In other cases like fast neutron irradiation, most of the displacements result from higher-energy PKAs colliding with other atoms as they slow down to rest.
As the atom slows down, the cross section for producing PKAs increases, resulting in groups of vacancies and interstitials concentrated at the end of the track.
Upon melting, former interstitials and vacancies become “density fluctuations,” since the surrounding lattice points no longer exist in liquid.
In the case of a thermal spike, the temperature is not high enough to maintain the liquid state long enough for density fluctuations to relax and interatomic exchange to occur.
Towards the end of the path of a PKA, the rate of energy loss becomes high enough to heat up the material well above its melting point.
This releases stored energy from these strains that raises the temperature even higher, maintaining the liquid state briefly after most of the density fluctuations disappear.
There should be a displacement spike towards the end of the path, a low-energy region where atoms have been moved to new lattice sites but no vacancy-interstitial pairs are retained.