Jin-Seok Chung

Automated indexing for texture and strain measurement with broad-bandpass x-ray microbeams

Methods are derived for measuring local strain, stress, and crystallographic texture (orientation) in polycrystalline samples when 1–10 grains are simultaneously illuminated by an energy scanable or broad-bandpass x-ray beam. The orientation and unit-cell shape for each illuminated grain can be determined from the diffracted directions of four Bragg reflections. The unit-cell volume is determined by measuring the energy (wavelength) of one reflection. The methods derived include an algorithm for simultaneously indexing the reflections from overlapping crystal Laue patterns and for determining the average strain and stress tensor of each grain. This approach allows measurements of the local strain and stress tensors which are impractical with traditional techniques.

Elliptical x-ray microprobe mirrors by differential deposition

A differential coating method is described for fabricating high-performance x-ray microfocusing mirrors. With this method, the figure of ultrasmooth spherical mirrors can be modified to produce elliptical surfaces with low roughness and low figure errors. Submicron focusing is demonstrated with prototype mirrors. The differential deposition method creates stiff monolithic mirrors which are compact, robust, and easy to cool and align. Prototype mirrors have demonstrated gains of more than 104 in beam intensity while maintaining submilliradian divergence on the sample. This method of producing elliptical mirrors is well matched to the requirements of an x-ray microdiffraction Kirkpatrick–Baez focusing system.

X-ray fluorescence microtomography study of trace elements in a SiC nuclear fuel shell

X-ray fluorescence microtomography has been used to measure the trace element spatial distribution in a TRISO SiC shell after exposure to 1.9×1025 neutrons/m2. The bare SiC shell was prepared by laser drilling the TRISO particle and leaching away the core and C. The exposed shell was measured with an X-ray probe having a size of approximately 1×3 μm2 on beamline 2-ID at the Advanced Photon Source. The trace element distribution in the SiC shell was reconstructed after correcting the data for artifacts arising from absorption and experimental restrictions. The trace elements were found in small (<2 μm) regions through the SiC shell and appear to have been introduced during fabrication. X-ray fluorescence microtomography is an ideal tool for this work because it is a penetrating non-destructive probe with good sensitivity to high Z trace elements in a low Z matrix and because it provides a picture of the elemental distribution in the shell.

Small-displacement monochromator for microdiffraction experiments

We describe the design, construction, and performance of the MHATT-CAT microdiffraction x-ray monochromator. This monochromator is specially engineered for x-ray microdiffraction experiments with a high brilliance undulator source. The monochromator passes a small emittance beam, suitable for focusing to submicron size with submilliradian divergence. Over its energy range of 8–22 keV the absolute energy calibration is better than 2 eV and scans of ± 1 keV show no measurable hysterisis. The monochromator operates with a simple water-cooled first crystal and shows no measurable warm-up time. Horizontal linear bearings allow the monochromator crystals to be rapidly inserted or removed from the beam. Slits before and after the monochromator work to pass broad bandpass or monochromatic x-ray beams at the same vertical height. The monochromatic beam direction is adjusted so the monochromatic and broad bandpass beams are coaxial. The design and performance of the monochromator allows efficient collection of micr...

Strain and Texture in Al-Interconnect Wires Weasured by X-Xay Microbeam Diffraction

The local strain and texture in Al interconnect wires have been investigated using white and monochromatic x-ray microbeams on the MHATTCAT undulator beam line at the Advanced Photon Source. Intergrain and intragrain orientations were obtained with ~0.01 degree sensitivity using white beam measurements on wide Al pads (~100 Mu-m) and thin (2 Mu-m) Al wires. Orientation changes of up to 1 degree were found within individual grains of the (111) textured Al interconnects. Deviatoric strain measurements indicate small intragranular strain variations, but intergranular strain variations were found to be quite large.

X-Ray Microbeam Measurement of Local Texture and Strain in Metals

Synchrotron x-ray sources provide high-brilliance beams that can be focused to submicron sizes with Fresnel zone-plate and x-ray mirror optics. With these intense, tunable or broad-bandpass x-ray microbeams, it is now possible to study texture and strain distributions in surfaces, and in buried or encapsulated thin films. The full strain tensor and local texture can be determined by measuring the unit cell parameters of strained material. With monochromatic or tunable radiation, at least three independent reflections are needed to determine the orientation and unit cell parameters of an unknown crystal. With broad-bandpass or white radiation, at least four reflections and one measured energy are required to determine the orientation and the unit cell parameters of an unknown crystal. Routine measurement of local texture and strain is made possible by automatic indexing of the Laue reflections combined with precision calibration of the monochromator-focusing mirrors-CCD detector system. Methods used in implementing these techniques on the MHA-IT-CAT beam line at the Advanced Photon Source will be discussed.

3-D Measurement of Deformation Microstructure of Al(0.2%)Mg Using Submicron Resolution White X-Ray Microbeams

We have used submicron-resolution white x-ray microbeams on the MHATT-CAT beamline 7-ID at the Advanced Photon Source to develop techniques for three-dimensional investigation of the deformation microstructure in a 20% plane strain compressed Al(0.2%)Mg tri-crystal. Kirkpatrick-Baez mirrors were used to focus white radiation from an undulator to a 0.7 x 0.7 {micro}m{sup 2} beam that was scanned over bi- and tri-crystal regions near the triple-junction of the tri-crystal. Depth resolution along the x-ray microbeam of less than 5 microns was achieved by triangulation to the diffractibn source point using images taken at a series of CCD distances from the microbeam. Computer indexing of the deformation cell structure in the bi-crystal region provided orientations of individual subgrains to {approximately}0.01 degrees, making possible detailed measurements of the rotation axes between individual cells.

Design and performance of x-ray optics optimized for polycrystalline microdiffraction

The design considerations are described for a new class of instrumentation optimized for x-ray microdiffraction measurements of polycrystalline amples. A prototype station on the MHATT-Cat beamline 7 at the Advanced Photon Source is used to illustrate the current level of performance of this instrumentation. This station allows for efficient measurements of x-ray microdiffraction from polycrystalline materials with submicron spatial resolution and with strain resolution below 1 part in 104. The station utilizes a specially designed monochromator/non-dispersive Kirkpatrick-Baez focusing system that allows for rapid oscillation between monochromatic and white beam conditions. This allows the deviatoric and full strain tensors of polycrystalline grains to be rapidly determined.

X-Ray Fluorescence Microtomography on a SiC Nuclear Fuel Shell

TRISO fuel particles, used in high-temperature gas-cooled reactors (HTGR) are composite structures with a nuclear fuel kernel surrounded by alternating layers designed to contain fission products and compensate for radiation damage. As shown in Figure 1, a typical fuel particle contains an inner kernel of nuclear fuel, a low-density buffer layer of pyrocarbon, a dense layer of pyrocarbon coating, an interlayer of SiC, and a dense outer layer of pyrocarbon. The fuel kernel size, the thickness of the various layers, and the overall size of the TRISO fuel particle can vary with the type of fuel kernel. The SiC layer provides the primary barrier for both radioactive elements in the kernel and gaseous and metallic fission products.

Comparison of a mosaic-crystal spectrometer to a high-performance solid-state detector for x-ray microfluorescence analysis

The minimum-detectable-limit of a compact double-focusing graphite mosaic-crystal spectrometer is compared to the minimum-detectable-limit from a high-performance Ge solid-state detector. The solid angle and efficiency of the solid-state detector is much greater than for the crystal spectrometer. However, the better signal-to-noise of the spectrometer and its insensitivity to matrix fluorescence and scattering can give it a better minimum-detectable-limit for trace element analysis. The relative advantages of the two detectors are illustrated for some simple test samples. The performance of the crystal spectrometer compared to the solid-state detector increases as the flux in the x-ray probe increases. This makes crystal spectrometers especially interesting for use with new high intensity 3rd generation synchrotron microprobes. An estimate is made of the source and sample conditions favored for each detector.