Refractive Index of Silicon at γ Ray Energies

For x rays the real part of the refractive index, dominated by Rayleigh scattering, is negative and converges to zero for higher energies. For $\ensuremath{\gamma}$ rays a positive component, related to Delbr\"uck scattering, increases with energy and becomes dominating. The deflection of a monochromatic $\ensuremath{\gamma}$ beam due to refraction was measured by placing a Si wedge into a flat double crystal spectrometer. Data were obtained in an energy range from 0.18 MeV to 2 MeV. The data are compared to theory, taking into account elastic and inelastic Delbr\"uck scattering as well as recent results on the energy dependence of the pair creation cross section. Probably a new field of $\ensuremath{\gamma}$ optics with many new applications opens up.