X-ray tube
From Freepedia
An X-Ray tube is a vacuum tube designed to produce man made X-Ray photons on demand. X-ray tubes are part of X-ray machines. By convention, photons of similar, and sometimes higher energy, but emanating from natural sources are called gamma rays.
X-Rays photons are part of the electromagnetic spectrum, historically named as extending from radio waves to microwaves, infrared, visible light, ultraviolet and gamma rays.
The development of X-Ray tubes was an important historical event which enabled the field of radiology, for both imaging and treatment applications. Over decades, the development and usefulness of internal imaging using X-Rays spread into development and use of other physical modalities, e.g. ultrasound, radioactive isotope decay (nuclear medicine), nuclear magnetic resonance (MRI), etc.
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X-Ray tube function
As with any vacuum tube, there is a cathode to emit electrons into the vacuum and an anode to collect the electrons, thus establishing a flow of electrical current through the tube. A high voltage electrical current power source is connected across cathode and anode, e.g. 20-100 kV (Kilovolts), such that the voltage can be quickly turned on & off for precise amounts of time, e.g. 0.001 second, and the current flow, often in the 0.1 to 10.0 mA range, once started, can be controlled.
Electrons focused on the anode collide with and accelerate other electrons, ions and nuclei within the anode material and about 1% of the energy generated is emitted/radiated, perpendicular to the path of the electron current, as X-Ray photons.
This X-Ray photon-generating effect is generally called the Bremsstrahlung effect, a contraction of the German brems for braking, and strahlung for radiation.
The range of photonic energies emitted by the system can be adjusted by changing the applied voltage. The number of emitted X-Ray photons, dose, adjusted by controlling the current flow.
Crookes tube
Historically, the first X-ray was invented by sir William Crookes. It was used to make a visible fluorescence on minerals. The Crookes tube is also called discharge tube or cold cathode tube.
It is a glass bulb with a small pressure of air, about 100 Pa (i.e. about 1 torr). It contains an aluminum cathode with a curved shape to concentrate the electron flow on the anode, or "target".
A high tension is made between the electrodes; this induces an ionization of the residual air, and thus an electron flow or "discharge" from the cathode to the anode. When these electrons hit the target, they are slowed down, producing the X-rays (Bremsstrahlung and X-ray fluorescence of the target).
This tube can not produce X-rays continuously. It is no longer used on modern devices.
See also Crookes tube.
Coolidge tube
The Crookes tube was improved by William Coolidge in 1913. The Coolidge tube, also called hot cathode tube, is the most widely used. It works with a very good quality vacuum (about 10-4 Pa, env. 10-6 torr).
In the Coolidge tube, the electrons are produced by a tungsten filament heated by an electric current (thermionic effect, also used for the television devices). The filament is the cathode of the tube. The high voltage is made between the cathode and the anode, the electrons are thus accelerated, and then hit the anode.
There are two designs: end-window tubes and side-window tubes.
In the end-window tubes, the filament is around the anode, the electrons have a curved path.
What is special about side-windows tubes is that the electron flow within the vacuum is
- focused onto a very small spot on the anode, using a Wehnelt: this device has a special shape and an electric potential lower than the filament to repel electrons in its middle;
- the anode is specially designed (often mechanically spun, occasionally external circulating oil radiator cooled, so as to dissipate the heat and wear resulting from this intense focused barrage of electrons,
- the anode is precisely angled at 1-20 degrees off perpendicular to the electron current so as to allow escape of some of the X-ray photons which are emitted essentially perpendicular to the direction of the electron current,
- the anode is usually made out of tungsten or molybdenum and
- the vacuum tube has a window designed for escape of the generated X-ray photons.
The power of a Coolidge tube usually ranges from 1 to 4 kW.
Rotating anode tube
The rotating anode tube is an improvement of the Coolidge tube. The heat produced by the tube (99% of the energy) is one of the main limitations on the power which can be applied. To reduce the localized temperature rise, the anode is larger and rotates so that the radiated portion continuously changes, aiding in the absorption of heat without the anode melting.
It is thus possible to build more powerful X-ray generators, e.g. 9 or 15 kW generators.
Related topics
Patents
- Coolidge, U.S. Patent 1211092, "X-ray tube"
- Langmuir, U.S. Patent 1251388, "Method of and apparatus for controlling x-ray tubes
- Coolidge, U.S. Patent 1917099, "X-ray tube"
- Coolidge, U.S. Patent 1946312, "X-ray tube"
