Fast migration of alkali ions in glass irradiated by electrons

Alkali-ion-containing glass irradiated by an electron probe undergoes chang es in the glass structure. During irradiation, the exposed volume of the gl ass rapidly changes structure that reflects in a fast decay of alkali X-ray intensities. The decay normally follows after some incubation period that depends on the characteristics of the probe. Incubation periods and the slo pes of the decay curves are determined experimentally for a K2O-CaO-SiO2 te rnary glass. A model explaining the nature of the observed phenomena is sug gested. The bombardment by high energy electrons causes the production of ' doubly' ionized alkalis. This new species of alkalis is accelerated by the electric field being created by the electron beam. A spontaneous migration begins as soon as the number of released alkalis reaches a critical percola tion point. The new activation energies, percolation thresholds and effecti ve ionization cross sections at the percolation point are calculated accord ing to the suggested model. (C) 1999 Elsevier Science B.V. All rights reser ved.