X-rays are used in various fields nowadays. It has vast use in various fields including biological researches, treatment, etc.
HOW IT WAS DISCOVERED?
In 1895, German scientist Roentgen observed that when fast-moving cathode rays strike a metal-piece of high atomic weight and high melting point, a new kind of rays are produced. These rays are invisible to eye, but affect the photographic plate in the same way as do the light rays. He named these unknown and invisible rays as ‘X-rays’. They are also callled ‘Roentgen rays’.
HOW X-RAYS ARE PRODUCED?
Coolidge tube or modern X-rays tube is used for the production of X-rays. It has a hard-glass bulb having a high vaccum of about ten to the power negative six. Two tubes are connected to it. One of the tubes has a tungsten filament F through which current is passed by means of a battery B. The filament, on being heated, emits thermionic electrons whose number per second depends upon the temperature of the filament. Around the filament is a molybdenum cylinder C which is kept at a negative potential relative to the filament.
It concentrates the electrons emitted by the filament in the form of a fine beam. Just infront of the filament F is a copper block whose front surface is inclined at 45 degree with respect to the electron-beam. On this surface is fixed a piece T of some metal of high atomic weight and high melting point like tungsten or molybdenum. It is called the ‘anti-cathode’ or ‘target’. The copper block is fixed at one end of a hollow copper tube in which a stream of water flows. The whole tube is enclosed by a lead envelope.
Working: When an alternating potential difference of about 20,000 volt is applied between the filament F and the target T by means of a step-up transf0rmer, then electrons emitting from the filament strike the target with a very high speed and X-rays are emitted. Due to the continuous striking of electrons, the target T becomes very hot. Actually, only 0.2 percent energy electrons is utilised for the production of X-rays, the rest is converted into heat. If there be no provision for the removal of this heat, the target may melt. This is why the target is kept cool by running cold water in the copper tube around it.
The electrons emitted from the filament strike the target only during one half-cycle of the alternating potential difference, when the target is positive with respect to the filament. During the other half-cycle, when the target is positive with respect to the filament, electrons are repelled by the target. Thus, the tube acts as its own rectifier.
For the production of X-rays, it is essential that the electrons emitted by the filament do not collide with the gas atoms before they reach the target. This is why high vacuum is kept in the tube. If it is not so, electrons will lose their energy by ionising the gas, and the positive ions so produced will be attracted towards the filament and damage it by collision.
SPECTRUM OF X-RAYS
When X-rays emitted by the target of an X-ray tube are analysed by a Bragg’s crystal spectrometer, two distinct types of spectra are obtained:
(i) a continuous spectrum, and
(ii) a line spectrum superposed on the continuous spectrum.
This shows that X-rays from the target contain a range of wavelengths of varying intensity; and that the intensity rises sharply at certain definite wavelengths.
The continuous spectrum has a definite short wavelength limit, below which there is no radiation. The value of limit is independent of the material of the target and depends on the voltage applied to the X-ray tube. Higher the voltage, smaller is the value of limit. The radiation having the continuous wavelengths forms the ‘continuous X-rays’.The continuous curve has pronounced intensity peaks at certain wavelengths. That indicates the production of X-rays.