A typical x ray spectrum is shown below.

The graph shows the relative intensity of x rays emitted at different wavelengths. It can be divided into two parts, a continuous spectrum (the curve) and a line spectrum (the peaks).

Continuous Spectrum

When a bombarding electron passes close to a nucleus it is deflected (see diagram below). The change of direction means that the electron has been accelerated. An accelerating charged particle emits electro-magnetic radiation. If the acceleration is great enough, the quantum of radiation emitted is an x ray.

If an electron passes very close to a nucleus (for example, electron e₃ in the diagram) it can be accelerated so much that it gives out all its energy in one quantum. This is therefore the biggest quantum (shortest wavelength) x ray emitted. The minimum wavelength therefore depends on the accelerating voltage. If the accelerating voltage is V, then the kinetic energy possessed by an electron when it reaches the target is eV.

Therefore, the minimum wavelength, l_min, is given by

An electron in the beam can have a collision with an electron in an atom of the target metal. If an electron in a low energy level is excited to a higher energy level, an x ray quantum can be emitted when an electron falls to fill the "space" in the lower energy level.

The wavelengths at which the peaks in the spectrum occur thus depend on the material of the target.

These "lines" in the spectrum are named after the energy level to which an electron falls, as shown below.

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