17.4.1 X-Ray Spectra

In 1895, W.C. Roentgen reported the discovery of X-rays. He even took an X-ray picture of his wife’s hand (and wedding ring), demonstrating the penetrative power of the radiation. He called it X-ray to underline the fact that the nature of this radiation was unknown.

Later, it was determined that X-ray is an electromagnetic wave with very short wavelength of the order of 10-10 m. This translates into very energetic photons of the order of 10 keV in the photon model of light.

In the laboratories, X-ray is produced using X-ray tubes. An x-ray tube is an evacuated tube containing a filament and a target metal (most commonly tungsten). A voltage is applied to the filament to heat it up until electrons are released[1]. A very high accelerating voltage VA (typically a few hundred kV) is applied between the filament and the target metal. This voltage provides a strong electric field that accelerates the electrons (released from the filament) to a very high speed before they smash into the target metal. X-ray is produced out of the carnage. Basically, the KE of the filament electrons has being converted into x-ray photons (and a lot of unwanted heat).

The spectrum of the x-ray radiation is pretty interesting; it is a combination of discrete spectral lines and a continuous spectrum. The discrete spectral lines are known as the characteristic lines. The continuous spectrum is called the bremsstrahlung (German for braking) radiation, or simply the background radiation. The continuous spectrum is actually range bound; the minimum wavelength is called the cut-off wavelength λmin.

It turns out that x-ray photons are being produced through two different kinds of interaction between the filament electrons and the target metal atoms.

  1. Interactions between the filament electrons and the inner shell electrons produce characteristic x-ray photons (depicted by (1) in diagram) which form the characteristic line spectrum.
  2. Interactions between the filament electrons and the atomic nuclei produce bremsstrahlung (German for braking) x-ray photons (depicted by (2) in diagram) which form the background radiation.

We will now look at these two mechanisms in more detail.


[1] It is similar to photoelectric effect, except that heat is used instead of photons to liberate the bound electrons

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