Amorphization has been studied in electron-(e~) and ion-irradiated Si. Si irradiated at < 10 K with 1.0-or 1.5-MeV Kr + became amorphous at < 0.4 displacement per atom (dpa), whereas Si irradiated at 10 K to a fluence of ~ 14 dpa of 1-MeV e ~, in an electron microscope, failed to amorphize. However, Si subjected to a simultaneous e ~ and Kr + in situ irradiation at < 10 K to a Kr + fluence of 1.5 dpa retained crystallinity. The critical ratio, at < 10 K, of the e ~ to Kr + ion displacement rates to maintain a degree of crystallinity is ~ 0.5. Atomistic models for these phenomena are presented.