Collision Cascade - Heat Spikes (thermal Spikes)

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.

Computer simulation-based animations of collision cascades in the heat spike regime are available on YouTube.

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. After the cascade, the overdense region becomes interstitial defects, and the underdense region typically becomes a region of vacancies.

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·k_BT), 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). The heat spikes cools down to the ambient temperature in 1–100 ps, so the "temperature" here does not correspond to thermodynamic equilibrium temperature. However, it has been shown that after about 3 lattice vibrations, the kinetic energy distribution of the atoms in a heat spike has the Maxwell-Boltzmann distribution, making the use of the concept of temperature somewhat justified. Moreover, experiments have shown that a heat spike can induce a phase transition which is known to require a very high temperature, showing that the concept of a (non-equilibrium) temperature is indeed useful in describing collision cascades.

In many cases, the same irradiation condition is a combination of linear cascades and heat spikes. For example, 10 MeV Cu ions bombarding Cu would initially move in the lattice in a linear cascade regime, since the nuclear stopping power is low. 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.

For instance, for copper irradiation of copper, recoil energies of around 5–20 keV are almost guaranteed to produce heat spikes. At lower energies, the cascade energy is too low to produce a liquid-like zone. At much higher energies, the Cu ions would most likely lead initially to a linear cascade, but the recoils could lead to heat spikes, as would the initial ion once it has slowed down enough. The concept subcascade breakdown threshold energy signifies the energy above which a recoil in a material is likely to produce several isolated heat spikes rather than a single dense one.