Modeling of X-ray-induced refractive index changes in poly(methyl methacrylate)

The refractive index of bulk poly(methyl methocrylate) samples is modified by X-ray exposure. The intensity of the radiation is distributed over an energy range of I to 20 keV with a maximum at 3 keV For an absorbed dose of about I kJcm(-3), an increase of the refractive index of up to 3 x 10(-4) is observed. However, for much lower doses of about 10 jcm(-3) a decrease of 4 x 10-4 ap- pears. These refractive index changes, as well as the simultaneously arising radiation-induced thickness changes, are detected interferometrically. Based on these observations, on dose calculations, and on literature data, a model is proposed which describes quantitatively the observed refractive index changes. The refractive index increase is caused by a radiation-induced chemical alteration of the polymer composition, while the refractive index decrease is correlated with a decrease of mass density.