Placeholder Image

字幕表 動画を再生する

  • What is X-ray diffraction, and what is it used for?

  • In an X-ray diffraction experiment, a sample is placed into the center of an instrument and illuminated with a beam of X-rays.

  • The X-ray tube and detector move in a synchronized motion.

  • The signal coming from the sample is recorded and graphed,

  • where peaks are observed related to the atomic structure of the sample.

  • Most materials are made up of many small crystals like sand on a beach.

  • Each of these crystals is composed of a regular arrangement of atoms,

  • and each atom is composed of a nucleus surrounded by a cloud of electrons.

  • It's at this scale that the story of X-ray diffraction begins.

  • X-rays are high-energy light with a repeating period called the wavelength.

  • Since the wavelength of an X-ray is similar to the distance between atoms in a crystal,

  • a special interference effect called "diffraction" can be used to measure the distance between the atoms.

  • Interference occurs when X-rays interact with each other.

  • If the waves are in alignment, the signal is amplified. This is called "constructive interference".

  • If the waves are out of alignment, the signal is destroyed. This is called "destructive interference".

  • When an X-ray encounters an atom, its energy is absorbed by the electrons.

  • Electrons occupy special energy states around an atom. Since this is not enough energy for the electron to be released,

  • the energy must be re-emitted in the form of a new X-ray, but the same energy as the original.

  • This process is called "elastic scattering".

  • In a crystal, the repeating arrangement of atoms form distinct planes separated by well-defined distances.

  • When the atomic planes are exposed to an X-ray beam, X-rays are scattered by the regularly spaced atoms.

  • Strong amplification of the emitted signal occurs at very specific angles

  • where the scattered waves constructively interfere. This effect is called "diffraction".

  • The angle between the incident and the scattered beam is called 2-theta.

  • In order for constructive interference to occur, the scattered waves must be in alignment,

  • meaning that the second wave must travel a whole number of wavelengths.

  • In this case, one half of a wavelength is traveled on the incident side,

  • and one half on the scattered side, yielding one additional wavelength.

  • In the case of the next X-ray, one wavelength has traveled on

  • both the incident and the scattered side resulting in two wavelengths.

  • This reinforcement occurs throughout the crystal.

  • The exact angle at which diffraction occurs will be determined from the red triangle.

  • The angle at the top is theta, half the angle between the incident and scattered beams.

  • The long side is the distance between the atomic planes and the short side we know is one half of a wavelength.

  • The relationship between the diffraction angle,

  • and the spacing between the atoms can be determined by applying the sine function.

  • Rearranging this equation yields an equation commonly known as "Bragg's Law",

  • named after Sir William Henry and William Lawrence Bragg, the father-son team who won the Nobel Prize in 1915

  • for their work analyzing crystal structures with X-ray diffraction.

  • This technique of X-ray diffraction is used today for a wide variety of materials, ranging from

  • single crystal epitaxial thin films, to polycrystalline mixtures of powders, and even randomly oriented amorphous materials.

  • X-ray diffraction helps scientists to develop new pharmaceuticals, classify rock formations

  • based on their mineral components and understand how the arrangement of atoms

  • affects the behavior of energy storage materials.

  • As scientists push their ability to engineer materials on the atomic level,

  • X-ray diffraction becomes an increasingly important tool in their toolbox.

  • Advances in equipment design have made X-ray diffraction easier to use, and more powerful than ever.

What is X-ray diffraction, and what is it used for?


ワンタップで英和辞典検索 単語をクリックすると、意味が表示されます

B2 中上級

What is X-ray Diffraction?

  • 5 2
    lovegastar に公開 2022 年 08 月 04 日