The Coolidge tube, also called hot cathode tube, is the most widely used device for the production of X-rays.
In the Coolidge tube, the electrons are produced by thermionic effect from a tungsten filament heated by an electric current. The filament is the cathode of the tube. The high voltage potential is between the cathode and the anode, the electrons are thus accelerated, then hit the anode and X-ray is produced.
It consists of an evacuated chamber where electrons are accelerated using high voltage of about 104 to 105 volts. These electrons are allowed to strike a metal target such as Tungsten or Molybdenum (Mo) having high melting point and atomic weight. Due to this the metal target gets excited to higher energy state and when the excited atoms return to their ground state X-rays are emitted.
About 98% of the energy is converted into heat and remaining 2% come out as X-rays. This large amount of heat can melt the metal target. So a copper tube is placed alongside the metal target where cold water is circulated continuously.
Frequency and wavelength of X-ray produced:
If V be the accelerating voltage applied between two electrodes, then the kinetic energy acquired by the electrons is given by:
eV = ½mv2max
Where m, e and v be the mass, charge and velocity of the electrons.
Electrons with energy ½mv2max strike the target and exit to their ground state and this energy is released as photon (hf).
½ mv2max = hfmax
or, eV = hf
or fmax =eV/h………(i)
Equation (i) gives the frequency of x-rays.
Now, we know f = ch/λ
eV = ch/λmin
∴ λmin = ch/eV……(i)
Equation (ii) gives the wavelength of the electron.
Control of intensity and quality of X-rays:
a. Intensity control: The intensity of x-rays depends on the number of electrons emitted by the filament. This can be controlled by varying the electric current flowing through the filament.
b. Quality control: The wavelength of X-rays determines the quality of them. We have,
λmin = ch/eV
Here, c, h and e are constant. So, by adjusting the accelerating potential V we can control the quality of X-rays.