X-rays tubes offer control over the type of x-ray produced.

Early x-ray tubes relied on the gas becoming ionised to produce electrons to hit the target.

Coolidge used heat to release electrons from the cathode.

Coolidge biography

Coolidge used a cooling system to remove unwanted heat from the target.

Short slide-show o x-ray tubes used for diagnosis

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Filament current determines x-ray intensity.

Anode voltage determines penetrating power of x-rays.

Any high-speed electron that strikes a metal target might release an x-ray. However x-ray tubes were developed
1. to increase the efficiency of production
2. to provide control over the type of x-rays that emerge.

Early X-ray tubes were gas discharge tubes made essentially for producing cathode rays. They consisted of a partially evacuated glass bulb containing two electrodes. This was commonly known as a Crooke’s tube. By connecting a voltage across the electrodes, the gas became ionised. Positive ions were then attracted to the cathode and upon striking it, imparted their energy to it, causing electrons to escape. These electrons, in the form of a beam of cathode rays, bombard the glass walls of the tube and produce X-rays. Such tubes produced only soft X-rays of low energy.

An early improvement in the x-ray tube was the introduction of a curved cathode to focus the beam of electrons on a metal target (of high atomic number), called the anode. This type generates harder rays of shorter wavelengths and of greater energy than those produced by the original Crookes tube.

The next great improvement was made in 1913 by the American physicist William David Coolidge. 

He designed an x-ray tube, which incorporated a number of improvements.

1. It contained a heated filament to release electrons from the cathode (by a process called thermionic emission).
2. It contained a cooling system to remove unwanted heat from the target.
3. It was more highly evacuated.
The Coolidge tube greatly increased the efficiency of x-ray production. It also offered independent control of the intensity and penetrating power of x-rays. Most of the x-ray tubes in present-day usage are modified Coolidge tubes.

How x-ray tubes control the quality of x-rays released
1. If the filament current is increased, more heat is released, therefore more electrons are released and so more electrons travel to the target. Consequently, more x-rays are released, and so the intensity of the x-ray beam is increased.
2. If the voltage between cathode and anode is increased, the electrons are accelerated to higher speeds and so have greater energy when they hit the target. Consequently when their energy is converted to a photon of radiation, the photon has greater energy (E = hf). A higher energy photon corresponds to a higher frequency of radiation and so to a more penetrating x-ray.
The intensity of the x-rays refers to the energy passing through unit area per second.
The ability of x-rays to penetrate matter is related to their frequency.