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Beamline and Microscope Overview The general layout of the x-ray microscope is shown above. It operates with the same principle as a visible light microscope, consisting of a source providing the radiation (wiggler through slit, S3), a condenser (focusing mirror, C) relaying the radiation onto an object (sample), an objective (micro zone plate, MZP) forming a magnified image, and a detector (CCD) receiving the image BL6-2 has been designed to accept the full vertical emittance of the wiggler and 1.2 mrad in the horizontal, limited by a slit located 10 m from the source. The beam is then collimated by a vertically reflecting Pt coated mirror (M0) located 13.86 m from the source and followed by the liquid nitrogen (LN) cooled, double crystal monochromator. The LN cooled monochromator is equipped with a set of Si(111) crystals giving an energy resolution of ΔE/E = 5 x 10-4 which meets the requirements of the zone plate objective of the TXM. Although this monochromator is not specifically designed to have a fixed exit beam height for different photon energies, the vertical beam motion is only a few microns over an energy range of 100 eV, which is the typical tuning range for near edge spectroscopy applications. This performance is a result of the post-monochromator toroidal M1 mirror, which is configured to focus the beam onto the S3 slit and thus reduces the vertical translation of the focused beam significantly. In addition, a mirror pitch-feedback system also serves to reduce the magnitude of these beam translations. The S3 slit, located 30 m from the wiggler source, serves as the effective source for the TXM. The slit system is mounted permanently on a support with micron level thermal stability. The slit assemblies consist of both horizontal and vertical blades with 1 micron reproducibility and are remotely operated. TXM Overview and Setup  In this photograph of the TXM inside the hutch, the beam direction as well as the location of the condensers and zone plates are indicated by arrows. After the beam is monochromated and focused onto the virtual source, the beam is then focused onto the sample by a condenser consisting of an elliptically shaped capillary tube located 1 meter from the virtual source. The condenser illuminates a spot on the sample providing a field of view of approximately 14 µm in diameter at 8 keV with a central stop that provides a hollow cone illumination. The sample is placed on a stage with high precision xyz/theta motion to allow for tomography and wider area survey images. An objective lens consisting of a high resolution Fresnel zone plate then projects the image of the sample onto a thin single crystal scintillator, which is optically coupled to a cooled (-70°C) charge-coupled device (CCD) in magnifying geometry yielding an effective pixel size of approximately 0.6µm on the scintillator screen. In addition, a phase ring can be placed in the back focal plane of the objective zone plate to provide Zernike phase contrast. The resolution of the image is determined by the outer zone width of the zone plate and the steep hollow cone illumination. The present generation of zone plates has an outermost zone width of 45 nm, but further improvements of the smallest zone width in zone plate fabrication technology are anticipated. A PC workstation controls the motions of all the system optics, sample manipulation, image display, 3D reconstruction and 3D rendering. The 3D reconstruction is performed using a digital signal processor card (DSP) that is optimized for high speed tomographic reconstruction and takes approximately 7 seconds. The vertical divergence of the x-ray beam at the virtual source position is about 0.25 mrad, thus the condenser optics were designed to increase the vertical divergence to match the numerical aperture of the objective zone plate. For applications requiring larger fields of view at the sample position, the capability of translating the sample over many fields of view obtain a tiled 2D image covering on the order of 100x100 µm2 (note also the superscript) was implemented. Such mosaic images are especially useful when trying to identify a promising area for higher resolution or tomographic imaging.. |
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