction of the heavy charged particle with the absorbing nucleus is negligible and as a result wiU not be considered in this section. 

Since neutrons are neutral they interact differently to charged particles. The primary interaction occurs with the nucleus of the absorber and little interaction is present with its orbital electrons. Neutrons interact with the nucleus through elastic and inelastic scattering and neutron capture. In the latter mechanism, the neutron is absorbed by the nucleus which in turn excites the nucleus to higher energy levels. As the nucleus returns back to the ground state a particle is emitted (dependent on the incident energy this could be a-particle, neutron etc), and a new radioactive nuclide is produced. 

---