Crystal reflectivity for bent crystal spectrometers

The reflectivity properties of a bent silicon crystal, used as the diffraction crystal in a transmission type Bent-Crystal Diffraction (BCD) spectrometer, were investigated. In particular the energy dependence of the integrated reflecting power was studied. It was found that the integrated reflecting power stayed constant up to unexpectedly high energies, depending on the diffraction order and on the quality of the crystal bending. Beyond an inflexion point this reflecting power decreased with only E^-1 instead of E^-2 with quartz crystals. Both this diffraction behaviour and the improved energy resolution extend the usefulness of bent-crystal diffraction spectrometers beyond 1 MeV gamma ray energy. The results are discussed and interpreted in terms of the diffraction theory for perfect crystals.