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In 1895 Roentgen was working with discharge tubes when he discovered that photographic plates placed near the tubes had become fogged although they had not been exposed to light. He decided that this effect must be due to the emission of some form of radiation from the discharge tube, and he named the radiation X- rays.

He deduced that these rays were electromagnetic in nature (it was later shown that their wavelength was much shorter than that of visible light). He realised that X- rays were produced when a beam of high energy electrons hits a metal target: the greater the electron energy, the higher the frequency of the X-rays.

X-ray tubes

A diagram of a modern X-ray tube is shown below.

This type of tube was devised by Coolidge in 1913; it can operate with either a hot or a cold cathode. In the hot-cathode tube electrons are emitted by thermionic emission and then accelerated by voltages usually of the order of 20 kV, giving relatively long-wavelength X-rays called 'soft' X-rays. With a cold cathode, however, the voltages required to cause electron emission are much greater - around 100 kV - and these tubes produce 'hard' X-rays of much shorter wavelength, between 10-9 and 10-13 m, depending on the voltages used. For some applications potential differences of up to 1 000 000 V are used.

The intensity of the X-ray beam depends on the number of electrons striking the target per second and in the hot-cathode tubes this is controlled by the heater current. The wavelength depends on the voltage across the tube. The penetrating power of the X- rays is thus dependent on the accelerating voltage and the intensity of the beam on the heater voltage.

When the electrons collide with the target anode they lose their kinetic energy; some of this energy is converted into X-radiation, but much of it produces heat. In fact, less than 0.05 per cent of the kinetic energy of the electrons becomes X-ray energy.

To prevent damage to the anode it has to be cooled, either by air cooling, using fins, or by pumping cooling liquid through it. It may even be rotated during use to spread the wear over a larger area.

Uses of X-rays

X- rays were put to practical use less than three months after Roentgen discovered them. A few of their present-day applications are as follows:

In art: detecting covered paintings;
In engineering: checking metal castings for defects; crystal analysis.
In medicine: there are two main areas
(a) diagnostic. In a simple form, this would be the detection of a broken bone or a tooth cavity. With the addition of an absorber such as barium or iodine, X-rays may be used to check respiratory or digestive disorders.
(b) theraputic. This use is almost completely restricted to the treatment of malignant cancers.

It is great importance in the use of X-rays for medical purposes that the dose given to both the patient and the operator is carefully controlled. X-rays can damage living tissue - hence their use for the destruction of tumours.

Properties of X-rays

X-rays were shown to have the following properties:
(a) they pass through many materials more or less unchanged (but see the discussion of their absorption, below);
(b) they cause fluorescence in materials such as rock salt, calcium compounds or uranium glass;
(c) they affect photographic plates, causing fogging;
(d) they cannot be refracted;
(e) they are unaffected by electric and magnetic fields;
(f) they discharge electrified bodies by ionising the surrounding air;
(g) they can cause photoelectric emission;
(h) they are produced when a beam of high-energy electrons strike a metal target (the higher the nucleon number of the target the greater the intensity of X rays produced).

© Keith Gibbs 2020