Ionization dilatation effects in fused silica from 2 to 18-keV electron irradiation

We have used low-energy electron irradiations in conjunction with a bending-cantilever-plate technique to investigate the volume dilatations produced by ionization effects in fused silica. Our irradiations generally cause compaction in agreement with previous data from high-energy electron irradiations, but we consistently observe a slowly saturating compaction-dose relationship rather than the sublinear power-law relationship seen in the high-energy data. Our results show agreement with the apparent ionization compaction effect extracted from low-dose-rate 1H+-ion-implantation experiments. We find no dose-rate dependence in the electron ionization compaction effect, in marked contrast with the apparent ionization compaction effect in the implantation experiments. We therefore conclude that the enhancement of the apparent ionization compaction effect at high dose rates in the implantation experiments results from an interaction between the ionization and the atomic displacements accompanying the implanation. By probing to various depths at a constant dose and dose rate, we find that the electron irradiation compaction effect is generally enhanced within several hundred angstroms of the sample surface. There is no apparent threshold for onset of the compaction mechanism for incident electron energies between 2.2 and 18 keV. Finally, we observe the low-fluence expansion effect in impure fused silica as seen by other workers but establish here that it is primarily an ionization-stimulated phenomenon that occurs only when the irradiation current varies with time.