Abstract
The design and construction of an instrument for full-field imaging of the X-ray fluorescence emitted by a fully illuminated sample are presented. The aim is to produce an X-ray microscope with a few micrometers spatial resolution, which does not need to scan the sample. Since the fluorescence from a spatially inhomogeneous sample may contain many fluorescence lines, the optic which will provide the magnification of the emissions must be achromatic, i.e. its optical properties must be energy-independent. The only optics which fulfill this requirement in the X-ray regime are mirrors and pinholes. The throughput of a simple pinhole is very low, so the concept of coded apertures is an attractive extension which improves the throughput by having many pinholes, and retains the achromatic property. Modified uniformly redundant arrays (MURAs) with 10 µm openings and 50% open area have been fabricated using gold in a lithographic technique, fabricated on a 1 µm-thick silicon nitride membrane. The gold is 25 µm thick, offering good contrast up to 20 keV. The silicon nitride is transparent down into the soft X-ray region. MURAs with various orders, from 19 up to 73, as well as their respective negative (a mask where open and closed positions are inversed compared with the original mask), have been made. Having both signs of mask will reduce near-field artifacts and make it possible to correct for any lack of contrast.
Highlights
Current X-ray microprobe beamlines raster-scan the sample through a focused X-ray beam, and use an energy-resolving point detector to separate the contributions of the various elements at each point on the sample
In this paper we demonstrate an achromatic imaging system, the coded aperture array
When the photons generated by an object O(x, y) propagate through the coded aperture mask of binary function M(x, y), the resulting image projected onto the detector is I(x,y), Iðx; yÞ 1⁄4 Oðx; yÞ Mðx; yÞ; ð1Þ
Summary
Current X-ray microprobe beamlines raster-scan the sample through a focused X-ray beam, and use an energy-resolving point detector to separate the contributions of the various elements at each point on the sample. This method has many advantages, it suffers from the need to mechanically move the sample through the beam, which is inevitably rather slow, if the requirement is for 3D tomographic imaging, since this requires two translational scans for each of many rotational positions. (i) An achromatic imaging optical element to magnify the fluorescence signal to match the desired resolution to the detector pixel size. The hyperspectral imaging detector is under development at Brookhaven National Laboratory, but is the subject of another work
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