Roentgen's discovery of X-rays
Roentgen’s discovery of X-rays :- In 1895, the German scientist Rontgen observed that when a fast moving cathode ray collides with a piece of metal having a higher atomic mass and a higher melting point.
A new type of ray is produced which is not visible by the eye. but in plate of photography It acts the same way as light rays do.
He named these unknown and invisible rays as X-Rays. In the name of roentgen or röntgen, these rays are also called Rontgen Rays. Professor Röntgen was awarded the noble prize of physics in 1901 for this important discovery.
Production of X Rays by Coolidge Tube
Nowadays Coolidge tubes are used to make X-rays. It was created in 1913 by Dr. Coolidge. It consists of a spherical bulb made of hard glass. Inside the glass there is a high vacuum of about 10-6 cm (Mercury). It has two drains connected.
A tube has a filament (F) of tungsten in which current is passed through a battery (B). When the filament is heated, electrons start coming out of it, due to thermic effect, whose number depends on the temperature of the filament.
There is a cylinder (C) of molybdenum around the filament which is kept at negative potential relative to the filament. Due to this the electrons coming out of the filament get concentrated in the form of a fine beam.
There is a block of copper just in front of the filament F whose plane is inclined at 45° to the path of the electron beam. A piece of metal with high melting point and high atomic mass like tungsten or molybdenum (T) is deposited on this plane.
It is called Anti-Cathode or Target. The copper block is located at the end of a hollow copper tube in which a stream of cold water is passed. The entire tubule is surrounded by a lead shell.
HowX-Ray is produced
working : When an alternating potential difference of about 20000 volts is applied between filament (F) and target (T) by a high-voltage transformer. The electrons coming out of the filament hit the target with a very high velocity and X-Rays start coming out.
The target (T) becomes very hot due to the constant collision of electrons.
In fact, only 2% of the energy of electrons is used to produce x rays, the remaining part gets converted into heat. If there is no arrangement to remove this heat, the target may melt.
For this reason, it is kept cool by flowing water in a copper tube around the target.
In the half cycle of alternating potential difference, when the target (T) is positive with respect to the filament (F), then the electron gets attracted and hits the target.
In the remaining half cycle, the target becomes negative with respect to the filament, due to which the electrons are repelled. In this way the electrons hit the target only in half the cycle and the tube itself acts like a rectifier.
For the generation of x rays it is necessary that the electrons from the filament do not collide with the atoms of the gas of the tube before reaching the target. That is why a high vacuum is maintained in the tube.
If this is not the case, the electron will lose its energy by ionizing the gas and the generated positive ions will collide with filament and damage the filament.
Control Over Intensity and Penetration of X-Rays
Soft and Hard X-Rays
In generating X-rays, their two properties have to be taken care of: Intensity and Penetration
Control over Intensity: The intensity of x-rays is called their rate of production i.e. the number of x-rays produced per second. Their value is directly proportional to the number of electrons producing x rays.
If the number of electrons hitting the target per second is increased, then the number of X rays produced will also increase i.e. X rays of more intensity will be produced.
To increase the intensity of X rays received from the Coolidge tube, the current flowing in the filament is increased. This increases the number of electrons per second from the filament, that is, the number of electrons hitting the target per second. As a result, the intensity of the generated X rays increases.
Control over Penetration: The penetrability of X rays depends on their wavelength. Rays with larger wavelength (approximately 4Å) have less frequency (v = c/λ), so their energy is also less.
When the energy (hv) is less, their penetrating power is less and they are called soft or less penetrating x rays.
These rays can pass through only thin sheets of matter. In contrast, rays with shorter wavelength (about 1Å) have higher penetrating power and are called hard or highly penetrating X rays.
The wavelength of X-Rays depends on the kinetic energy of the electrons producing them and this kinetic energy depends on the potential difference between the filament (F) and the target (T) inside the tube.
As this potential difference is increased, the wavelength of the X rays produced in the tube becomes smaller, that is, the penetrating power increases.
Therefore, the power of X rays can be increased by increasing the potential difference applied at the ends of the tube and it can be reduced by decreasing.