  Some results on damage build up in, and amorphization of, Si, induced by
25-30 keV Al$_5^-$, Si$_5^-$ and Cs$^-$ ions, at room temperature, are
reported. We show that at low energy, amorphization is a nucleation and growth
process, based on the direct impact mechanism. With an Avrami exponent $\sim
1.6$, the growth towards amorphization seems to be diffusion limited. A
transition to a completely amorphized state is indicated at a dose exceeding 17
eV/atom, which is higher than 6-12 eV/atom as predicted by simulations. The
observed higher threshold could be due to temperature effects although an
underestimation of keV-energy recoils, in simulation, may not be ruled out.
