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X-Ray Spectra
See diagram of typical x-ray tube here.  
   
The part of the electro-magnetic spectrum which is referred to as x-rays is from wavelengths of about 10-11m to 10-8m.  
   
On hearing the word spectrum we tend to picture the component colours of the light emitted by a source (see here).  
For example, red, orange, yellow, green, blue, violet for a white light source.  
As x-rays are not visible, an x-ray spectrum is represented by a graph showing the relative intensity of x-rays emitted by a source at different wavelengths.  
(This can be done for visible light as well, of course.)  
   
The graph below shows a typical x-ray spectrum.  
 
   
The graph can be considered to consist of two parts, a continuous spectrum (the curve) and a line spectrum (the peaks).  
 
The Continuous Spectrum  
This part of the spectrum does not depend on what type of metal is used for the target.  
 
   
When a fast moving electron passes close to a nucleus it is deflected as shown 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 virtually 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 given by  
 
where e is the charge on the electron.  
To calculate the minimum wavelength, λmin, we use Planck's formula for the energy of a quantum of radiation  
 
Therefore  
 
   
Line or Characteristic Spectrum  
The peaks of intensity occur at wavelengths which depend on the type of metal used for the target.  
 
To explain the existence of these peaks we consider collisions between electrons in the beam (from the cathode) and electrons in low energy orbits in atoms of the target.  
The electron in the atom can be excited to a higher energy state by the collision.  
An electron in the atom can then fall down to the lower energy state which has been made available.  
When an electron falls to the lower energy level a quantum of radiation is emitted.  
The energy possessed by this quantum (and therefore its wavelength) depends on the energy difference between the two levels in the atom.  
This depends on what type of metal it is.  
   
These lines in the spectrum (the peaks of intensity) are named after the energy level to which an electron falls, as shown below.  
   
 
 
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