Advanced Modern Physics
1.0 X-Rays
1.0 X-Rays
X-rays are electromagnetic radiation of a very short wavelength and high energy. They are emitted when electrons (or cathode rays) moving at high-speed strike a target of high atomic mass.
Discovery of X-rays
X-rays were discovered by Roentgen in 1985. He carried out an experiment where a discharge tube operating at low pressure and high voltage, emitted a radiation that caused a fluorescent screen in the neighbourhood to glow brightly.
To check whether the glow is due to visible light, he wrapped the entire discharge tube in a black paper to prevent visible light. However, results were the same.
Thus, some unknown radiation was responsible for fluorescence. It was then confirmed that X-rays were emitted when cathode rays (consisting of fast moving electrons) strike the wall of the discharge tube.
Production of X-rays (Coolidge's Tube)
X-rays are produced when fast moving electrons strike a target (like a metal piece).
When electrons strike the atoms, they lose their kinetic energy which is converted into radiant energy in the form of X-rays.
The setup developed by Coolidge for the production of X-ray is shown in Fig. 1.
The filament is heated to undergo thermionic emission with the help of a supply. The electrons released are given a shape of a beam with the use of collimator and then are accelerated through a high DC voltage which is maintained between cathode and anode. These fast moving electrons then strike the metal target which is embedded at the end of the copper rod. Some part of the energy is lost and converted to heat while the rest is converted into X-rays.
Only about 1-10% of the energy of the electrons striking the target produce X-rays and the rest is converted to heat.
The target, as a result, becomes very hot and therefore should have high melting point, specific heat capacity and atomic number. The heat generated is dissipated through the copper rod and the anode is cooled by water flowing through the anode.
Clearly, for more energetic electrons, accelerating voltage is increased.
For more number of photons, the voltage across the filament is increased.
The nature of emitted X-rays depends on:
- The current in the filament
- The voltage between the filament and the anode
An increase in the current in the filament means an increase in the number of electrons emitted implying a more intense beam of electrons and hence a more intense beam of X-rays is produced. As a result through this we can control the quantity of X-rays.
An increase in the accelerating voltage implies more energetic beam electrons and a more energetic X-rays. As a result through this, we can control the quality of X-ray produced i.e, penetration power of X-rays.
Based on penetration power, X-rays are classified into two types:
- Soft X-rays - Low energy and hence low penetration power
- Hard X-rays - High energy and hence high penetration power
