Impurity Effect in the Ionization Dilatation of Vitreous Silica

The normal effect of ionizing radiation is to cause a contraction. Vitreous silica which colors in the visible (presumably because of its aluminum content) shows a very rapid radiation‐induced expansion which soon saturates. Thus, depending on the impurity composition, the vitreous silica may show an initial decreased contraction or an initial expansion followed by subsequent contraction. Data are given for the optical absorption and the dilatation, for x‐ray, gamma‐ray, and electron irradiations. The dilatation was measured by interferometric and photoelastic methods, and for the latter, the method was further developed. The expansion was found to have a different distribution along the range of electrons than the contraction. Spontaneous annealing was found for the expansion, but not for the contraction. The thermal annealing of the dilatation and optical absorption were studied. In the coloring vitreous silica, the expansion annealed at a lower temperature than the contraction. The expansion and the contraction appear to be quite independent. The expansion is explained as caused by breaking Al–O links in the structure, the contraction by a compaction of the Si–O structure. Variation in absorption behavior associated with OH content is attributed largely to the kinds of centers formed in the vicinity of impurities rather than the number of radiation‐induced atomic configurations which can form centers.