Material optimization for hard x-ray Fresnel zone plates

Fresnel zone plates have recently been used as the focusing optic for hard x-ray (5 - 11 keV) microscopy techniques. Fresnel zone plates used in the hard x-ray regime focus by constructive interference effects based on the phase modulation of the incident x-ray beam and have experimentally been shown to focus 20 - 30% of the incident photons to less than a one- micron focal spot. The materials of choice for these zone plates have been Al, Cu, Ni, and Au. The focus of this work is the theoretical optimization of the focusing efficiency of phase- modulating Fresnel zone plates in the hard x-ray regime by appropriate material selection. The optimal materials for three different energy ranges will be examined (1 - 5 keV, 5 - 20 keV, and > 20 keV) and a discussion of the selection criteria involved will be presented.