It is known that radiation produces microscopic and macroscopic changes in silicas and other silicate glasses that result in densifications of several percent [1], This change in density is matched by a corresponding linear change in refractive index via the Lorentz-Lorenz relation such that where R is the molar polarizablilty, V the molar volume and n the refractive index.- Measurements of neutron irradiated bulk a-Si02 yield an exact correlation between the refractive index and density as shown by Fig. 1 such that dn/dp =185 m3/kg [ 1 ].

Utilizing a VG HB603 STEM fitted with a parallel EELS, measurement of irradiation-induced density changes in silicas were attempted using the 0-100 eV low-loss region of the electron energy-loss spectrum. The dielectric function and refractive index changes were extracted from these data using a modified Kramers-Kronig analysis (Fig. 2).