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.