How does x ray diffraction work

A famous example of this technique was the determination of the structure of DNA. In that case, growing true single crystals proved to be challenging and analyzing the data from single crystals was also an unsolved problem at the time , but the additional orientation of the diffraction pattern due to the fiber geometry was enough to deduce the helical form of the DNA molecule.

Fiber diffraction is often used when studying long-chain molecules such as DNA, or columnar structures such as discotic liquid crystals. Due to the curvature of the Ewald sphere, the diffraction pattern observed on a flat detector is distorted, and some portions of the Ewald sphere are actually inaccessible. The Fraser correction R. Fraser, T. Macrae, A. Miller, R. Rowlands, J. Fiber diffraction pattern from dendrimers self-assembled in a columnar hexagonal phase with helical correlations.

This approach is therefore ideal for measuring the properties of thin films or multilayers on solid or liquid substrates. The resultant intensity profile can be analyzed to establish the two-dimensional crystal structure within the plane of the film. The resultant intensity profile can be analyzed to the thickness of the layer or, layers in a multilayer film , and in some cases to say something about the electron density profile within each layer.

Geometry for grazing incidence and X-ray reflectivity measurements. Following Bragg's Law , this implies that the length scale of the objects being probed is fairly large, typically in the range between 3 and nm. Historically, this technique was primarily used to study relatively large "objects" dispersed in a medium, such proteins dissolved in an aqueous medium, colloidal particles, micelles, or voids in porous media.

More recently, SAXS has been used to study self-assembled systems such as block copolymers that have periodic order with repeat distances much larger than a single molecule.

The image to the right shows a small-angle powder diffraction pattern from branched molecules called dendrimers. Many tens of molecules self-assemble into spheres, and these spheres then form a cubic structure that may be 20 or more nm across. In this case there is considerable disorder in the atomic positions, but long range order in the positions of the spheres.

Measuring such systems requires instrumentation optimized for scattering at small angles but analysis techniques closer to those traditionally used for crystallographic analysis.

Left: Small-angle diffraction pattern from dendrimers self-assembled in the Pm-3n cubic phase. Right: Small angle scattering patterns from carbide-derived porous carbons as a function of chlorination temperature, providing quantitative information on the size distribution of pore sizes. Production of X-rays : There are a variety of methods for producing a beam of x-rays.

X-ray Tube. This is the simplest and oldest approach, and is still occasionally used. A beam of electrons strikes a metallic target and X-rays are emitted. The intensity of the X-ray beam is limited by the heat released into the target by the electron beam.

Rotating anode X-ray Generator. This variant of the traditional X-ray tube, which became widely available in the 's, addresses the heat loading problem by replacing the fixed target with a rotating cylinder, water-cooled on the inside.

Considerably more X-ray intensity is thereby made possible, but there are both literal and figurative costs: the engineering requirements are considerably more stringent, and rotating-anode generators are subject to breakdowns and require frequent maintenance. Microfocus Tube. Microsource tubes started to become available around , and are gradually replacing rotating anode generators. A synchrotron X-ray source uses a totally different mechanism from the tube sources described above: the radiation emitted from a relativistic beam of electrons or positrons accelerated by a magnetic field.

The resulting beam is generally many orders of magnitude more intense than that produced by the tabletop sources described above. However, such a beam can be produced only at a large centralized facility, obliging most users to travel substantial distances and plan their usage well in advance. Collimation : The radiation produced by any of the above mechanisms consists in general of rays traveling in a variety of directions and consisting of a spread of wavelengths.

The purpose of the collimation portion of an XRD instrument is to produce a relatively thin beam of X-rays with a narrow spread of wavelengths, all traveling in essentially the same direction. Some commonly used components are described below. Slits or pinholes. A filament of tungsten is heated to produce electrons which are then accelerated towards an anode material i.

Cu, Co, Mo, etc. On impact electrons are dislodged from the inner shell and when electrons from higher shells fall down to take their place, an x-ray characteristic of the anode material is released. See figure below. A set of optics focus the x-ray beam on the sample which then reflect up into the detector which is positioned at the same angle opposite to the tube See figure below. This technique sends x-ray beams through it.

X-ray beams are chosen because their wavelength is similar to the spacing between atoms in the sample, so the angle of diffraction will be affected by the spacing of the atoms in the molecule, as opposed to using much larger wavelengths, which would be unaltered by the spacing between atoms.

This is the angle of diffraction. Some of these diffracted beams cancel each other out, but if the beams have similar wavelengths, then constructive interference occurs. Constructive interference is when the x-ray beams that are whole number integers of the same wavelength add together to create a new beam with a higher amplitude.

The greater amplitude of the wave translates into a greater signal for this specific angle of diffraction. The distance between atomic plates can then be used to determine composition or crystalline structure.