X unit

The X unit is an obsolete unit of X-ray wavelength, approximately 1.0021 × 10⁻¹³ meters, defined using the spacing of calcite crystals.

Manne Siegbahn: A Swedish physicist who developed precision X-ray spectroscopy and introduced the X unit. Siegbahn established the unit for reporting X-ray wavelengths.
Wavelength: The distance between successive corresponding points of a wave. The X unit measures this property of X-rays.
Ångström: A length unit equal to 10⁻¹⁰ meters, commonly used for atomic and X-ray scales. It remains familiar for X-ray wavelengths, while the X unit is obsolete and much smaller.
X-ray emission spectroscopy: A method that analyzes characteristic X-rays emitted by excited atoms. X units expressed the short wavelengths of characteristic spectral lines.
X-ray spectroscopy: A family of methods that measures how materials emit, absorb, or scatter X-rays. The unit arose in spectroscopy, where precise wavelength comparisons were needed.
X-rays: High-energy electromagnetic radiation with wavelengths shorter than ultraviolet light. The unit was designed specifically to express their wavelengths.
Picometer: A length unit equal to 10⁻¹² meters. It offers an SI-compatible scale for wavelengths without relying on a crystal reference.
X-ray absorption spectroscopy: A technique that measures how a material absorbs X-rays across a range of energies. Its edge positions can be reported as wavelengths as well as energies.
Calcite: A crystalline form of calcium carbonate with a well-characterized lattice spacing. Its crystal spacing supplied the physical reference used to define the unit.
Crystal lattice: A repeating three-dimensional arrangement of atoms or ions in a crystal. The calcite definition depends on regularly repeated atomic planes.

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