Collision cascade

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).

This kind of a cascade can be theoretically well treated using the binary collision approximation (BCA) simulation approach.

The electronic stopping power can be readily included in binary collision approximation[4] or molecular dynamics (MD) simulations.

If the kinetic energy of the atoms in the region of dense collisions is recalculated into temperature (using the basic equation E = 3/2·N·kBT), one finds that the kinetic energy in units of temperature is initially of the order of 10,000 K. Because of this, the region can be considered to be very hot, and is therefore called a heat spike or thermal spike (the two terms are usually considered to be equivalent).

But once the Cu ion would slow down enough, the nuclear stopping power would increase and a heat spike would be produced.

Moreover, many of the primary and secondary recoils of the incoming ions would likely have energies in the keV range and thus produce a heat spike.

[23] Swift heavy ions, i.e. MeV and GeV heavy ions which produce damage by a very strong electronic stopping, can also be considered to produce thermal spikes[24][25] in the sense that they lead to strong lattice heating and a transient disordered atom zone.

[33] Prolonged irradiation of many materials can lead to their full amorphization, an effect which occurs regularly during the ion implantation doping of silicon chips.

[34] The defects production can be harmful, such as in nuclear fission and fusion reactors where the neutrons slowly degrade the mechanical properties of the materials, or a useful and desired materials modification effect, e.g., when ions are introduced into semiconductor quantum well structures to speed up the operation of a laser.

[36] A curious feature of collision cascades is that the final amount of damage produced may be much less than the number of atoms initially affected by the heat spikes.

[1] On the other hand, in semiconductors and other covalently bonded materials the damage production is usually similar to the number of displaced atoms.

A classical molecular dynamics computer simulation of a collision cascade in Au induced by a 10 keV Au self-recoil. This is a typical case of a collision cascade in the heat spike regime. Each small sphere illustrates the position of an atom, in a 2-atom-layer-thick cross section of a three-dimensional simulation cell. The colors show (on a logarithmic scale) the kinetic energy of the atoms, with white and red being high kinetic energy from 10 keV downwards, and blue being low.
Schematic illustration of independent binary collisions between atoms
Schematic illustration of a linear collision cascade. The thick line illustrates the position of the surface, and the thinner lines the ballistic movement paths of the atoms from beginning until they stop in the material. The purple circle is the incoming ion. Red, blue, green and yellow circles illustrate primary, secondary, tertiary and quaternary recoils, respectively. In between the ballistic collisions the ions move in a straight path.
As above, but in the middle the region of collisions has become so dense that multiple collisions occur simultaneously, which is called a heat spike. In this region the ions move in complex paths, and it is not possible to distinguish the numerical order of recoils - hence the atoms are colored with a mixture of red and blue.
Image sequence of the time development of a collision cascade in the heat spike regime produced by a 30 keV Xe ion impacting on Au under channeling conditions. The image is produced by a classical molecular dynamics simulation of a collision cascade. The image shows a cross section of two atomic layers in the middle of a threedimensional simulation cell. Each sphere illustrates the position of an atom, and the colors show the kinetic energy of each atom as indicated by the scale on the right. At the end, both point defects and dislocation loops remain.