In an X-ray diffraction (XRD) instrument, X-rays are generated using an X-ray tube, which works like this:
- Electron generation: Electrons are emitted from a heated filament called the cathode.
- Electron acceleration: These electrons are accelerated towards a metal target (the anode) using a high voltage.
- X-ray production: When the high-speed electrons hit the target, their energy is converted into X-rays. This happens in two ways:
- Characteristic X-rays: Electrons knock out inner-shell electrons of the target atoms, and when outer electrons fill the vacancy, X-rays with specific energies are emitted.
- Bremsstrahlung (continuous) X-rays: The electrons slow down rapidly upon hitting the target, producing a broad spectrum of X-rays.
- Filtering and selection: A monochromator or filter is often used to select X-rays of a specific wavelength, which are then directed at the sample for diffraction.
Common targets in XRD are metals like copper (Cu), molybdenum (Mo), or cobalt (Co) because they produce strong X-rays suitable for diffraction.
In short, X-rays in XRD are made by smashing high-speed electrons into a metal target, which emits X-rays that are then used to study crystals.