The different elements of a diffractometer

T able 2.1. Emission wavelength and excitation threshold cf a few elements 

This type of tube was the one used by Rbntgen, who discovered the existence of X-rays in 1895. 

The intensity of the discharge current in a tube of this kind is highly dependent on the gas pressure, hence the need to be able to control it precisely and maintain it a constant level. This feature constitutes the main weakness of this kind of tube. The anticathode s lifetime is short (roughly 100 hours) but, on the other hand, its surface is not polluted, unlike in Coolidge tubes, and therefore the X-rays emitted are pure. Currently these tubes ate in practice no longer used. 

The intensity of an X-ray beam emitted by the anticathode is directly proportional to the current inside the tube and to the square of the applied voltage. However, the efficiency of the emission process is very low, and even in the best conditions, with a voltage of 100 kV, only 2% of the energy provided is transmitted to the X-rays. 

Roughly 98% of the electron beam s energy is consumed by the Joule effect and heats up the anticathode, which is why it is important to set up a system to evacuate heat. A cooling system by water circulation is used. 

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