High Miller-index germanium crystals for high-energy x-ray imaging applications

Near-normal-incidence bent crystals are widely used for x-ray imaging applications. Advantages include high collection solid angle and potentially high efficiency for narrow-band sources, while disadvantages include relatively large (several Å) interatomic spacings and a limited number of suitable matches between a crystal 2d value and an integral multiple of useful emission line wavelengths. The disadvantages become more significant at x-ray energies >10  keV. The former disadvantage can be mitigated by using high-order reflections from crystal planes having low Miller indices, but both disadvantages can be mitigated by using low-order reflections from crystal planes having high Miller indices. We report here on integrated reflectivity measurements we performed of Ge (15,7,7) (2d=0.6296  Å), a candidate for imaging Ru He-α (θ(B)=87°). We find good agreement with calculations, and the data show a multitude of closely spaced reflections with slightly different Bragg angles including a fifth-order reflection of Ge (3,1,1) that has comparable reflectivity. This demonstrates that arbitrary choices of Miller indices in Ge crystals can be used to fine-tune Bragg angles for near-normal-incidence x-ray imaging at tens of kiloelectron volt x-ray energies with minimal lower-energy contamination from lower-order reflections, and that existing calculational tools can be used to reliably estimate integrated reflectivity.