In condensed-matter physics, a collision cascade (also known as a displacement cascade or a displacement spike) is a set of nearby adjacent energetic (much higher than ordinary thermal energies) collisions of atoms induced by an energetic particle in a solid or liquid.
If the maximum atom or ion energies in a collision cascade are higher than the threshold displacement energy of the material (tens of eVs or more), the collisions can permanently displace atoms from their lattice sites and produce defects. The initial energetic atom can be, e.g., an ion from a particle accelerator, an atomic recoil produced by a passing high-energy neutron, electron or photon, or be produced when a radioactive nucleus decays and gives the atom a recoil energy.
The nature of collision cascades can vary strongly depending on the energy and mass of the recoil/incoming ion and density of the material (stopping power).
Linear cascades
When the initial recoil/ion mass is low, and the material where the cascade occurs has a low density (i.e. the recoil-material combination has a low stopping power), the collisions between the initial recoil and sample atoms occur rarely, and can be understood well as a sequence of independent binary collisions between atoms. This kind of a cascade can be theoretically well treated using the binary collision approximation (BCA) simulation approach. For instance, H and He ions with energies below 10 keV can be expected to lead to purely linear cascades in all materials.
The most commonly used BCA code SRIM can be used to simulate linear collision cascades in disordered materials for all ion in all materials up to ion energies of 1 GeV. Note, however, that SRIM does not treat effects such as damage due to electronic energy deposition or damage produced by excited electrons. The nuclear and electronic stopping powers used are averaging fits to experiments, and are thus not perfectly accurate either. The electronic stopping power can be readily included in binary collision approximation or molecular dynamics (MD) simulations. In MD simulations they can be included either as a frictional force or in a more advanced manner by also following the heating of the electronic systems and coupling the electronic and atomic degrees of freedom. However, uncertainties remain on what is the appropriate low-energy limit of electronic stopping power or electron-phonon coupling is.
In linear cascades the set of recoils produced in the sample can be described as a sequence of recoil generations depending on how many collision steps have passed since the original collision: primary knock-on atoms (PKA), secondary knock-on atoms (SKA), tertiary knock-on atoms (TKA), etc. Since it is extremely unlikely that all energy would be transferred to a knock-on atom, each generation of recoil atoms has on average less energy than the previous, and eventually the knock-on atom energies go below the threshold displacement energy for damage production, at which point no more damage can be produced.
Heat spikes (thermal spikes)
When the ion is heavy and energetic enough, and the material is dense, the collisions between the ions may occur so near to each other that they can not be considered independent of each other. In this case the process becomes a complicated process of many-body interactions between hundreds and tens of thousands of atoms, which can not be treated with the BCA, but can be modelled using molecular dynamics methods.
Typically, a heat spike is characterized by the formation of a transient underdense region in the center of the cascade, and an overdense region around it.<ref name=Sei56> { { cite book |author1=F. Seitz |author2=J. S. Koehler |year=1956 |chapter=Displacement of Atoms during Irradiation |editor1=F. Seitz |editor2=D. Turnbull |title=Solid State Physics |volume=2 |page=307 |publisher=Academic Press |oclc= |isbn= }}</ref> After the cascade, the overdense region becomes interstitial defects, and the underdense region typically becomes a region of vacancies.