Electron irradiation induced crystalline-to-amorphous transition in quartz single crystals

The electron irradiation induced crystalline-to-amorphous (C-A) transition in quartz was studied as a function of incident electron energy over a range from 100 to 2250 keV, by means of high voltage electron microscopy. Special attention was paid to the C-A transition at electron energies greater than 650 keV for which there are no earlier reports. The C-A transition is induced via the ionization mechanism at least up to an electron energy of about 1500 keV. At electron energies greater than 1500 keV, however, the knock-on displacement mechanism seems to make an increasing contribution towards the C-A transition until it becomes dominant over the ionization mechanism. The behavior of the C-A transition as observed by bright field imaging is essentially the same for the transitions induced by both 2 MeV and 125 keV electron irradiation, in spite of the change in dominant mechanism for the C-A transition at around 1500 keV. Namely, in both cases the transition proceeds in such a manner that strain centers, which are amorphous inclusions embedded in the crystalline matrix, are formed at the beginning stage of irradiation and with continued irradiation grow in size until they coalesce into large amorphous inclusions covering the entire irradiated volume. Based on the results, the mechanism behind the C-A transition is discussed. (orig.)