Crystal monochromator

Neutron monochromator for the ECHIDNA powder diffractometer at OPAL in Australia. It is made by slabs of [113] oriented Germanium crystals which are inclined towards each other in order to focus down the Bragg reflected beam.

A crystal monochromator is a device in neutron and X-ray optics to select a defined wavelength of the radiation for further purpose on a dedicated instrument or beamline.[1] It operates through the diffraction process according to Bragg's law.

Similar devices are called crystal analyzer for the examination of scattered radiation.

Crystal Monochromator

A crystal monochromator is an optical device used in X-ray and neutron spectroscopy to isolate a specific wavelength or a narrow range of wavelengths from a broader spectrum of radiation. It operates based on the principle of diffraction through a crystalline structure, governed by Bragg's Law. Crystal monochromators are integral to applications in materials science, structural biology, and high-energy physics.

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Principles of Operation

Crystal monochromators utilize the atomic lattice structure of a crystal to diffract incident radiation at specific angles. The diffraction condition is defined by Bragg’s Law: nλ=2dsinθ Where:

• n: Order of diffraction,
• λ: Wavelength of the incident radiation,
• d: Spacing between atomic planes in the crystal,
• θ: Angle of incidence.

By adjusting the angle of the crystal, the monochromator selectively allows radiation of a desired wavelength to pass while filtering out others.

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Materials

Commonly used materials for crystal monochromators include:

• Silicon (Si): Offers high purity and stability, ideal for synchrotron radiation.
• Germanium (Ge): Suitable for specific wavelength ranges due to its lattice properties.
• Quartz: Used for its thermal stability in certain applications.

These materials are chosen for their well-defined crystal lattice structures and their ability to withstand the operational environment.

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Configurations

Crystal monochromators can be designed in various configurations:

• Flat Crystal Monochromators: Use a single, flat crystal to diffract radiation. They are simple and commonly employed in laboratory X-ray setups.
• Double-Crystal Monochromators: Consist of two crystals aligned to improve wavelength purity and minimize beam divergence.
• Bent Crystal Monochromators: Use curved crystals to focus the diffracted beam, enhancing intensity and resolution.

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Applications

Crystal monochromators are widely used in scientific and industrial research:

• X-ray Diffraction (XRD): To study the atomic and molecular structure of materials.
• Synchrotron Beamlines: For producing monochromatic X-rays in synchrotron radiation facilities.
• Neutron Scattering: To isolate specific neutron wavelengths for scattering experiments.
• Spectroscopy: In high-resolution X-ray and neutron spectrometers to filter out unwanted wavelengths.

References

1. Streli, Christina; Wobrauschek, P.; Kregsamer, P. (2017-01-01), Lindon, John C.; Tranter, George E.; Koppenaal, David W. (eds.), "X-Ray Fluorescence Spectroscopy, Applications", Encyclopedia of Spectroscopy and Spectrometry (Third Edition), Oxford: Academic Press, pp. 707–715, ISBN 978-0-12-803224-4, retrieved 2024-11-09