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.