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.
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.