Discovery of X-rays and some of their uses

The Nobel prize for physics was awarded to Roentgen in 1901 for his discovery of X-rays.



            The discovery of X-rays was by a German physicist called Wilhelm Roentgen.  In 1895 Roentgen discovered X-rays almost by accident.  Whilst doing some experiments in which he passed an electric current through Crookes tubes (special tubes containing a cathode and electrode from which the air has been removed), Roentgen noticed that photographic plates nearby began to grow fogged.  To discover why this occurred he placed black paper on the tube and then switched on the current.  Nearby a screen coated with barium began to glow.  This caused Roentgen to believe that unknown rays produced inside the tube were passing through the paper to make this fluorescent substance give out light.  These he named X-rays since x is a scientific number for anything that is unknown.         


What are X-rays?


X-rays are electromagnetic waves like radio and light waves.  These all travel at the speed of light, which is 300 000km per second.  The wavelength of an X-ray however is one hundredth that of the light rays that you can see, at around 10¯¹ºm.  This means that they have a lot more energy. X-ray fluorescence is the emission of X-rays from excited atoms produced by the impact of high-energy electrons, other particles or a primary beam of other X-rays.  Atoms of all the elements emit a characteristic X-ray spectrum when they are bombarded with electrons.  The X-ray photons are emitted when the incident electrons knock an inner orbital electron out of an atom.  When this happens, an outer energy electron falls into the inner shell to replace it, losing potential energy in doing so.

            The occurrence of X-rays is not just on the Earth.  For instance it has been found that X-rays reach the Earth from outer space.  Mysterious objects called quasars give out huge amounts of X-ray energy.   X-rays can pass through many forms of matter and therefore have many uses.  X-rays are produced for these purposes by an X-ray tube.


Uses of X-rays


The main use of X-rays is in medicine.  A comman application is in the form of X-ray machines, which take photos of a patient’s body.  If an arm or leg were broken for example, then this limb would be put in front of the X-ray with a piece of photographic film behind.  The X –ray is turned on briefly and goes through to the film.  The rays go through the skin and flesh easily, showing up as dark areas on the film, but with more difficulty through bone.  They are slowed down and so these areas are much lighter.  X-rays can also be used to kill cancer cells, but also kill healthy cells, so must be used with much care.

            Other uses are in industry, at airports to check customers and baggage and by art historians to see if a picture has been painted on top of an older one.  X-ray diffraction is also very important in spectroscopy and as a basis for X-ray crystallography.  The diffraction of X-rays by a crystal where the wavelength of X-rays is comparable in size to the distances between atoms in most crystals is used to disperse X-rays in a spectrometer and to determine the structure of crystals or molecules.