The index of refraction n(E_{ \gamma})=1+\delta(E_{ \gamma})+i\beta(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 Delbr\"uck scattering. Second-order elastic scattering cross sections \sigma_{ sca} with Rayleigh scattering (virtual photo effect), virtual Compton effect and Delbr\"uck 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.
Categories: physics.atom-ph physics.optics