Amorphization has been studied in electron- (${e}^{\mathrm{\ensuremath{-}}}$) and ion-irradiated Si. Si irradiated at l10 K with 1.0- or 1.5-MeV ${\mathrm{Kr}}^{+}$ became amorphous at l0.4 displacement per atom (dpa), whereas Si irradiated at 10 K to a fluence of \ensuremath{\approxeq}14 dpa of 1-MeV ${e}^{\mathrm{\ensuremath{-}}}$ in an electron microscope failed to amorphize. However, Si subjected to a simultaneous ${e}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{Kr}}^{+}$ in situ irradiation, at l10 K, to a ${\mathrm{Kr}}^{+}$ fluence of 1.5 dpa retained crystallinity. The critical ratio, at l10 K, of the ${e}^{\mathrm{\ensuremath{-}}}$ to ${\mathrm{Kr}}^{+}$ ion displacement rates to maintain a degree of crystallinity is \ensuremath{\approxeq}0.5. Atomistic models for these phenomena are presented.