Multilayer Thin Films for Imaging X-Rays

Characterization of film materials is crucial to narrowband filter performance and consequently to x -ray instrumentation. A technique is presented to include all of the imperfections in film materials such as contamination, oxidation, volume scattering, inhomogeneities and anisotropies into the optical constants or more appropriately the "optical behavior" values. These values are the signature of the x-ray material that represent how the medium interacts with the incident radiation. A theoretical model and experimental techniques are described to determine the optical behavior values of substrate and film materials used in the x-ray region. A fitting routine is used to match the calculated and measured reflectance to obtain accurate optical behavior values. Sources of errors in reflectance are investigated and errors in the optical behavior values associated with these sources of error are given. Reflectance filters designed using the optical behavior values of the above approach will significantly improve the ability to correctly design and fabricate filters for x-ray instrumentation. X-ray filter design approaches are described and several x-ray filters shown. A new approach is explained to vary the density of a film material to achieve the same effect as having two materials with different optical constants. This technique allows thousands of layers of different densities to be deposited. Finally, several applications of the instrumentation where narrowband x-ray filters play a significant role are described. Some of these applications include a multilayer acting as a dispersion element in a spectrograph, an x-ray filter that aids in the detection of cancerous breast tissue, as well as an x-ray filter used in luggage inspection equipment to detect explosive material.