X-ray tube
Overview
 
An X-ray tube is a vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

 that produces X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

s. They are used in X-ray machines. X-rays are part of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

, an ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

 with wavelengths shorter than ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 light. X-ray tubes evolved from experimental Crookes tube
Crookes tube
A Crookes tube is an early experimental electrical discharge tube, invented by English physicist William Crookes and others around 1869-1875, in which cathode rays, that is electrons, were discovered....

s with which X-rays were first discovered in the late 19th century, and the availability of this controllable source of X-rays created the field of radiography
Radiography
Radiography is the use of X-rays to view a non-uniformly composed material such as the human body. By using the physical properties of the ray an image can be developed which displays areas of different density and composition....

, the imaging of opaque objects with penetrating radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

. X-ray tubes are also used in CAT scanners, airport luggage scanners, X-ray crystallography
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...

, and for industrial inspection.

As with any vacuum tube, there is a cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...

, which emits electrons into the vacuum and an anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

 to collect the electrons, thus establishing a flow of electrical current, known as the beam, through the tube.
Encyclopedia
An X-ray tube is a vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

 that produces X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

s. They are used in X-ray machines. X-rays are part of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

, an ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

 with wavelengths shorter than ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 light. X-ray tubes evolved from experimental Crookes tube
Crookes tube
A Crookes tube is an early experimental electrical discharge tube, invented by English physicist William Crookes and others around 1869-1875, in which cathode rays, that is electrons, were discovered....

s with which X-rays were first discovered in the late 19th century, and the availability of this controllable source of X-rays created the field of radiography
Radiography
Radiography is the use of X-rays to view a non-uniformly composed material such as the human body. By using the physical properties of the ray an image can be developed which displays areas of different density and composition....

, the imaging of opaque objects with penetrating radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

. X-ray tubes are also used in CAT scanners, airport luggage scanners, X-ray crystallography
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...

, and for industrial inspection.

X-ray tube function

As with any vacuum tube, there is a cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...

, which emits electrons into the vacuum and an anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

 to collect the electrons, thus establishing a flow of electrical current, known as the beam, through the tube. A high voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

 power source, for example 30 to 150 kilovolts (kV), is connected across cathode and anode to accelerate the electrons. The X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

 spectrum depends on the anode material and the accelerating voltage.

In many applications, the current flow (typically in the range 1 mA to 1 A
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...

) is able to be pulsed on for between about 1 ms
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....

 to 1 s. This enables consistent doses of X-rays, and taking snapshots of motion. Until the late 1980s, X-ray generators were merely high-voltage, AC to DC variable power supplies. In the late 1980s a different method of control was emerging, called high speed switching. This followed the electronics technology of switching power supplies (aka switch mode power supply), and allowed for more accurate control of the X-ray unit, higher quality results, and reduced X-ray exposures.

Electrons from the cathode collide with the anode material, usually tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...

, molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...

 or copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

, and accelerate other electrons, ions and nuclei within the anode material. About 1% of the energy generated is emitted/radiated, usually perpendicular to the path of the electron beam, as X-rays. The rest of the energy is released as heat. Over time, tungsten will be deposited from the target onto the interior surface of the tube, including the glass surface. This will slowly darken the tube and was thought to degrade the quality of the X-ray beam, but research has suggested there is no effect. Eventually, the tungsten deposit may become sufficiently conductive that at high enough voltages, arcing occurs. The arc will jump from the cathode to the tungsten deposit, and then to the anode. This arcing causes an effect called "crazing
Crazing
Crazing is a network of fine cracks on the surface of a material, for example in a glaze layer.Crazing is a phenomenon that frequently precedes fracture in some glassy thermoplastic polymers. Crazing occurs in regions of high hydrostatic tension, or in regions of very localized yielding, which...

" on the interior glass of the X-ray window. As time goes on, the tube becomes unstable even at lower voltages, and must be replaced. At this point, the tube assembly (also called the "tube head") is removed from the X-ray system, and replaced with a new tube assembly. The old tube assembly is shipped to a company that reloads it with a new X-ray tube.

The X-ray photon-generating effect is generally called the Bremsstrahlung
Bremsstrahlung
Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...

 effect, a contraction of the German bremsen for braking, and strahlung for radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

.

The range of photonic energies emitted by the system can be adjusted by changing the applied voltage, and installing aluminum filters of varying thicknesses. Aluminum filters are installed in the path of the X-ray beam to remove "soft" (non-penetrating) radiation. The number of emitted X-ray photons, or dose, are adjusted by controlling the current flow and exposure time.

Simply put, the high voltage controls X-ray penetration, and thus the contrast of the image. The tube current and exposure time affect the dose and therefore the darkness of the image.

Crookes tube

Historically, X-rays were discovered radiating from experimental discharge tubes called Crookes tube
Crookes tube
A Crookes tube is an early experimental electrical discharge tube, invented by English physicist William Crookes and others around 1869-1875, in which cathode rays, that is electrons, were discovered....

s invented by British physicist William Crookes
William Crookes
Sir William Crookes, OM, FRS was a British chemist and physicist who attended the Royal College of Chemistry, London, and worked on spectroscopy...

 and others. As the medical and other uses of X-rays became apparent, workshops began to manufacture specialized Crookes tubes to produce X-rays. These were the first X-ray tubes. These first generation cold cathode
Cold cathode
A cold cathode is a cathode used within nixie tubes, gas discharge lamps, discharge tubes, and some types of vacuum tube which is not electrically heated by the circuit to which it is connected...

or Crookes X-ray tubes were used until the 1920s.

Crookes tubes generated the electrons needed to create X-rays by ionization
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...

 of the residual air in the tube, instead of a heated filament
Hot cathode
In vacuum tubes, a hot cathode is a cathode electrode which emits electrons due to thermionic emission. In the accelerator community, these are referred to as thermionic cathodes. The heating element is usually an electrical filament...

, so they were partially but not completely evacuated
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

. They consisted of a glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 bulb with around 10-6 to 5×10-8 atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

 of air (0.1 to 0.005 Pa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

). An aluminum cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...

 plate at one end of the tube created a beam of electrons, which struck a platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

 anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

 target at the center generating X-rays. The anode surface was angled so that the X-rays would radiate through the side of the tube. The cathode was concave so that the electrons were focused on a small (~1 mm) spot on the anode, approximating a point source
Point source
A point source is a localised, relatively small source of something.Point source may also refer to:*Point source , a localised source of pollution**Point source water pollution, water pollution with a localized source...

 of X-rays, which resulted in sharper images. The tube had a third electrode, an anticathode connected to the anode. It improved the X-ray output, but the method by which it achieved this is not understood. A more common arrangement used a cupped plate anticathode (similar in construction to the cathode) in line with the anode such that the anode was between the cathode and the anticathode.

To operate, a DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

 voltage of a few kilovolts to as much as 100 kV was applied between the anodes and the cathode, usually generated by an induction coil
Induction coil
An induction coil or "spark coil" is a type of disruptive discharge coil. It is a type of electrical transformer used to produce high-voltage pulses from a low-voltage direct current supply...

, or for larger tubes, an electrostatic machine. This created and then accelerated a small number of ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s from the low pressure gas in the tube. These struck further gas atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s, knocking electrons off them, creating more positive ions in a chain reaction. All the positive ions were attracted to the cathode. When they struck it, they knocked electrons out of the metal, which were accelerated along with the electrons knocked from the gas atoms toward the anode target. When these high speed electrons struck the atoms of the anode, they created X-rays by one of two processes, either Bremsstrahlung
Bremsstrahlung
Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...

or X-ray fluorescence
X-ray fluorescence
X-ray fluorescence is the emission of characteristic "secondary" X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays...

.

Crookes tubes were unreliable. As time passed, the residual air would be absorbed by the walls of the tube, reducing the pressure. This increased the voltage across the tube, generating 'harder' X-rays, until eventually the tube stopped working. To prevent this, 'softener' devices were used (see picture). A small tube attached to the side of the main tube contained a mica sleeve or chemical that released a small amount of gas when heated, restoring the correct pressure.

The glass envelope of the tube would blacken in use due to the X-rays affecting its structure.

Coolidge tube

The Crookes tube
Crookes tube
A Crookes tube is an early experimental electrical discharge tube, invented by English physicist William Crookes and others around 1869-1875, in which cathode rays, that is electrons, were discovered....

 was improved by William Coolidge
William David Coolidge
William David Coolidge was an American physicist, who made major contributions to X-ray machines. He was the director of the General Electric Research Laboratory and a vice-president of the corporation...

 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, or 10-6 Torr).

In the Coolidge tube, the electrons are produced by thermionic effect
Thermionic emission
Thermionic emission is the heat-induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the binding potential, also known as work function of the metal. The charge carriers can be electrons or ions, and...

 from a tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...

 filament heated by an electric current. The filament is the cathode of the tube. The high voltage potential is between the cathode and the anode, the electrons are thus accelerated
Acceleration
In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...

, and then hit the anode.

There are two designs: end-window tubes and side-window tubes.

End window tubes usually have "transmission target" which is thin enough to allow X-rays to pass through the target (X-rays are emitted in the same direction as the electrons are moving.)
In one common type of end-window tube, the filament is around the anode ("annular" or ring-shaped), the electrons have a curved path.(half of a toroid)

What is special about side-window tubes is:
  • An Electrostatic Lens
    Electrostatic lens
    An electrostatic lens is a device that assists in the transport of charged particles. For instance, it can guide electrons emitted from a sample to an electron analyzer, analogous to the way an optical lens assists in the transport of light in an optical instrument. The recent development of...

     to focus the beam onto a very small spot on the anode
  • The anode is specially designed to dissipate the heat and wear resulting from this intense focused barrage of electrons. Some anodes are:
    • Mechanically spun to increase the area heated by the beam.(Medical "rotating anode")
    • Cooled by circulating coolant. (indirectly on most rotating anodes)

  • 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.
  • The tube has a window designed for escape of the generated X-ray photons.


The power of a Coolidge tube usually ranges from 0.1 to 18 kW.

Rotating anode tube

Imagine thinning the tube. The anode can then be rotated by electromagnetic induction
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....

 from a series of stator
Stator
The stator is the stationary part of a rotor system, found in an electric generator, electric motor and biological rotors.Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the...

 windings outside the evacuated tube.

Because the entire anode assembly has to be contained within the evacuated tube, heat removal is a serious problem, further exacerbated by the higher power rating available. Direct cooling by conduction
Heat conduction
In heat transfer, conduction is a mode of transfer of energy within and between bodies of matter, due to a temperature gradient. Conduction means collisional and diffusive transfer of kinetic energy of particles of ponderable matter . Conduction takes place in all forms of ponderable matter, viz....

 or convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

, as in the Coolidge tube, is difficult. In most tubes, the anode is suspended on ball bearings with silver powder lubrication which provide almost negligible cooling by conduction.

A recent development has been liquid gallium
Gallium
Gallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the gallium salt in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquefies...

 lubricated fluid dynamic bearings
Fluid bearing
Fluid bearings are bearings which support the bearing's loads solely on a thin layer of liquid or gas.They can be broadly classified as fluid dynamic bearings or hydrostatic bearings. Hydrostatic bearings are externally pressurized fluid bearings, where the fluid is usually oil, water or air, and...

 which can withstand very high temperatures without contaminating the tube vacuum. The large bearing contact surface and metal lubricant provide an effective method for conduction of heat from the anode.

The anode must be constructed of high temperature materials. The focal spot temperature can reach 2500 °C (4,532 °F) during an exposure, and the anode assembly can reach 1000 °C (1,832 °F) following a series of large exposures. Typical materials are a tungsten-rhenium target on a molybdenum core, backed with graphite. The rhenium
Rhenium
Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-white, heavy, third-row transition metal in group 7 of the periodic table. With an average concentration of 1 part per billion , rhenium is one of the rarest elements in the Earth's crust. The free element has...

 makes the tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...

 more ductile and resistant to wear from the impact of the electron beams. The molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...

 conducts heat from the target. The graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

 provides thermal storage for the anode, and minimizes the rotating mass of the anode.

Increasing demand for high-performance Computed tomography
Computed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...

(CT) scanning and angiography systems has driven development of very high performance medical X-ray tubes. Contemporary CT tubes have power ratings of up to 100 kW and anode heat capacity of 6 MJ, yet retain an effective focal spot area of less than 1 mm2.

Microfocus X-ray tubes

Some X-ray examinations (such as, e.g., non-destructive testing
Nondestructive testing
Nondestructive testing or Non-destructive testing is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage....

 and 3-D microtomography
Microtomography
Microtomography , like tomography, uses x-rays to create cross-sections of a 3D-object that later can be used to recreate a virtual model without destroying the original model....

) need very high-resolution images and do therefore require X-ray tubes that can generate very small focal spot sizes, typically below 50 µm in diameter. These tubes are called microfocus X-ray tubes.

There are two basic types of microfocus X-ray tubes: solid-anode tubes and metal-jet-anode tubes.

Solid-anode microfocus X-ray tubes are in principle very similar to the Coolidge tube, but with the important distinction that care has been taken to be able to focus the electron beam into a very small spot on the anode. Many microfocus X-ray sources operate with focus spots in the range 5-20 µm, but in the extreme cases spots smaller than 1 µm may be produced.

The major drawback of solid-anode microfocus X-ray tubes is the very low power they operate at. In order to avoid melting of the anode the electron-beam power density must be below a maximum value. This value is somewhere in the range 0.4-0.8 W/µm depending on the anode material. This means that a solid-anode microfocus source with a 10 µm electron-beam focus can operate at a power in the range 4-8 W.

In metal-jet-anode microfocus X-ray tubes the solid metal anode is replaced with a jet of liquid metal, which acts as the electron-beam target. The advantage of the metal-jet anode is that the maximum electron-beam power density is significantly increased. Values in the range 3-6 W/µm have been reported for different anode materials (gallium and tin). In the case with a 10 µm electron-beam focus a metal-jet-anode microfocus X-ray source may operate at 30-60 W.

The major benefit of the increased power density level for the metal-jet X-ray tube is the possibility to operate with a smaller focal spot, say 5 µm, to increase image resolution and at the same time acquire the image faster, since the power is higher (15-30 W) than for solid-anode tubes with 10 µm focal spots.

Hazards of X-ray production from vacuum tubes

Any vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

 operating at several thousand volts or more can produce X-rays as an unwanted byproduct, raising safety issues. The higher the voltage, the more penetrating the resulting radiation and the more the hazard. Color televisions and computer CRT
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...

 displays operate at 30-40 kilovolts, making them the main concern among household appliances. Historically, concern has focused less on the cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...

, since its thick glass envelope is impregnated with several pounds of lead for shielding, than on high voltage (HV) rectifier
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...

 and voltage regulator
Voltage regulator
A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. A voltage regulator may be a simple "feed-forward" design or may include negative feedback control loops. It may use an electromechanical mechanism, or electronic components...

 tubes inside. In the late 1960s it was found that a failure in the HV supply circuit of some GE
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...

 TVs could leave excessive voltages on the regulator tube, causing it to emit X-rays. The models were recalled and the ensuing scandal caused the US agency responsible for regulating this hazard, the Center for Devices and Radiological Health
Center for Devices and Radiological Health
The Center for Devices and Radiological Health is the branch of the United States Food and Drug Administration responsible for the premarket approval of all medical devices, as well as overseeing the manufacturing, performance and safety of these devices...

 of the Food and Drug Administration
Food and Drug Administration
The Food and Drug Administration is an agency of the United States Department of Health and Human Services, one of the United States federal executive departments...

 (FDA), to require that all TVs include circuits to prevent excessive voltages in the event of failure. This hazard was eliminated with the advent of all solid state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...

 TVs, which have no tubes beside the CRT. Since 1969 the FDA has limited TV X-ray emission to 0.5 mR (milliroentgen) per hour. The tubes in the image to the right have a brown discoloration or "sunburn". This is caused by the condensation of vaporised anode material on the glass enclosure, which is effectively unwanted Sputter deposition
Sputter deposition
Sputter deposition is a physical vapor deposition method of depositing thin films by sputtering, that is ejecting, material from a "target," that is source, which then deposits onto a "substrate," such as a silicon wafer...

.

See also

  • Computed axial tomography
  • Electron beam tomography
    Electron beam tomography
    Electron beam tomography , now owned by the General Electric company , is a specific form of computed tomography in which the X-ray tube is not mechanically spun in order to rotate the source of X-ray photons...

  • Angiography
  • Coronary angiography
  • Synchrotron radiation
    Synchrotron radiation
    The electromagnetic radiation emitted when charged particles are accelerated radially is called synchrotron radiation. It is produced in synchrotrons using bending magnets, undulators and/or wigglers...

  • X-ray fluorescence
    X-ray fluorescence
    X-ray fluorescence is the emission of characteristic "secondary" X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays...

  • X-ray generator

Patents

  • Coolidge
    William David Coolidge
    William David Coolidge was an American physicist, who made major contributions to X-ray machines. He was the director of the General Electric Research Laboratory and a vice-president of the corporation...

    , , "X-ray tube"
  • Langmuir
    Irving Langmuir
    Irving Langmuir was an American chemist and physicist. His most noted publication was the famous 1919 article "The Arrangement of Electrons in Atoms and Molecules" in which, building on Gilbert N. Lewis's cubical atom theory and Walther Kossel's chemical bonding theory, he outlined his...

    , , "Method of and apparatus for controlling X-ray tubes
  • Coolidge, , "X-ray tube"
  • Coolidge, , "X-ray tube"

External links

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