The Refractive Index of Silicon at Gamma Ray Energies

TL;DR

This paper investigates the refractive index of silicon at gamma ray energies, revealing that pair creation effects dominate the index, which opens new possibilities for gamma-ray optics and nuclear photonics.

Contribution

It provides the first experimental and theoretical demonstration that the pair creation contribution to the refractive index surpasses photo effects at gamma-ray energies.

Findings

Pair creation dominates the refractive index at gamma-ray energies.

Experimental confirmation of positive elta_{pair} at gamma rays.

Potential for new gamma-ray optics applications.

Abstract

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.