X-rays are produced when fast moving electrons are suddenly stopped by matter. An X-ray (Röntgen radiation) is a form of high-energy radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. X-rays were discovered in science as a type of unidentified radiation emanating from discharge tubes by experimenters investigating cathode rays. They were accidently noticed by a German scientist known as Wilhelm Röntgen. Röntgen, a professor of physics discovered X-rays while experimenting with Crookes tubes and began studying them on November 1985.
How x-rays are produced
X-rays are produced in x-ray tube where highly accelerated electrons are suddenly stopped by a metal target. Figure below shows the structure of an X-ray tube.
How x-ray tube works
When a current flows through the filament in the cathode, electrons are produced by thermionic emission. Thermionic emission is the release of charged particles, usually electrons, from a heated surface, such as a metal. Electrons are then accelerated towards the target by a high potential difference of about 100Kv between the cathode and the anode. The metal target is the anode. Cathode is concave shaped in order to focus the electron beam produced towards the metal target. The metal target is usually composed of tungsten embedded onto copper.
Tungsten is preferred because of it’s high melting point. Copper is also preferred as it is a good conductor of heat. This ensures efficient dissipation of heat. A lot of heat is produced during x-ray production. This is because most of kinetic energy of the electrons are converted to heat when they heat a metal target. Only about 0.5 % of the kinetic energy is converted to x-ray radiations. Because heat produced is a lot, cooling is enhanced by the cooling fins on the outside of the tube and circulation of oil through the channels in the copper block(node).
modern x-ray tube
In an improved recent model of an x-ray tube, the target is designed to rotate during operation so as to change the point of impact. This reduces wear and tear resulting from frictional forces. The figure below shows the more modern x-ray tube.
X-ray tube is made up of strong highly evacuated glass. This ensures that no gaseous particles are inside to collide with the fast moving electrons. Collisions with air particles would cause electrons to loose some of their energy before they reach their target hence lacking enough energy to produce x-ray.
The target is usually set at an angle of 45o to the electron beam so that to direct x-rays out of the tube through a window on the lead shield. see figure 1 above. Incase some x-rays are produced at an angle that would make them unable to exit the tube, then they will be safely absorbed by the lead shield surrounding the tube. A step-up transformer is used to increase voltage from main supply so as to provide the high voltage required to accelerate electrons.
properties of X-rays
- They are not deflected by magnetic or electric fields. This shows that they carry no charge.
- X-rays penetrate matter according to the density of matters. High density materials like lead are least penetrated by x-rays.
- Affects photographic emulsions. This makes x-rays useful in x-ray photography.
- They ionizes air molecules
- causes fluorescence in certain substances such as zinc sulphide
- X-rays are of shorter wavelengths compared to visible light.
- They can be plane-polarized, be diffracted and be reflected
- travels in a straight line at a speed of light