X - Ray Spectra

X – rays are produced when electrons are accelerated through a high voltage and hit a target. The x – rays emitted from a material contain a range of frequencies. A typical X – ray spectrum is shown below.

The X - ray spectrum consists of a continuous part – the smooth curve in the diagram above – and a series of sharp spikes characteristic of the material that is the source of the X – rays.

The Continuous Spectrum

As the incoming electrons collide with the atoms in the target, they are decelerated, producing X – rays. The energy or wavelengths of the X - ray photons depends on the energy lost in the collision, with the maximum photon energy equal to the kinetic energy of the incident electron. This maximum photon energy corresponds to a maximum frequency (via the relationship)or minimum wavelength (via the relationship), labelledon the graph above.

Characteristic Features

When a electron hits an atom in the target, it may excite an electron from an inner orbital in the atom to a higher energy level. The electron soon drops down to the lower energy level, emitting an X - ray photon with energy equal to the difference between the two energy levels. Because the energy levels are characteristic of the atom, so are the energies and wavelengths of the emitted photons. These produce the spikes in the graph above. If the accelerating voltage is increased, the continuous part of the curve may move up and to the left, indicating a higher intensity of radiation and a higher maximum photon energy, but the spikes will remain at the same wavelength because these indicate a property of the material, which does not change with the energy of the incident electrons.

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