# Ultra high precision refractive index measurement of Si at $\gamma$-ray energies up to 2 MeV

## TL;DR

This study precisely measures silicon's refractive index at gamma-ray energies up to 2 MeV, confirming classical scattering predictions and resolving previous discrepancies caused by systematic errors.

## Contribution

It provides the first high-precision measurements of silicon's refractive index at gamma-ray energies, clarifying earlier conflicting results.

## Key findings

- Refractive index follows classical scattering model predictions.
- Systematic errors explained previous sign-change observations.
- High-precision data up to 2 MeV for silicon's refractive index.

## Abstract

The refractive index of silicon at $\gamma$-ray energies from 181 - 1959 keV was investigated using the GAMS6 double crystal spectrometer and found to follow the predictions of the classical scattering model. This is in contrast to earlier measurements on the GAMS5 spectrometer, which suggested a sign-change in the refractive index for photon energies above 500 keV. We present a re-evaluation of the original data from 2011 as well as data from a 2013 campaign in which we show that systematic errors due to diffraction effects of the prism can explain the earlier data.