* Astronomy

Title: The Refractive Index of Silicon at Gamma Ray Energies
Authors: D. Habs, M.M Günther, M. Jentschel, W. Urban

The index of refraction n(E_gamma)=1+delta(E_gamma)+ibeta(E_gamma) is split into a real part delta and an absorptive part beta. The absorptive part has the three well-known contributions to the cross section sigma_{abs}: the photo effect, the Compton effect and the pair creation, but there is also the inelastic Delbrück scattering. Second-order elastic scattering cross sections sigma_{sca} with Rayleigh scattering (virtual photo effect), virtual Compton effect and Delbrück scattering (virtual pair creation) can be calculated by integrals of the Kramers-Kronig dispersion relations from the cross section sigma_{abs}. The real elastic scattering amplitudes are proportional to the refractive indices delta_{photo}, delta_{Compton} and delta_{pair}. While for X-rays the negative delta_{photo} dominates, we show for the first time experimentally and theoretically that the positive delta_{pair} dominates for gamma rays, opening a new era of gamma optics applications, i.e. of nuclear photonics.

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