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.