As the X-Ray diffraction pattern of a crystalline substance is unique, it is possible to characterize and thus to identify any polycrystalline substance (phase).

In order to understand the diffraction pattern, either the incident beam is monochromatic or the X-Ray detector is able to resolve the energy from the Kα1, Kα2 doublet to the Kβ1 line. Alternatively, Sollers slits / optics can be used in order to select the corresponding angular range.

A resolution better than 450eV is necessary (FWHM of the measured Cu Kα1, Kα2 doublet).

Diffraction patterns consists of rings, high intensity spots due to crystallized materials, which are mixed to the existing phases are averaged over continuous sample rotations. Intensity integration over those rings allows pattern indexation.

Near photon counting sensitivity maybe required for standard laboratory X-ray sources whereas high brilliance sources such as microfocus / synchrotons will require good dynamic range: typically 15,000:1 and large area 100 x100mm. One to two megapixel detectors with spatial resolution of 60-120 microns is usually sufficient.

A typical experimental set-up requires a highly collimated X-ray source and a detector with photon counting sensitivity as the intrinsic process behind small angle x-ray scattering is very inefficient. Ideally both small angle and wide angle detectors are combined in order to characterize simultaneously short and longer ranges of scattering vectors.
For instance, WAXS will be used to determinate the degree of crystallinity of polymer samples. SAXS is capable of delivering structural information of macromolecules between 5 and 25 nm with averaged particle sizes, shapes, distribution, and surface-to-volume ratio, of repeat distances in partially ordered systems of up to 150 nm. USAXS (ultra-small angle X-ray scattering) can resolve even larger dimensions.
SAXS WAXS patterns consist of low intensity patterns acquired over minutes of integration requiring very low background noise and good signal discrimination. Coexistence of bright and very bright signals on the same image could require dynamic range up to 106:1 using multiple exposures.

Large area detectors up to 200x200mm and 16 megapixel resolution are used in synchrotrons whereas smaller input size detectors: typically 60 to 90mm and 1 megapixel resolution are used with laboratory sources.

SAXS / WAXS

Small molecules (unit cell containing 100 atoms or less) and macromolecular (unit cell containing 10000 atoms or more) are crystallized and exposed to high brilliance X-ray beam on a synchrotron or X-ray lab source.

The experimental set up consists of gradually rotating the samples over 0.1 to 0.25 degrees in order to record Bragg reflections from each orientation of the crystal.

A good dynamic range is required, typically > 15,000:1 with potentially the possibility to read and expose at the same time in order to be able to rotate the sample at continuous speed over fine angular ranges: this is the fine Phi slicing technique.

Depending on beam delivery conditions as well as crystallization quality, the data collected can reveal conformal properties of a materialin addition to its electron density that will shed some light onto binding mechanisms of enzymes or proteins.

Large area detectors up to 270x270mm and 16 megapixel resolution can be used in synchrotrons, whereas 165mm diagonal detectors are more commonly used with laboratory sources.

Grazing incidence geometry is used near the total external reflection angle of the sample material. Measurements of X-ray intensity reflected from the sample as a function of angle gives a pattern of interference fringes, which is analyzed to determine the properties of the film layers responsible for creating the fringe pattern.

The detector usually records more than 10 reflection orders, this requires high dynamic range with the ability to detect very low flux for unveiling the very last reflections whilst coping with the strongest first order reflections. Integrating intensities over a 2D detector allows a large angle collection at once: ie typically >4 degrees without any scanning requirements.

Detectors with 56x28mm or up to 80x30mm can be offered.

Linear scanning will allow fast acquisition routines with a few millisecond read out time and 100% duty cycle. This delivers optimum sensitivity and dynamic range whereas area scanning will allow large angular collection at the expense of a longer read out cycle.

Only discrete photon energy can be absorbed by the sample, this corresponds to the characteristic binding energy of electrons in the material that is excited. It will unveil its local composition as well as its electronic state.

Tuneable X-ray sources are required in order to identify discrete K absorption edges of complex or multiple ionized materials presence within a given structure.

Integrating intensities over a 2D detector allows a rapid mapping over large areas.

Very good spatial resolution is required for matching the microbeam dimensions that are available in both synchrotrons and laboratory sources.

This application requires detectors that can ideally offer simultaneous energy resolution down to 150 eV and 2 dimensional mapping response with no read out dead period.

Detector format of 13x13mm are already available with energy response covering the Vacuum UV, the water window up to 8 keV. Typical duty cycle achieved: 163ms per frame.

The single crystal, the specimen and the detector are mounted precisely on a goniometer with the distance from the source of X-rays (the specimen) and the crystal equal to the distance from the crystal to the detector.

The technique is often used in conjunction with EDS, where the general chemical make-up of an unknown can be learned from its entire spectrum.

WDS is mainly used in chemical analysis, in an X-ray fluorescence spectrometer or in an electron microprobe.

The detector geometry must allow good angular coverage for mapping all wavelengths in a single acquisition without having to move the detector.

Detectors with 80x30mm active area can be offered with both linear scanning or area scan modes, 100% duty cycle, optimum sensitivity and dynamic range. Temporal resolution down to <100 nanoseconds with 30KHz repetition rate can be offered for pump probed experiments.

The Laue back reflection mode records X-rays scattered backwards from a broad spectrum source. This is useful if the sample is too thick or bulky for X-rays to transmit through it.

The diffracting planes in the crystal are determined by knowing that the normal to the diffracting plane bisects the angle between the incident beam and the diffracted beam. Crystal orientation is determined from the position of the spots.

Each spot can be indexed, i.e. attributed to a particular plane, using special charts.

The Laue technique can also be used to assess crystal perfection from the size and shape of the spots. If the crystal has been bent or twisted in anyway, the spots become distorted and smeared out.

With modern synchrotron and laboratory optics able to deliver micrometer beam size, it is possible to highlight the grain orientation and strain distribution of individual grains in a polycrystalline alloy before and after tensile loading.

thema : X-RAY cameras - Laue camera - powder diffraction - dual laue camera - protein crystallography - single crystal orientation - SAXS WAXS - laue detector - X-RAY diffraction.