Spark image

X rays

You have probably seen that in the thermionic diode, electrons are attracted across from the cathode to the anode and a current flows. If we now increase this attracting voltage the electrons will cross the tube with greater and greater speeds and so have much greater energy when they collide with the anode

If this attracting voltage is above about 20 000 V, the electrons will be moving at about one third the speed of light and so when they crash into the anode a lot of heat energy will be produced. However, not all the electron's energy is turned into heat. About 1% becomes a radiation called X rays, first discovered in 1895 by a man called Roentgen. These X rays are electromagnetic radiation of short wavelength, not as short as gamma rays but of shorter wavelength than ultra violet. They are emitted from the anode of the X ray tube when high-energy electrons strike it.
The higher the voltage across the tube the greater the energy of the electrons and the shorted the wavelength of the X rays produced.

Higher electron accelerating voltages give shorter wavelength X rays

For 'hard' X rays a voltage of about 200 kV gives a wavelength of 0.0062 nm or 0.62x10-11 m. 'Soft' X rays are produced by accelerating voltages of some 30 kV and have wavelengths of around 0.041 nm (4.1x10-11 m).
The diagram at the top of the page is an X-ray tube and it shows that the anode has to be tubes use cooling fins; others pump water or oil through the anode.

Experiments with X-rays show that they have the following properties:
1. They are short wavelength electromagnetic waves.
2. They do not pass through thick metal.
3. They pass through paper, wood, glass, flesh and cloth.
4. They cause ions to be produced.
5. They travel in straight lines.
6. They affect photographic paper.
7. Excessive exposure can cause damage to cells and people who use then regularly must protect their bodies

Uses of X-rays
You will all know that X-rays are used to take pictures of your insides but they do have other uses:
1. medicine or dentists photographs of your insides or your teeth
2. detecting cracks in metals or in bones
3. discovering the structure of a crystal
4. looking for paintings below other pictures

© Keith Gibbs 2011