Crystal monochromator

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.

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.

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