User:Ulflund/X-ray tube

Todo:
 * Add sources
 * Add section with details on the different components including subsections on
 * cathodes, including filaments and crystals
 * anodes, including common materials, cooling methods, take-off angles
 * x-ray windows, common materials, filtering
 * vacuum vessel, common materials
 * electron optics (electrostatic vs. magnetic)
 * Compare to other types of x-ray sources (synchrotrons, laser-based, XFELs, radionuclides)
 * Line focus and heel effect
 * High voltage on cathode, anode or both

An X-ray tube is a vacuum tube that converts electrical input power into X-rays. In the vacuum of the X-ray tube, electrons are accelerated by a high-voltage electric field. X-rays are produced when these electrons quickly decelerate as they strike a target material. The X-ray tube is the core of an X-ray generator which typically also contains a high-voltage power supply, cooling system and other components required to generate X-rays in a safe way.

The X-ray tube is the most common source of X-rays for a wide range of applications including medical imaging, non-destructive testing , airport luggage scanners, X-ray crystallography , X-ray fluorescence , and many more. They vary in size and power from the few watt X-ray tubes that are integrated in handheld X-ray fluorescence tools to the 100+ kW medical rotating-anode X-ray tubes developed for high-performance computed tomography and angiography.

Physics of X-ray production


The X-ray tube contains a cathode, which emits electrons into the vacuum and an anode or target to collect the electrons, thus establishing a flow of electrical current, known as the beam. A high voltage power supply, for example 30 to 150 kilovolts (kV), is connected across cathode and anode to create a strong electric field that accelerate the electrons. When these high-energy electrons hit the anode X-rays are generated by two separate processes described below.

Bremsstrahlung


When an electron interact with the electric field of an atomic nucleus it is deflected, loose some of its kinetic energy and emit an X-ray photon. The energy of the photon is equal to that lost by the electron and can be any value between zero and the initial energy of the electron. This process is responsible for the continuous part of the X-ray spectrum with photons of all energies up to the tube voltage times the electron charge. For a tube voltage of 120 kV as in the figure to the right, the maximum photon energy is thus 120 keV.

The efficiency of bremsstrahlung generation is proportional to the atomic number of the target material. Applications that mostly use bremsstrahlung, such as medical X-ray imaging, therefore use high-atomic-number targets, most commonly tungsten.

The upper limit on X-ray energy is set through the tube voltage. The lower limit can be adjusted by adding filters, often out of aluminium, to the X-ray path. The filters will absorb the less penetrating low energy X-rays thus increasing the average X-ray energy. The number of emitted X-ray photons, and thus the dose, are adjusted by controlling the current and exposure time.

Characteristic radiation
The incoming electrons can also interact with the electrons in the atoms of the target, knocking them out of their orbits. When an inner shell electron is knocked out an outer shell electron will quickly take its place. An X-ray photon with an energy equal to the difference in energy between these two shells can then be created. This process creates X-rays of a few different discrete energies giving the sharp peaks in the X-ray spectrum to the right. This is called characteristic radiation since the photon energies uniquely depend on the elements in the target. Higher atomic number elements can create higher energy photons assuming the beam energy is higher than the inner shell electron binding energy.

Applications requiring monochromatic radiation, e.g., X-ray crystallography and SAXS, use X-ray tubes where the target material is selected to produce characteristic radiation with a photon energy suitable for that application. E.g., the 8 keV Kα radiation from copper is often used in macromolecular crystallography.

Heat Released
Less than 1% of the electron beam power is converted into X-rays in normal X-ray tubes, with the rest being converted to heat. How quickly this heat can be removed without damaging the target is what limits the performance of most X-ray tube designs. There are many designs that mitigate this problem in different ways. Rotating-anode and liquid-metal-jet tubes use a moving target to remove the heat. Other designs can have water cooling very close to the interaction point or simply run at a very low power. Medical rotating anode tubes usually rely on a graphite block that can absorb large amounts of heat and then slowly radiate that away when the tube is not producing X-rays.

Types of X-ray tubes
X-ray tubes can differ in many ways but they are often classified based on the geometry of their anode, or e-beam target, since this has a large impact on the X-ray tube performance. The anode can be either of reflection or transmission type depending on the direction of outgoing X-rays. The performance can be improved at the cost of complexity by having a moving anode, typically implemented either as a rotating anode or a liquid metal jet. Below are descriptions of these four major types of X-ray tubes as well as a comparison between closed and open X-ray tubes.

Reflection-target X-ray tubes


The first X-ray tubes made were all reflection-target side-window X-ray tubes. In this design electrons hit the target surface at an angle and X-rays are taken out at the side of the tube. Due to its simplicity this is still a very common tube design. The design allows for efficient heat dissipation from the focal spot through heat conduction into the anode block. While the first tubes were air cooled or radiatively cooled, modern side-window X-ray tubes are often water or oil-cooled.

Microfocus X-ray tubes
Some X-ray examinations (such as, e.g., non-destructive testing and 3-D microtomography) need very high-resolution images and therefore require X-ray tubes that can generate very small focal spot sizes. X-ray tubes with a source spot size in the range from a few to several tens of μm in diameter are generally refered to as microfocus X-ray tubes. They 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.

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 must be lower for smaller X-ray spot sizes. The maximum power is roughly proportional to the electron-beam diameter with the proportionality constant 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.

While for high-resolution imaging as mentioned above, a small X-ray spot is necessary to resolve small features, many analytical X-ray applications such as crystallography, powder diffraction, small-angle X-ray scattering, and fluorescence imaging use X-ray optics to focus the X-ray beam. The optics can then often be designed to handle the x-ray spot size available, and the important X-ray source parameter becomes the X-ray brightness. The X-ray brightness is the number of X-ray photons emitted from the source per source spot area and solid angle, corresponding to radiance in radiometry. In the same way as radiance cannot be increased in an optical system, neither can the brightness with X-ray optics. The optics can magnify or demagnify the focus but it cannot increase the brightness of the X-ray beam. The brightness of an X-ray source is mainly determined by the e-beam power divided by the focal spot area. Since the power one can put in the focal spot before melting the target is proportional to the diameter of the focal spot a smaller spot allows for a higher brightness. Because of the higher brightness and low power consumption of microfocus X-ray tubes compared to conventional stationary-anode X-ray tubes they are now widely used for analytical applications.

Transmission-target X-ray tubes
Transmission target X-ray tubes use a thin target that also acts as the vacuum barrier of the X-ray tube. X-rays are then taken out in the forward direction of the electron beam. This approach allows for closer access to the X-ray source spot and allows for smaller X-ray spot size than can be achieved with the side-window X-ray tubes.

In a side-window X-ray tube there needs to be space between the target and the window for electrons to be able to reach the target. This makes it difficult to design the X-ray tube to allow very close access to the X-ray source. For applications were samples need to be within a few mm from the X-ray spot, such as very high resolution X-ray tomography, transmission-target X-ray tubes are the best option.

To make a smaller X-ray spot, the first step is to focus the electron beam into a tighter focus on the target. This works well X-ray spots larger than about 5-50 μm depending on acceleration voltage and target density. For smaller spots than this, however, diffusion of electrons in the target material puts a lower limit on the spot size. When the electrons impinge on the target of an X-ray tube they randomly interact with the atoms in the target multiple times changing speed and direction. Due to these random interactions the electrons spread out generating x-rays over multiple-μm-sized volume even if the electrons are all focused to the exact same point. This limitation can be overcome in the transmission-target X-ray tube by making the target material very thin. Electrons will then leave this material after a small number of interactions, preventing spread of the electrons from impacting the x-ray spot size too much.

The material generating X-rays in transmission target X-ray tubes is often a 1 to 10 μm thick layer of tungsten. The tungsten layer is deposited onto a substrate to make the target strong enough to hold vacuum and thick enough to efficiently conduct heat away from the focal point. The substrate needs to have a low atomic number to transmit the X-rays and avoid generating more X-rays but also needs to be strong and have a high thermal conductivity. For these reasons beryllium or diamond are typically used for the substrate.

The downsides with the transmission targets compared to reflection targets is that they cannot be cooled as efficiently and for the same e-beam current they produce less X-rays since some of the electrons penetrate into the substrate of the target.

Transmission target X-ray tubes are also called end-window X-ray tubes. This, however, also includes a reflection-target design where the filament is around the anode ("annular" or ring-shaped), the electrons have a curved path (half of a toroid).

Rotating-anode X-ray tubes


To cool the focal spot more efficiently than what can be done with heat conduction alone, one can move the target to expose new cool material to the electron beam. The most common way of achieving this is to rotate the anode often at speeds up to 10000 rpm. This continuously moves new cool target material into the focal spot and moves the heated material out. Each point on the target then has a full revolution to conduct heat away to the rest of the anode.

A considerable amount of heat is generated in the focal spot (the area where the beam of electrons coming from the cathode strike to) of a stationary anode. Rather, a rotating anode lets the electron beam sweep a larger area of the anode, thus redeeming the advantage of a higher intensity of emitted radiation, along with reduced damage to anode compared to its stationary state.

The focal spot temperature can reach 2,500 C during an exposure, and the anode assembly can reach 1,000 C following a series of large exposures. Typical anodes are a tungsten-rhenium target on a molybdenum core, backed with graphite. The rhenium makes the tungsten more ductile and resistant to wear from the impact of the electron beams. The molybdenum conducts heat from the target. The graphite provides thermal storage for the anode, and minimizes the rotating mass of the anode.

Heat capacity

Metal-jet-anode X-ray tubes
In metal-jet-anode X-ray tubes the solid metal anode is replaced with a jet of liquid metal. The high speed of the jet, on the order of 100 m/s, efficiently transports heat away from the interaction point, which together with the regenerative nature of the jet allows for very high e-beam power densities.

The advantage of the metal-jet anode is that the maximum electron-beam power density is significantly increased compared to other X-ray tubes. Power up to 1 kW can be applied with a 30 um X-ray spot size. Metal-jet-anode available as of 2017 are limited to spot sizes in the range from 5 to 40 um and target materials consisting of gallium, indium and tin alloys.

The metal-jet-anode X-ray tube is the newest type of X-ray tube mentioned here with the first patent appearing in 2002 and the first demonstration of a working X-ray source in 2003.

Sealed and open X-ray tubes
X-ray tubes can also be classified based on their approach for maintaining a vacuum. Sealed or closed tubes have a vacuum envelope that is evacuated during production of the tube and then sealed off. This has the advantage of not requiring an active pumping system but the disadvantage that the entire tube needs to be replaced if some part of it breaks or wears out. Open tubes on the other hand typically use a turbomolecular pump to achieve the high vacuum required. This allows service to be performed on the X-ray tube but increases its complexity.

Hazards of X-ray production from vacuum tubes
Any vacuum tube 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. CRT displays, once common in color televisions and computer displays, operate at 3-40 kilovolts, making them the main concern among household appliances. Historically, concern has focused less on the cathode ray tube, since its thick glass envelope is impregnated with several pounds of lead for shielding, than on high voltage (HV) rectifier and voltage regulator tubes inside. In the late 1960s it was found that a failure in the HV supply circuit of some General Electric 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 of the Food and Drug Administration (FDA), to require that all TVs include circuits to prevent excessive voltages in the event of failure. The hazard associated with excessive voltages was eliminated with the advent of all solid state 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 flat screens used today do not have any vacuum tubes capable of emitting X-rays.

Crookes tube (cold cathode tube)


X-ray tubes evolved from experimental Crookes tubes with which X-rays were first discovered on November 8, 1895, by the German physicist Wilhelm Conrad Röntgen. Crookes tubes generated the electrons needed to create X-rays by ionization of the residual air in the tube, so they were partially but not completely evacuated. They consisted of a glass bulb with around 10−6 to 5×10−8 atmospheric pressure of air (0.1 to 0.005 Pa). An aluminum cathode plate at one end of the tube, a platinum anode target. 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 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 copper 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 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, or for larger tubes, an electrostatic machine.

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.

These first generation cold cathode or Crookes X-ray tubes were used until the 1920s.

Coolidge tube (hot cathode 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, or 10−6 Torr).

In the Coolidge tube, the electrons are produced by thermionic effect from a tungsten 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, and then hit the anode.

Other X-ray sources
The vast majority of manmade X-ray sources are X-ray tubes but there are also other ways of producing X-rays with some advantages. Synchrotron light sources are often used to produce the very intense X-ray beams required in some research fields. The high brightness of these X-ray sources comes with the downside of them being very big, often hundreds of meters in circumference, and expensive to build. These, as well as X-ray free-electron lasers, are large scale facilities where one can go to perform experiments, in comparison to X-ray tubes that are pieces of equipment used in many labs and hospitals.

Laser-based sources

Radionuclides

List of X-ray tube manufacturers
GE Philips (https://www.usa.philips.com/healthcare/solutions/radiography) Siemens Excillum (https://excillum.com) Brucker Rigaku Yxlon X-ray works Hammamatsu Angell (https://en.szangell.com/) Balteau NDT (https://www.balteau-ndt.com/) Xenocs (https://www.xenocs.com/) Gulmay (https://www.gulmay.com/x-ray-tubes/) Lohmann (https://www.lohmannx-ray.com/) Lucem (https://lucem.co.kr/ENG/main/index.php) Zeiss (https://www.zeiss.com/metrology/products/systems/computed-tomography.html) Varex (https://www.vareximaging.com/products/security-industrial/industrial-x-ray-tubes) Malvern Panalytical (https://www.malvernpanalytical.com/en/products/category/x-ray-tubes) Canon (https://etd.canon/en/product/category/xray/index.html) MXR (https://microxray.com/) Superior (https://superiorxraytube.com/) DUNLEE (https://www.dunlee.com/) Oxford (https://xray.oxinst.com/x-ray-tube-products/) Petrick (https://www.petrickgmbh.de/index_en.html) rtw (https://www.rtwxray.de/index.php?id=home) IAE (https://www.iae.it/manufacturing/) MOXTEK

Patents

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