Editing X-ray (section)

==Production==
Whenever charged particles (electrons or ions) of sufficient energy hit a material, X-rays are produced.

===Production by electrons===
{|align=right class="wikitable"
|+ Characteristic X-ray emission lines for some common anode materials.<ref>{{Cite web |url=http://physics.nist.gov/PhysRefData/XrayTrans/Html/search.html |title=X-ray Transition Energies Database |publisher=NIST Physical Measurement Laboratory |date= 9 December 2011 |access-date=2016-02-19}}</ref><ref>{{Cite web |url=http://xdb.lbl.gov/Section1/Table_1-3.pdf |title=X-Ray Data Booklet Table 1-3 |publisher=Center for X-ray Optics and Advanced Light Source, Lawrence Berkeley National Laboratory |date= 1 October 2009 |access-date=2016-02-19 |archive-url=https://web.archive.org/web/20090423224919/http://xdb.lbl.gov/Section1/Table_1-3.pdf | archive-date=23 April 2009}}</ref>
! rowspan= 2 |Anode<br />material !! rowspan= 2 |Atomic<br />number !! colspan=2 |Photon energy [keV] !! colspan=2 |Wavelength [nm]
|-
! [[K-alpha|K<sub>α1</sub>]] !! K<sub>β1</sub> !! K<sub>α1</sub> !! K<sub>β1</sub>
|-
! [[tungsten|W]]
 |74 ||59.3 ||67.2 ||0.0209 ||0.0184
|-
! [[molybdenum|Mo]]
 |42 ||17.5 ||19.6 ||0.0709 ||0.0632
|-
! [[copper|Cu]]
 |29 ||8.05 ||8.91 ||0.154 ||0.139
|-
! [[silver|Ag]]
 |47 ||22.2 ||24.9 ||0.0559 ||0.0497
|-
! [[gallium|Ga]]
 |31 ||9.25 ||10.26 ||0.134 ||0.121
|-
! [[indium|In]]
 |49 ||24.2 ||27.3 ||0.0512 ||0.0455
|-
! [[aluminium|Al]]
 |13 ||1.4867 ||1.5574 ||0.8340 ||0.7961
|}

[[File:TubeSpectrum-en.svg|thumb|Spectrum of the X-rays emitted by an X-ray tube with a [[rhodium]] target, operated at 60&nbsp;[[Kilovolt|kV]]. The smooth, continuous curve is due to ''[[bremsstrahlung]]'', and the spikes are [[energy-dispersive X-ray spectroscopy|characteristic K lines]] for rhodium atoms.]]

X-rays can be generated by an [[X-ray tube]], a [[vacuum tube]] that uses a high voltage to accelerate the [[electron]]s released by a [[hot cathode]] to a high velocity. The high velocity electrons collide with a metal target, the [[anode]], creating the X-rays.<ref>{{Cite book |vauthors = Whaites E, Cawson R |title= Essentials of Dental Radiography and Radiology |publisher= Elsevier Health Sciences |date= 2002 |pages= 15–20 |url= https://books.google.com/books?id=x6ThiifBPcsC&q=radiography+kilovolt+x-ray+machine |isbn= 978-0-443-07027-3}}</ref> In medical X-ray tubes the target is usually [[tungsten]] or a more crack-resistant alloy of [[rhenium]] (5%) and tungsten (95%), but sometimes [[molybdenum]] for more specialized applications, such as when softer X-rays are needed as in mammography. In crystallography, a copper target is most common, with [[cobalt]] often being used when fluorescence from iron content in the sample might otherwise present a problem. When even lower energies are needed, as in [[X-ray photoelectron spectroscopy]], the K<sub>α</sub> X-rays from an aluminium or magnesium target are often used.{{cn|date=December 2024}}

The maximum energy of the produced X-ray [[photon]] is limited by the energy of the incident electron, which is equal to the voltage on the tube times the electron charge, so an 80&nbsp;kV tube cannot create X-rays with an energy greater than 80&nbsp;keV. When the electrons hit the target, X-rays are created by two different atomic processes:{{cn|date=December 2024}}
# ''[[Characteristic X-ray]] emission'' (X-ray electroluminescence): If the electron has enough energy, it can knock an orbital electron out of the inner [[electron shell]] of the target atom. After that, electrons from higher energy levels fill the vacancies, and X-ray photons are emitted. This process produces an [[emission spectrum]] of X-rays at a few discrete frequencies, sometimes referred to as spectral lines. Usually, these are transitions from the upper shells to the K shell (called K lines), to the L shell (called L lines) and so on. If the transition is from 2p to 1s, it is called Kα, while if it is from 3p to 1s it is Kβ. The frequencies of these lines depend on the material of the target and are therefore called characteristic lines. The Kα line usually has greater intensity than the Kβ one and is more desirable in diffraction experiments. Thus the Kβ line is filtered out by a filter. The filter is usually made of a metal having one proton less than the anode material (e.g. Ni filter for Cu anode or Nb filter for Mo anode).
# ''[[Bremsstrahlung]]'': This is radiation given off by the electrons as they are scattered by the strong electric field near the nuclei. These X-rays have a [[continuous spectrum]]. The frequency of ''Bremsstrahlung'' is limited by the energy of incident electrons.

So, the resulting output of a tube consists of a continuous ''Bremsstrahlung'' spectrum falling off to zero at the tube voltage, plus several spikes at the characteristic lines. The voltages used in diagnostic X-ray tubes range from roughly 20&nbsp;kV to 150&nbsp;kV and thus the highest energies of the X-ray photons range from roughly 20&nbsp;keV to 150&nbsp;keV.<ref>{{Cite book |vauthors = Bushburg J, Seibert A, Leidholdt E, Boone J |title= The Essential Physics of Medical Imaging |publisher= Lippincott Williams & Wilkins |date= 2002 |location= US |page= 116 |url= https://books.google.com/books?id=VZvqqaQ5DvoC&q=radiography+kerma+rem+Sievert&pg=PT33 |isbn= 978-0-683-30118-2}}</ref>

Both of these X-ray production processes are inefficient, with only about one percent of the electrical energy used by the tube converted into X-rays, and thus most of the [[electric power]] consumed by the tube is released as waste heat. When producing a usable flux of X-rays, the X-ray tube must be designed to dissipate the excess heat.

A specialized source of X-rays which is becoming widely used in research is [[synchrotron radiation]], which is generated by [[particle accelerator]]s. Its unique features are X-ray outputs many orders of magnitude greater than those of X-ray tubes, wide X-ray spectra, excellent [[collimation]], and [[linear polarization]].<ref>{{Cite conference | vauthors = Emilio B, Ballerna A |title= Preface |book-title= Biomedical Applications of Synchrotron Radiation: Proceedings of the 128th Course at the International School of Physics -Enrico Fermi- 12–22 July 1994, Varenna, Italy |page= xv |publisher= IOS Press |date= 1994 |url= https://books.google.com/books?id=VEld4080nekC&pg=PA129 |isbn= 90-5199-248-3 }}</ref>

Short nanosecond bursts of X-rays peaking at 15&nbsp;keV in energy may be reliably produced by peeling pressure-sensitive adhesive tape from its backing in a moderate vacuum. This is likely to be the result of recombination of electrical charges produced by [[triboelectric effect|triboelectric charging]]. The intensity of X-ray [[triboluminescence]] is sufficient for it to be used as a source for X-ray imaging.<ref name="Camara2008">{{cite journal |last1=Camara |first1=Carlos G. |last2=Escobar |first2=Juan V. |last3=Hird |first3=Jonathan R. |last4=Putterman |first4=Seth J. |title=Correlation between nanosecond X-ray flashes and stick–slip friction in peeling tape |journal=Nature |date=October 2008 |volume=455 |issue=7216 |pages=1089–1092 |doi=10.1038/nature07378 |bibcode=2008Natur.455.1089C }}</ref>

===Production by fast positive ions===
X-rays can also be produced by fast protons or other positive ions. The proton-induced X-ray emission or [[particle-induced X-ray emission]] is widely used as an analytical procedure. For high energies, the production [[cross section (physics)|cross section]] is proportional to ''Z''<sub>1</sub><sup>2</sup>''Z''<sub>2</sub><sup>−4</sup>, where ''Z''<sub>1</sub> refers to the [[atomic number]] of the ion, ''Z''<sub>2</sub> refers to that of the target atom.<ref>{{Cite journal|doi = 10.1016/0370-1573(86)90149-3|title = Review of experimental cross sections for K-shell ionization by light ions|journal = Physics Reports|volume = 135|issue = 2|pages = 47–97|date= 1986| vauthors = Paul H, Muhr J |bibcode = 1986PhR...135...47P}}</ref> An overview of these cross sections is given in the same reference.

===Production in lightning and laboratory discharges===
X-rays are also produced in lightning accompanying [[terrestrial gamma-ray flash]]es. The underlying mechanism is the acceleration of electrons in lightning related electric fields and the subsequent production of photons through ''Bremsstrahlung''.<ref>{{cite journal |last1=Köhn |first1=Christoph |last2=Ebert |first2=Ute |title=Angular distribution of Bremsstrahlung photons and of positrons for calculations of terrestrial gamma-ray flashes and positron beams |journal=Atmospheric Research |date=January 2014 |volume=135-136 |pages=432–465 |doi=10.1016/j.atmosres.2013.03.012 |arxiv=1202.4879 |bibcode=2014AtmRe.135..432K }}</ref> This produces photons with energies of some few [[electronvolt|keV]] and several tens of MeV.<ref>{{cite journal |last1=Köhn |first1=Christoph |last2=Ebert |first2=Ute |title=Calculation of beams of positrons, neutrons, and protons associated with terrestrial gamma ray flashes |journal=Journal of Geophysical Research: Atmospheres |date=27 February 2015 |volume=120 |issue=4 |pages=1620–1635 |doi=10.1002/2014JD022229 |bibcode=2015JGRD..120.1620K |url=https://ir.cwi.nl/pub/23845 }}</ref> In laboratory discharges with a gap size of approximately 1&nbsp;meter length and a peak voltage of 1&nbsp;MV, X-rays with a characteristic energy of 160&nbsp;keV are observed.<ref>{{cite journal |last1=Kochkin |first1=Pavlo |last2=Köhn |first2=Christoph |last3=Ebert |first3=Ute |last4=van Deursen |first4=Lex |title=Analyzing x-ray emissions from meter-scale negative discharges in ambient air |journal=Plasma Sources Science and Technology |date=August 2016 |volume=25 |issue=4 |pages=044002 |doi=10.1088/0963-0252/25/4/044002 |bibcode=2016PSST...25d4002K |url=https://pure.tue.nl/ws/files/45766610/Kochkin_2016_Plasma_Sources_Sci._Technol._25_044002.pdf }}</ref> A possible explanation is the encounter of two [[streamer discharge|streamers]] and the production of high-energy [[Runaway electrons|run-away electrons]];<ref>{{Cite journal |vauthors = Cooray V, Arevalo L, Rahman M, Dwyer J, Rassoul H |doi = 10.1016/j.jastp.2009.07.010 |title = On the possible origin of X-rays in long laboratory sparks |journal = Journal of Atmospheric and Solar-Terrestrial Physics |volume = 71 |issue = 17–18 |pages = 1890–1898 |date= 2009 |bibcode = 2009JASTP..71.1890C}}</ref> however, microscopic simulations have shown that the duration of electric field enhancement between two streamers is too short to produce a significant number of run-away electrons.<ref>{{cite journal |vauthors = Köhn C, Chanrion O, Neubert T |title = Electron acceleration during streamer collisions in air |journal = Geophysical Research Letters |volume = 44 |issue = 5 |pages = 2604–2613 |date = March 2017 |pmid = 28503005 |pmc = 5405581 |doi = 10.1002/2016GL072216 |bibcode = 2017GeoRL..44.2604K }}</ref> Recently, it has been proposed that air perturbations in the vicinity of streamers can facilitate the production of run-away electrons and hence of X-rays from discharges.<ref>{{Cite journal |vauthors = Köhn C, Chanrion O, Babich LP, Neubert T |doi = 10.1088/1361-6595/aaa5d8 |title = Streamer properties and associated x-rays in perturbed air |journal = Plasma Sources Science and Technology |volume = 27 |pages = 015017 |date= 2018 |issue = 1 |bibcode = 2018PSST...27a5017K |doi-access = free}}</ref><ref>{{cite journal |vauthors = Köhn C, Chanrion O, Neubert T |title = High-Energy Emissions Induced by Air Density Fluctuations of Discharges |journal = Geophysical Research Letters |volume = 45 |issue = 10 |pages = 5194–5203 |date = May 2018 |pmid = 30034044 |pmc = 6049893 |doi = 10.1029/2018GL077788 |bibcode = 2018GeoRL..45.5194K }}</ref>