Coolidge X-ray tube was designed by W.D. Coolidge in 1913 for the production of X-rays.
The Coolidge tube consists of an evacuated glass bulb at low pressure. There is a tungsten filament F (also known as cathode or heater) connected to a low tension battery (L.T.) at voltage of about 6 volts which helps to heat the filament. There is a molybdenum shield M around the filament which prevents the spreading of electrons emitted from the filament.
There is a tungsten block T which is placed just opposite to the filament whose face is inclined at 45˚ to the horizontal. This block is also called target or anode because the accelerated electrons strike it. Due to the collision, much more heat is generated in the target (temperature raises up to nearly 2000˚C) that can melt it so a cooling arrangement is done by using cooling fans or circulating water with the help of copper tube AB adjusted with the target. The typical voltage 30kV to 150kV is applied between the filament and the target with the help of the high tension battery (H.T.).
Working: When a high potential difference in order of kV is applied between the filament and the target, the emitted electrons are accelerated towards the target by acquiring high kinetic energy. These fast moving electrons strike the target and get retarded. The retarded electrons lose energy which is converted in the form of radiation called X-rays. In this way, X-rays are produced by Coolidge tube.
Quality of X-rays [Soft and Hard X-rays]
The quality of X-ray is its penetrating power. Hard X-rays have more penetrating power than soft X-rays. The penetrating power depends upon the energy of the photons. Therefore, it further depends upon the frequency of the photons. Thus, frequency is given by, [hf=eV] Where, V is the applied potential difference between anode and cathode. As the p.d. (V) is increased, electrons will acquire more K.E. and hence more energetic (penetrating) X-rays are produced. Hence, quality of X-rays is controlled by the amount of voltage across the two electrodes. Clearly, hard X-rays have more energy than soft X-rays.
Intensity of X-rays
The intensity of X-ray beam means the number of photons flowing through unit area in unit time. The number of photons of X-rays depends on the number of electrons emitted by the filament that strike the target. The number of electrons emitted by the filament depends upon its temperature which can be controlled by changing the filament current. Hence, intensity of X-rays can be controlled by adjusting the p.d. across the filament.
Advantages of Modern Coolidge X-ray Tube
- The main advantage of Coolidge tube is that both quantity (penetrating power) and quantity (intensity) of X-rays can be controlled.
- This tube is suitable for working voltage 100 volts to 10⁶ volts.
- It produces X-rays continuosly.
[Also See: Bragg’s Law]