Feasibility of a spectral imager in the soft x-ray region

The development of Fourier Transform (FT) spectral techniques in the soft X-ray (100eV to 500eV spectral region) has been advocated in the past as a possible route to constructing a bench-top size spectral imager with high spatial and spectral resolution. The crux of the imager is the soft X-ray interferometer. The auxiliary subsystems include a soft X-ray source, focusing optics and a CCD-based detection system. When tuned over a sufficiently large range of path delays (frames), the interferometer will sinusoidally modulate a spectrum of a wide-band X-ray source centered at the core wavelength of interest with high resolving power. The spectrum illuminates a target, the reflected signal is imaged onto a CCD, and data acquired for different frames is converted to spectra in software by using FT methods similar to those used in IR spectrometry, producing spectral image per each pixel. The use of short wavelengths results in dramatic increase in imaging resolution over that for IR. Important for future NASA missions, and unlike X-ray Absorption Near Edge Structure (XANES) that uses intense and in monochromatic beams which only a synchrotron can deliver, FTXR plans to use a miniature, wide bandwidth X-ray source. By modulating the beam spectrum around the wavelength of interest, the beam energy is used much more efficiently than with gratings (when only a very small, monochromatized portion of the radiation is used at one time) facilitating construction of a bench-top instrument. With the predicted <0.1eV spectral and <100 nm spatial resolution, the imager would be able to map a core-level shift spectrum for each pixel of the image for elements such as C, Si, Ca, N (Kα-lines) which can be used as a chemical compound fingerprint and for imaging intracellular structures. For heavy elements it could provide "bonding maps" (L and M-shell lines), enabling to study fossils of microorganisms on space missions and in returned samples to Earth. We have initiated development of a Fourier Transform X-ray Reflection (FTXR) spectral imager based on the use of a Mach-Zender type interferometer. The enabling technology for the interferometer is the X-ray beam splitting mirrors. The mirrors are not available commercially; multi layers of quarter-wave films are not suitable, requiring a different approach to beam-splitters than in the visible or IR regions. Several efforts by other researchers used parallel slits or stripes for partial transmission, with only a very limited success. In contrast, our beam splitters are based on thin (about 200 nm) SiN membranes perforated with a large number of very small holes, prepared using state-of-art microfabrication techniques that have only recently become available in our laboratory at JPL. Precise control of surface roughness and high planarity are needed to achieve the wave coherency required for high-contrast fringe forming. The perforation design is expected to result in much greater surface flatness, facilitating greater wave coherence than for the other techniques. We report on our progress in the fabrication of beam splitting mirrors to-date, interferometer design, modeling, assembly, and experimental results.