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{{Short description|Form of electromagnetic radiation}}
{{About|the type of radiation|the medical specialty|Radiology|other uses}}
[[File:Color X-ray photogram.jpg|thumb|Natural color X-ray [[photogram]] of a wine scene. Note the edges of hollow cylinders as compared to the solid candle.]]
[[File:Taking the X out of X-Rays (Dr. William Coolidge, 1940).webm|thumb|[[William D. Coolidge|William Coolidge]]  explains medical imaging and X-rays.]]

An '''X-ray''' (also known in many languages as '''Röntgen radiation''') is a form of high-energy [[electromagnetic radiation]] with a [[wavelength]] shorter than those of [[ultraviolet]] rays and longer than those of [[gamma ray]]s. Roughly, X-rays have a [[wavelength]] ranging from 10&nbsp;[[Nanometre|nanometers]] to 10&nbsp;[[Picometre|picometers]], corresponding to [[frequency|frequencies]] in the range of 30&nbsp;[[Hertz|petahertz]] to 30&nbsp;[[Hertz|exahertz]] ({{val|3|e=16|u=Hz}} to {{val|3|e=19|u=Hz}}) and photon energies in the range of 100&nbsp;[[electronvolt|eV]] to 100&nbsp;[[keV]], respectively.<ref>{{Cite web |title=Figure 7.1, Wavelengths and frequencies of the different groups of electromagnetic radiation. X-rays lie in the range of 0.01 nm up to 10 nm - Medical Imaging Systems - NCBI Bookshelf |author= |work=ncbi.nlm.nih.gov |date= 3 August 2018|access-date=8 October 2024 |url= https://www.ncbi.nlm.nih.gov/books/NBK546155/figure/ch7.fig1/}}</ref>

X-rays were discovered in [[1895 in science|1895]] by the German scientist [[Wilhelm Röntgen|Wilhelm Conrad Röntgen]],<ref>{{Cite web |title=X-Rays |url=https://science.nasa.gov/ems/11_xrays |publisher=[[NASA]] |website = Science Mission Directorate|date=10 August 2016 }}</ref> who named it ''X-radiation'' to signify an unknown type of radiation.<ref name="squires">Novelline, Robert (1997). ''Squire's Fundamentals of Radiology''. Harvard University Press. 5th edition. {{ISBN|0-674-83339-2}}.</ref>

X-rays can penetrate many solid substances such as construction materials and living tissue,<ref>{{Cite book |url=https://books.google.com/books?id=pwbkBUi-918C&pg=PA205 |page=205 |title=The Manual of Dental Assisting |author=Dental Assistants' Association of Australia |publisher=Elsevier Australia |year=2005|isbn=978-0-7295-3737-7 }}</ref> so X-ray [[radiography]] is widely used in [[medical diagnostics]]  (e.g., checking for [[Bone fracture|broken bones]]) and [[material science]] (e.g., identification of some [[chemical elements]] and detecting weak points in construction materials).<ref>{{Cite book |last1=Caldwell |first1=Wallace E. |title=History of the World |last2=Merrill |first2=Edward H. |publisher=The Greystone Press |year=1964 |volume=1 |location=United States |pages=394}}</ref>  However X-rays are [[ionizing radiation]] and exposure can be hazardous to health, causing [[DNA]] damage, cancer and, at higher intensities, burns and [[radiation sickness]].  Their generation and use is strictly controlled by public health authorities.

==History==

=== Pre-Röntgen observations and research ===
[[File:Crookes' type discharge tubes Wellcome M0015832EA.jpg|thumb|upright|Example of a [[Crookes tube]], a type of [[discharge tube]] that emitted X-rays]]

X-rays were originally noticed in science as a type of unidentified [[radiation]] emanating from [[discharge tube]]s by experimenters investigating [[cathode ray]]s produced by such tubes, which are energetic [[electron]] beams that were first observed in 1869. Early researchers noticed effects that were attributable to them in many of the early [[Crookes tube]]s (invented around [[1875 in science|1875]]). Crookes tubes created free electrons by [[ionization]] of the residual air in the tube by a high [[Direct current|DC]] [[voltage]] of anywhere between a few [[kilovolt]]s and 100&nbsp;kV. This voltage accelerated the electrons coming from the [[cathode]] to a high enough velocity that they created X-rays when they struck the [[anode]] or the glass wall of the tube.<ref>{{cite journal |doi=10.1038/npre.2009.3267.4|title=The History, Development and Impact of Computed Imaging in Neurological Diagnosis and Neurosurgery: CT, MRI, and DTI|journal=Nature Precedings|date=2009| vauthors = Filler A |doi-access=free}}</ref>

The earliest experimenter thought to have (unknowingly) produced X-rays was [[William Morgan (actuary)|William Morgan]]. In 1785, he presented a paper to the [[Royal Society of London]] describing the effects of passing [[Electric current|electrical currents]] through a partially evacuated glass tube, producing a glow created by X-rays.<ref name="Morg1785">{{cite journal |title=Electrical Experiments Made in Order to Ascertain the Non-Conducting Power of a Perfect Vacuum, &c. | vauthors = Morgan W |journal=Philosophical Transactions of the Royal Society |volume=75 |pages=272–278 |publisher=Royal Society of London  |date=24 February 1785 |url=https://archive.org/details/philtrans00580668 |doi=10.1098/rstl.1785.0014|doi-access=free }}</ref><ref>{{cite journal | vauthors = Anderson JG |title=William Morgan and X-rays |journal=Transactions of the Faculty of Actuaries |volume=17 |pages=219–221 |date =January 1945 |url = https://www.actuaries.org.uk/documents/william-morgan-and-x-rays |doi=10.1017/s0071368600003001}}</ref> This work was further explored by [[Humphry Davy]] and his assistant [[Michael Faraday]].{{cn|date=December 2024}}

Starting in 1888, Philipp Lenard conducted experiments to see whether cathode rays could pass out of the Crookes tube into the air. He built a Crookes tube with a "window" at the end made of thin aluminium, facing the cathode so the cathode rays would strike it (later called a "Lenard tube"). He found that something came through, that would expose photographic plates and cause fluorescence. He measured the penetrating power of these rays through various materials. It has been suggested that at least some of these "Lenard rays" were actually X-rays.<ref name="Thomson182-186">{{Cite book | vauthors = Thomson JJ |title= The Discharge of Electricity through Gasses |publisher= Charles Scribner's Sons |date= 1903 |location= US |pages= 182–186 |url= https://books.google.com/books?id=Ryw4AAAAMAAJ&pg=PA182}}</ref>

Helmholtz formulated mathematical equations for X-rays. He postulated a dispersion theory before Röntgen made his discovery and announcement. He based it on the [[electromagnetic theory of light]].<ref>''Wiedmann's Annalen'', Vol. XLVIII.</ref>{{Full citation needed|date=October 2021}} However, he did not work with actual X-rays.

In early 1890, photographer [[William Nicholson Jennings|William Jennings]] and associate professor of the [[University of Pennsylvania]] Arthur W. Goodspeed were making photographs of coins with electric sparks. On 22 February after the end of their experiments two coins were left on a stack of photographic plates before Goodspeed demonstrated to Jennings the operation of [[Crookes tube]]s. While developing the plates, Jennings noticed disks of unknown origin on some of the plates, but nobody could explain them, and they moved on. Only in 1896 they realized that they accidentally made an X-ray photograph (they didn't claim a discovery).<ref>{{Cite journal |last=Walden |first=T L |date=December 1991 |title=The first radiation accident in America: a centennial account of the x-ray photograph made in 1890. |url=http://dx.doi.org/10.1148/radiology.181.3.1947073 |journal=Radiology |volume=181 |issue=3 |pages=635–639 |doi=10.1148/radiology.181.3.1947073 |pmid=1947073 |issn=0033-8419}}</ref>

Also in 1890, Roentgen's assistant [[Ludwig Zehnder]] noticed a flash of light from a fluorescent screen immediately before the covered tube he was switching on punctured.<ref>http://www.smj.org.sg/sites/default/files/3605/3605hdxray1.pdf</ref>

When [[Stanford University]] physics professor [[Fernando Sanford]] conducted his "electric photography" experiments in 1891–1893 by photographing coins in the light of electric sparks,<ref name=":2">{{Cite book |url=https://books.google.com/books?id=pjwiKBTJsQkC&pg=PA63 |title=Illustrated Electrical Review: A Journal of Scientific and Electrical Progress |date=1894 |publisher=Electrical Review Publishing Company |language=en}}</ref> like Jennings and Goodspeed, he may have unknowingly generated and detected X-rays. His letter of [[1893#January–March|6 January 1893]] to the ''[[Physical Review]]'' was duly published<ref name=":2" /> and an article entitled ''Without Lens or Light, Photographs Taken With Plate and Object in Darkness'' appeared in the ''[[San Francisco Examiner]]''.<ref>{{Cite journal |vauthors=Wyman T |date=Spring 2005 |title=Fernando Sanford and the Discovery of X-rays |journal="Imprint", from the Associates of the Stanford University Libraries |pages=5–15}}</ref>

In [[1894 in science|1894]], [[Nikola Tesla]] noticed damaged film in his lab that seemed to be associated with Crookes tube experiments and began investigating this invisible, [[radiant energy]].<ref name="Scenes from the past: Nikola Tesla">{{cite journal | vauthors = Hrabak M, Padovan RS, Kralik M, Ozretic D, Potocki K | title = Scenes from the past: Nikola Tesla and the discovery of X-rays | journal = Radiographics | volume = 28 | issue = 4 | pages = 1189–1192 | date = July 2008 | pmid = 18635636 | doi = 10.1148/rg.284075206 | doi-access = free }}</ref><ref>{{Cite book | vauthors = Chadda PK | title = Hydroenergy and Its Energy Potential | date = 2009 | publisher = Pinnacle Technology | isbn = 978-1-61820-149-2 | page = 88 }}</ref> After Röntgen identified the X-ray, Tesla began making X-ray images of his own using high voltages and tubes of his own design,<ref>Tesla's technical publications indicate that he invented and developed a single-electrode X-ray tube. Morton, William James and Hammer, Edwin W. (1896) ''American Technical Book Co.'', p. 68. {{US patent|514170}}, "Incandescent Electric Light". {{US patent|454622}} "System of Electric Lighting". These differed from other X-ray tubes in having no target electrode and worked with the output of a [[Tesla coil]].</ref> as well as Crookes tubes.

===Discovery by Röntgen===
[[File:WilhelmRöntgen.JPG|thumb|upright|[[Wilhelm Röntgen]]]]

On [[1895#October–December|8 November 1895]], German physics professor [[Wilhelm Röntgen]] stumbled on X-rays while experimenting with Lenard tubes and [[Crookes tube]]s and began studying them. He wrote an initial report "On a new kind of ray: A preliminary communication" and on 28 December 1895, submitted it to [[Würzburg]]'s Physical-Medical Society journal.<ref>{{Cite journal | vauthors = Stanton A |title= Wilhelm Conrad Röntgen On a New Kind of Rays: translation of a paper read before the Würzburg Physical and Medical Society, 1895 |journal= [[Nature (journal)|Nature]] |volume= 53 |issue= 1369 |pages= 274–6 |date= 23 January 1896 |doi= 10.1038/053274b0 |bibcode= 1896Natur..53R.274.|doi-access= free }} see also pp. 268 and 276 of the same issue.</ref> This was the first paper written on X-rays. Röntgen referred to the radiation as "X", to indicate that it was an unknown type of radiation. Some early texts refer to them as Chi-rays, having interpreted "X" as the uppercase [[Chi (letter)|Greek letter Chi]], ''Χ''.<ref>{{Cite journal |last1=Garcia |first1=J. |last2=Buchwald |first2=N. A. |last3=Feder |first3=B. H. |last4=Koelling |first4=R. A. |last5=Tedrow |first5=L. |date=1964 |title=Sensitivity of the head to X-ray |url=https://pubmed.ncbi.nlm.nih.gov/14171545 |journal=Science |volume=144 |issue=3625 |pages=1470–1472 |doi=10.1126/science.144.3625.1470 |issn=0036-8075 |pmid=14171545 |bibcode=1964Sci...144.1470G |s2cid=44719943 |quote=Rats have been trained to respond to signals consisting of very low doses of chi-ray directed to the head.}}</ref><ref>{{Cite journal |last1=Baganha |first1=M. F. |last2=Marques |first2=M. A. |last3=Botelho |first3=M. F. |last4=Teixeira |first4=M. L. |last5=Carvalheira |first5=V. |last6=Calisto |first6=J. |last7=Silva |first7=A. |last8=Fernandes |first8=A. |last9=Torres |first9=M. |last10=Brito |first10=J. |date=1993 |title=Tomodensitometry and radioisotopic methods in the study of unilateral lung hyperlucencies of vascular origin |url=https://pubmed.ncbi.nlm.nih.gov/8475784 |journal=Acta Médica Portuguesa |volume=6 |issue=1 |pages=19–24 |issn=0870-399X |pmid=8475784}}</ref><ref>{{Cite journal |last1=Takahashi |first1=K. |last2=Case |first2=B. W. |last3=Dufresne |first3=A. |last4=Fraser |first4=R. |last5=Higashi |first5=T. |last6=Siemiatycki |first6=J. |date=1994 |title=Relation between lung asbestos fibre burden and exposure indices based on job history |journal=Occupational and Environmental Medicine |volume=51 |issue=7 |pages=461–469 |doi=10.1136/oem.51.7.461 |issn=1351-0711 |pmc=1128015 |pmid=8044245}}</ref>

There are conflicting accounts of his discovery because Röntgen had his [[Nachlass|lab notes]] burned after his death, but this is a likely reconstruction by his biographers:<ref>{{Cite web | vauthors = Peters P |date=1995 |title=W. C. Roentgen and the discovery of x-rays |website=Textbook of Radiology |publisher=Medcyclopedia.com, GE Healthcare |url=http://www.medcyclopaedia.com/library/radiology/chapter01.aspx |archive-url=https://archive.today/20080511205052/http://www.medcyclopaedia.com/library/radiology/chapter01.aspx |archive-date=11 May 2008 |access-date=5 May 2008 }}</ref><ref name="Glasser">{{Cite book | vauthors = Glasser O |title= Wilhelm Conrad Röntgen and the early history of the roentgen rays |publisher= Norman Publishing |date= 1993 |pages= 10–15 |url= https://books.google.com/books?id=5GJs4tyb7wEC&pg=PA10 |isbn= 978-0930405229}}</ref> Röntgen was investigating cathode rays from a Crookes tube which he had wrapped in black cardboard so that the visible light from the tube would not interfere, using a [[fluorescent]] screen painted with barium [[platinocyanide]]. He noticed a faint green glow from the screen, about {{convert|1|m|ft|sp=us}} away. Röntgen realized some invisible rays coming from the tube were passing through the cardboard to make the screen glow. He found they could also pass through books and papers on his desk. Röntgen threw himself into investigating these unknown rays systematically. Two months after his initial discovery, he published his paper.<ref>{{cite news | vauthors = Arthur C |title=Google doodle celebrates 115 years of X-rays |url=https://www.theguardian.com/technology/blog/2010/nov/08/google-doodle-x-ray-115-year-anniversary |newspaper=The Guardian |publisher=Guardian US |access-date=5 February 2019|date=8 November 2010 }}</ref>

[[File:First medical X-ray by Wilhelm Röntgen of his wife Anna Bertha Ludwig's hand - 18951222.jpg|thumb|upright=0.9|left|''Hand mit Ringen'' (Hand with Rings): print of Wilhelm Röntgen's first "medical" X-ray, of his wife's hand, taken on 22 December 1895 and presented to [[Ludwig Zehnder]] of the Physik Institut, [[University of Freiburg]], on 1 January 1896<ref>{{Cite book | vauthors = Kevles BH |title= Naked to the Bone Medical Imaging in the Twentieth Century |publisher= [[Rutgers University Press]] |date= 1996 |location= Camden, New Jersey |pages= [https://archive.org/details/isbn_9780813523583/page/19 19–22] |isbn= 978-0-8135-2358-3 |url= https://archive.org/details/isbn_9780813523583/page/19 }}</ref><ref>{{Cite web | vauthors = Sample S |title= X-Rays |website= The Electromagnetic Spectrum |publisher= [[NASA]] |date= 27 March 2007 |url= http://science.hq.nasa.gov/kids/imagers/ems/xrays.html |access-date= 3 December 2007}}</ref>]]

Röntgen discovered their medical use when he made a picture of his wife's hand on a photographic plate formed due to X-rays. The photograph of his wife's hand was the first photograph of a human body part using X-rays. When she saw the picture, she said "I have seen my death."<ref name="pbs">{{Cite web|url=https://www.pbs.org/newshour/health/i-have-seen-my-death-how-the-world-discovered-the-x-ray|title='I Have Seen My Death': How the World Discovered the X-Ray| vauthors = Markel H |date=20 December 2012|website=PBS NewsHour|publisher=PBS|access-date=23 March 2019}}</ref>

The discovery of X-rays generated significant interest. Röntgen's biographer [[Otto Glasser]] estimated that, in [[1896 in science|1896]] alone, as many as 49 essays and 1044 articles about the new rays were published.<ref>{{Cite book|title=Dr. W. C. Ro ̈ntgen| vauthors = Glasser O |publisher=Thomas|date=1958|location=Springfield}}</ref> This was probably a conservative estimate, if one considers that nearly every paper around the world extensively reported about the new discovery, with a magazine such as ''[[Science (journal)|Science]]'' dedicating as many as 23 articles to it in that year alone.<ref>{{Cite journal| vauthors = Natale S |date=1 November 2011|title=The Invisible Made Visible|journal=Media History|volume=17|issue=4|pages=345–358|doi=10.1080/13688804.2011.602856|hdl=2134/19408|s2cid=142518799|url=https://dspace.lboro.ac.uk/2134/19408|hdl-access=free}}</ref> Sensationalist reactions to the new discovery included publications linking the new kind of rays to occult and paranormal theories, such as telepathy.<ref>{{Cite journal| vauthors = Natale S |date=4 August 2011|title=A Cosmology of Invisible Fluids: Wireless, X-Rays, and Psychical Research Around 1900|journal=Canadian Journal of Communication|volume=36|issue=2|pages=263–276 |doi=10.22230/cjc.2011v36n2a2368|doi-access=free|hdl=2318/1770480|hdl-access=free}}</ref><ref>{{cite journal | vauthors = Grove AW | title = Röntgen's ghosts: photography, X-rays, and the Victorian imagination | journal = Literature and Medicine | volume = 16 | issue = 2 | pages = 141–173 | date = 1 January 1997 | pmid = 9368224 | doi = 10.1353/lm.1997.0016 | s2cid = 35604474 }}</ref>

The name X-rays stuck, although (over Röntgen's great objections) many of his colleagues suggested calling them ''Röntgen rays''. They are still referred to as such in many languages, including [[German language|German]], [[Hungarian language|Hungarian]], [[Ukrainian language|Ukrainian]], [[Danish language|Danish]], [[Polish language|Polish]], [[Czech language|Czech]], [[Bulgarian language|Bulgarian]], [[Swedish language|Swedish]], [[Finnish language|Finnish]], [[Portuguese Language|Portuguese]], [[Estonian language|Estonian]], [[Slovak language|Slovak]], [[Slovenian language|Slovenian]], [[Turkish language|Turkish]],  [[Russian language|Russian]], [[Latvian language|Latvian]], [[Lithuanian language|Lithuanian]], [[Albanian language|Albanian]], [[Japanese language|Japanese]], [[Dutch language|Dutch]], [[Georgian language|Georgian]], [[Hebrew language|Hebrew]], [[Icelandic language|Icelandic]], and [[Norwegian language|Norwegian]].{{original research inline|date=October 2024}}

Röntgen received the first [[Nobel Prize in Physics]] for his discovery.<ref>{{Cite web |url=https://www.nobelprize.org/nobel_prizes/physics/articles/karlsson/ |title=The Nobel Prizes in Physics 1901–2000 | vauthors = Karlsson EB |date=9 February 2000 |publisher=The Nobel Foundation |access-date=24 November 2011 |location=Stockholm }}</ref>

===Advances in radiology===
[[File:Crookes tube xray experiment.jpg|thumb|Taking an X-ray image with early [[Crookes tube]] apparatus, late 1800s. The Crookes tube is visible in center. The standing man is viewing his hand with a [[fluoroscope]] screen. The seated man is taking a [[radiograph]] of his hand by placing it on a [[photographic plate]]. No precautions against radiation exposure are taken; its hazards were not known at the time.]]
[[File:Professor-Karl-Gustav-Lennander-in-1897-removing-a-pistol-bullet-from-the-occipital-lobe-of-the-brain-in-a-young-man-aft.jpg|thumb|upright|Surgical removal of a bullet whose location was diagnosed with X-rays (see inset) in 1897]]

Röntgen immediately noticed X-rays could have medical applications. Along with his 28 December Physical-Medical Society submission, he sent a letter to physicians he knew around Europe (1 January 1896).<ref name=Feldman1989>{{cite journal | vauthors = Feldman A | title = A sketch of the technical history of radiology from 1896 to 1920 | journal = Radiographics | volume = 9 | issue = 6 | pages = 1113–1128 | date = November 1989 | pmid = 2685937 | doi = 10.1148/radiographics.9.6.2685937 }}</ref> News (and the creation of "shadowgrams") spread rapidly with Scottish electrical engineer [[Alan Archibald Campbell-Swinton]] being the first after Röntgen to create an X-ray photograph (of a hand). Through February, there were 46 experimenters taking up the technique in North America alone.<ref name=Feldman1989/>

The first use of X-rays under clinical conditions was by [[John Hall-Edwards]] in Birmingham, England on 11 January 1896, when he radiographed a needle stuck in the hand of an associate. On 14 February 1896, Hall-Edwards was also the first to use X-rays in a surgical operation.<ref>{{Cite web|url=http://www.birmingham.gov.uk/xray |title=Major John Hall-Edwards |access-date=2012-05-17 |publisher=Birmingham City Council |archive-url=https://web.archive.org/web/20120928204852/http://www.birmingham.gov.uk/xray |archive-date=28 September 2012 }}</ref>

[[File:James Green & James H. Gardiner - Sciagraphs of British Batrachians and Reptiles - 1897 - Ycba f6c56349-13da-4efc-a671-e40af53b0823.jpg|thumb|Images by James Green, from "Sciagraphs of British Batrachians and Reptiles" (1897), featuring (from left) ''Rana esculenta'' (now ''[[Pelophylax lessonae]]''), ''Lacerta vivipara'' (now ''[[Zootoca vivipara]]''), and ''[[Lacerta agilis]]'']]

In early 1896, several weeks after Röntgen's discovery, [[Ivan Romanovich Tarkhanov]] irradiated frogs and insects with X-rays, concluding that the rays "not only photograph, but also affect the living function".<ref>Kudriashov, Y. B. (2008). ''Radiation Biophysics''. Nova Publishers. p. xxi. {{ISBN|9781600212802}}.</ref> At around the same time, the zoological illustrator James Green began to use X-rays to examine fragile specimens. [[George Albert Boulenger]] first mentioned this work in a paper he delivered before the [[Zoological Society of London]] in May 1896. The book ''Sciagraphs of British Batrachians and Reptiles'' (sciagraph is an obsolete name for an X-ray photograph), by Green and James H. Gardiner, with a foreword by Boulenger, was published in 1897.<ref Name="YCBA">{{Cite web |title=Green, James (Zoological Artist), Sciagraphs of British batrachians and reptiles, 1897 |url=https://collections.britishart.yale.edu/catalog/orbis:12428971 |publisher=Yale Centre for British Art |access-date=24 November 2021}}</ref><ref>{{cite journal |title=Sciagraphs of British Batrachians and Reptiles1 |journal=Nature |date=1 April 1897 |volume=55 |issue=1432 |pages=539–540 |doi=10.1038/055539a0|bibcode=1897Natur..55..539. |s2cid=4054184 |doi-access=free }}</ref>

The first medical X-ray made in the United States was obtained using a discharge tube of [[Ivan Puluj]]'s design.<ref>{{Cite journal |last1=Mayba |first1=Ihor I. |last2=Gaiua |first2=Roman |last3=Kyle |first3=Robert A. |last4=Shampo |first4=Marc A. |date=July 1997 |title=Ukrainian Physicist Contributes to the Discovery of X-Rays |url=https://linkinghub.elsevier.com/retrieve/pii/S0025619611635738 |journal=Mayo Clinic Proceedings |language=en |volume=72 |issue=7 |pages=658 |doi=10.1016/S0025-6196(11)63573-8|pmid=9212769 }}</ref> In January 1896, on reading of Röntgen's discovery, Frank Austin of [[Dartmouth College]] tested all of the discharge tubes in the physics laboratory and found that only the Puluj tube produced X-rays. This was a result of Puluj's inclusion of an oblique "target" of [[mica]], used for holding samples of [[fluorescent]] material, within the tube. On 3 February 1896, Gilman Frost, professor of medicine at the college, and his brother Edwin Frost, professor of physics, exposed the wrist of Eddie McCarthy, whom Gilman had treated some weeks earlier for a fracture, to the X-rays and collected the resulting image of the broken bone on [[photographic plate|gelatin photographic plates]] obtained from Howard Langill, a local photographer also interested in Röntgen's work.<ref name= PKS>{{cite journal | vauthors = Spiegel PK | title = The first clinical X-ray made in America—100 years | journal = AJR. American Journal of Roentgenology | volume = 164 | issue = 1 | pages = 241–243 | date = January 1995 | pmid = 7998549 | doi = 10.2214/ajr.164.1.7998549 | doi-access = free }}</ref>

[[File:X-ray 1896 nouvelle iconographie de salpetriere.jpg|thumb|left|1896 plaque published in ''"Nouvelle Iconographie de la Salpetrière"'', a medical journal. In the left a hand deformity, in the right same hand seen using [[radiography]]. The authors named the technique ''Röntgen photography''.]]

Many experimenters, including Röntgen himself in his original experiments, came up with methods to view X-ray images "live" using some form of luminescent screen.<ref name=Feldman1989/> Röntgen used a screen coated with barium [[platinocyanide]]. On 5 February 1896, live imaging devices were developed by both Italian scientist Enrico Salvioni (his "cryptoscope") and [[William Francis Magie]] of [[Princeton University]] (his "Skiascope"), both using barium platinocyanide. American inventor [[Thomas Edison]] started research soon after Röntgen's discovery and investigated materials' ability to fluoresce when exposed to X-rays, finding that [[calcium tungstate]] was the most effective substance. In May 1896, he developed the first mass-produced live imaging device, his "Vitascope", later called the [[fluoroscopy|fluoroscope]], which became the standard for medical X-ray examinations.<ref name=Feldman1989/> Edison dropped X-ray research around 1903, before the death of [[Clarence Madison Dally]], one of his glassblowers. Dally had a habit of testing X-ray tubes on his own hands, developing a cancer in them so tenacious that both arms were [[amputation|amputated]] in a futile attempt to save his life; in 1904, he became the first known death attributed to X-ray exposure.<ref name=Feldman1989/> During the time the fluoroscope was being developed, Serbian American physicist [[Mihajlo Pupin]], using a calcium tungstate screen developed by Edison, found that using a fluorescent screen decreased the exposure time it took to create an X-ray for medical imaging from an hour to a few minutes.<ref>Nicolaas A. Rupke, ''Eminent Lives in Twentieth-Century Science and Religion'', page 300, Peter Lang, 2009 {{ISBN|3631581203}}</ref><ref name=Feldman1989/>

In 1901, [[assassination of William McKinley|U.S. President William McKinley was shot twice]] in an assassination attempt while attending the [[Pan-American Exposition|Pan American Exposition]] in [[Buffalo, New York]]. While one bullet only grazed his [[sternum]], another had lodged somewhere deep inside his [[abdomen]] and could not be found. A worried McKinley aide sent word to inventor Thomas Edison to rush an [[X-ray generator|X-ray machine]] to Buffalo to find the stray bullet. It arrived but was not used. While the shooting itself had not been lethal, [[gangrene]] had developed along the path of the bullet, and McKinley died of [[septic shock]] due to bacterial infection six days later.<ref>{{Cite web|title=Visible Proofs: Forensic Views of the Body: Galleries: Cases: Could X-rays Have Saved President William McKinley?|url=https://www.nlm.nih.gov/exhibition/visibleproofs/galleries/cases/mckinley.html|access-date=2022-01-24|website=NLM.NIH.gov}}</ref>

===Hazards discovered===
With the widespread experimentation with X‑rays after their discovery in [[1895 in science|1895]] by scientists, physicians, and inventors came many stories of burns, hair loss, and worse in technical journals of the time. In February 1896, Professor John Daniel and [[William Lofland Dudley]] of [[Vanderbilt University]] reported hair loss after Dudley was X-rayed. A child who had been shot in the head was brought to the Vanderbilt laboratory in 1896. Before trying to find the bullet, an experiment was attempted, for which Dudley "with his characteristic devotion to science"<ref>{{cite journal | vauthors = Daniel J | title = THE X-RAYS | journal = Science | volume = 3 | issue = 67 | pages = 562–563 | date = April 1896 | pmid = 17779817 | doi = 10.1126/science.3.67.562 | bibcode = 1896Sci.....3..562D | url = https://zenodo.org/record/1448086 }}</ref><ref>{{Cite book |title=The South in the Building of the Nation: Biography A-J | vauthors = Fleming WL |page=300 |publisher=Pelican Publishing |isbn=978-1589809468|date=1909 }}</ref><ref>{{Cite book |url=https://books.google.com/books?id=IioKBAAAQBAJ&pg=PA174 |title=Understanding Ionizing Radiation and Protection |date=Mar 2014 |page=174|author1=Ce4Rt }}</ref> volunteered. Daniel reported that 21 days after taking a picture of Dudley's [[human skull|skull]] (with an exposure time of one hour), he noticed a bald spot {{convert|2|in|cm|sp=us|order=flip|0}} in diameter on the part of his head nearest the X-ray tube: "A plate holder with the plates towards the side of the skull was fastened and a [[coin]] placed between the skull and the head. The tube was fastened at the other side at a distance of one-half-inch [{{convert|.5|in|cm|disp=out}}] from the hair."<ref>{{Cite book |url=https://books.google.com/books?id=5GJs4tyb7wEC&pg=PA294|title=Wilhelm Conrad Röntgen and the Early History of the Roentgen Rays | vauthors = Glasser O |page=294 |date=1934 |publisher=Norman Publishing |isbn=978-0930405229}}</ref> Beyond burns, hair loss, and cancer, X-rays can be linked to infertility in males based on the amount of radiation used.{{cn|date=December 2024}}

In August 1896, H. D. Hawks, a graduate of Columbia College, suffered severe hand and chest burns from an X-ray demonstration. It was reported in ''Electrical Review'' and led to many other reports of problems associated with X-rays being sent in to the publication.<ref>{{cite journal | vauthors = Sansare K, Khanna V, Karjodkar F | title = Early victims of X-rays: a tribute and current perception | journal = Dento Maxillo Facial Radiology | volume = 40 | issue = 2 | pages = 123–125 | date = February 2011 | pmid = 21239576 | pmc = 3520298 | doi = 10.1259/dmfr/73488299 }}</ref> Many experimenters including [[Elihu Thomson]] at Edison's lab, [[William J. Morton]], and [[Nikola Tesla]] also reported burns. Elihu Thomson deliberately exposed a finger to an X-ray tube over a period of time and suffered pain, swelling, and blistering.<ref name="physics.isu.edu">{{Cite web|title=ISU Health Physics Radinf – First 50 Years|url=https://sites.google.com/isu.edu/health-physics-radinf/history-of-radiation-and-radiation-protection/first-50-years|access-date=2022-01-24|website=Sites.Google.com }}</ref> Other effects were sometimes blamed for the damage including ultraviolet rays and (according to Tesla) ozone.<ref name="Scenes from the past: Nikola Tesla" /> Many physicians claimed there were no effects from X-ray exposure at all.<ref name="physics.isu.edu" /> On 3 August 1905, in San Francisco, California, [[Elizabeth Fleischman]], an American X-ray pioneer, died from complications as a result of her work with X-rays.<ref>California, San Francisco Area Funeral Home Records, 1835–1979. Database with images. FamilySearch. Jacob Fleischman in the entry for Elizabeth Aschheim. 3 August 1905. Citing funeral home J.S. Godeau, San Francisco, San Francisco, California. Record book Vol. 06, p. 1–400, 1904–1906. San Francisco Public Library. San Francisco History and Archive Center.</ref><ref>Editor. (5 August 1905). Aschheim. Obituaries. ''San Francisco Examiner''. San Francisco, California.</ref><ref>Editor. (5 August 1905). Obituary Notice. Elizabeth Fleischmann. ''San Francisco Chronicle''. Page 10.</ref>

Hall-Edwards developed a cancer (then called X-ray dermatitis) sufficiently advanced by 1904 to cause him to write papers and give public addresses on the dangers of X-rays. His left arm had to be amputated at the elbow in 1908,<ref name="BirminghamCouncil">{{cite web |title=Major John Hall-Edwards |url=http://www.birmingham.gov.uk/xray |publisher=Birmingham City Council |access-date=23 April 2010 |archive-url=https://web.archive.org/web/20120928204852/http://www.birmingham.gov.uk/xray |archive-date=28 September 2012}}</ref><ref>{{Cite web |date=15 June 2018 |title=JOHN HALL-EDWARDS |url=https://engole.info/john-hall-edwards/ |access-date=2023-10-27 |website=Engole the Elven for Knowledge}}</ref> and four fingers on his right arm soon thereafter, leaving only a thumb. He died of cancer in 1926. His left hand is kept at [[Birmingham University]].{{cn|date=December 2024}}

===20th century and beyond===
[[File:Historical X-ray nci-vol-1893-300.jpg|thumb|A patient being examined with a thoracic [[fluoroscope]] in [[1940 in science|1940]], which displayed continuous moving images. This image was used to argue that [[ionizing radiation|radiation exposure]] during the X-ray procedure would be negligible.]]

The many applications of X-rays immediately generated enormous interest. Workshops began making specialized versions of Crookes tubes for generating X-rays and these first-generation [[cold cathode]] or Crookes X-ray tubes were used until about 1920.{{cn|date=December 2024}}

A typical early 20th-century medical X-ray system consisted of a [[Induction coil|Ruhmkorff coil]] connected to a [[X-ray tube#Crookes tube (cold cathode tube)|cold cathode Crookes X-ray tube]]. A spark gap was typically connected to the high voltage side in parallel to the tube and used for diagnostic purposes.<ref name="Scha1905">{{cite book |title=Electro-medical Instruments and their Management | vauthors = Schall K |publisher=Bemrose & Sons Ltd. Printers |date=1905 |pages=[https://archive.org/details/electromedicali00ltdgoog/page/n106 96], 107 |url=https://archive.org/details/electromedicali00ltdgoog}}</ref> The spark gap allowed detecting the polarity of the sparks, measuring voltage by the length of the sparks thus determining the "hardness" of the vacuum of the tube, and it provided a load in the event the X-ray tube was disconnected. To detect the hardness of the tube, the spark gap was initially opened to the widest setting. While the coil was operating, the operator reduced the gap until sparks began to appear. A tube in which the spark gap began to spark at around {{convert|2.5|in|cm|sp=us|order=flip}} was considered soft (low vacuum) and suitable for thin body parts such as hands and arms. A {{convert|5|in|cm|sp=us|adj=on|order=flip}} spark indicated the tube was suitable for shoulders and knees. An {{convert|7|to|9|in|cm|sp=us|adj=on|order=flip}} spark would indicate a higher vacuum suitable for imaging the abdomen of larger individuals. Since the spark gap was connected in parallel to the tube, the spark gap had to be opened until the sparking ceased to operate the tube for imaging. Exposure time for photographic plates was around half a minute for a hand to a couple of minutes for a thorax. The plates may have a small addition of fluorescent salt to reduce exposure times.<ref name="Scha1905" />

Crookes tubes were unreliable. They had to contain a small quantity of gas (invariably air) as a current will not flow in such a tube if they are fully evacuated. However, as time passed, the X-rays caused the glass to absorb the gas, causing the tube to generate "harder" X-rays until it soon stopped operating. Larger and more frequently used tubes were provided with devices for restoring the air, known as "softeners". These often took the form of a small side tube that contained a small piece of [[mica]], a mineral that traps relatively large quantities of air within its structure. A small electrical heater heated the mica, causing it to release a small amount of air, thus restoring the tube's efficiency. However, the mica had a limited life, and the restoration process was difficult to control.{{cn|date=December 2024}}

In [[1904 in science|1904]], [[John Ambrose Fleming]] invented the [[thermionic diode]], the first kind of [[vacuum tube]]. This used a [[hot cathode]] that caused an [[electric current]] to flow in a [[vacuum]]. This idea was quickly applied to X-ray tubes, and hence heated-cathode X-ray tubes, called "Coolidge tubes", completely replaced the troublesome cold cathode tubes by about 1920.{{cn|date=December 2024}}

In about 1906, the physicist [[Charles Barkla]] discovered that X-rays could be scattered by gases, and that each element had a characteristic [[X-ray spectrum]]. He won the [[1917 in science|1917]] [[Nobel Prize in Physics]] for this discovery.<ref>{{Cite web |title=Nobel Prize in Physics 1917 |url=https://www.nobelprize.org/prizes/physics/1917/barkla/facts/ |access-date=2025-02-02 |website=NobelPrize.org |language=en-US}}</ref>

In [[1912 in science|1912]], [[Max von Laue]], Paul Knipping, and Walter Friedrich first observed the [[diffraction]] of X-rays by crystals. This discovery, along with the early work of [[Paul Peter Ewald]], [[William Henry Bragg]], and [[William Lawrence Bragg]], gave birth to the field of [[X-ray crystallography]].<ref name="Stoddart">{{cite journal | vauthors = Stoddart C |title=Structural biology: How proteins got their close-up |journal=Knowable Magazine |date=1 March 2022 |doi=10.1146/knowable-022822-1|doi-access=free |url=https://knowablemagazine.org/article/living-world/2022/structural-biology-how-proteins-got-their-closeup |access-date=25 March 2022}}</ref>

In [[1913 in science|1913]], [[Henry Moseley]] performed crystallography experiments with X-rays emanating from various metals and formulated [[Moseley's law]] which relates the frequency of the X-rays to the atomic number of the metal.<ref>{{Cite journal |last1=Soltis |first1=Tomas |last2=Folan |first2=Lorcan M. |last3=Eltareb |first3=Waleed |date=2017-05-01 |title=One hundred years of Moseley's law: An undergraduate experiment with relativistic effects |url=https://pubs.aip.org/aapt/ajp/article-abstract/85/5/352/1038149/One-hundred-years-of-Moseley-s-law-An?redirectedFrom=fulltext |journal=American Journal of Physics |volume=85 |issue=5 |pages=352–358 |doi=10.1119/1.4977793 |bibcode=2017AmJPh..85..352S |issn=0002-9505}}</ref>

The [[X-ray tube#Coolidge tube (hot cathode tube)|Coolidge X-ray tube]] was invented the same year by [[William D. Coolidge]]. It made possible the continuous emissions of X-rays. Modern X-ray tubes are based on this design, often employing the use of rotating targets which allow for significantly higher heat dissipation than static targets, further allowing higher quantity X-ray output for use in high-powered applications such as rotational CT scanners.{{cn|date=December 2024}}

[[File:Abell 2125.jpg|thumb|upright=0.8|left|Chandra's image of the galaxy cluster Abell 2125 reveals a complex of several massive multimillion-degree-Celsius gas clouds in the process of merging.]]

The use of X-rays for medical purposes (which developed into the field of [[radiation therapy]]) was pioneered by Major [[John Hall-Edwards]] in [[Birmingham, England|Birmingham]], England. Then in 1908, he had to have his left arm amputated because of the spread of [[radiation dermatitis|X-ray dermatitis]] on his arm.<ref>Birmingham City Council: [http://www.birmingham.gov.uk/xray Major John Hall-Edwards] {{webarchive |url=https://web.archive.org/web/20120928204852/http://www.birmingham.gov.uk/xray |date=28 September 2012 }}</ref>

Medical science also used the motion picture to study human physiology. In 1913, a motion picture was made in Detroit showing a hard-boiled egg inside a human stomach. This early X-ray movie was recorded at a rate of one still image every four seconds.<ref>{{Cite news|date=4 April 1913|title=X-ray movies show hard boiled egg fighting digestive organs (1913)|pages=2|work=The News-Palladium|url=https://www.newspapers.com/clip/64031702/x-ray-movies-show-hard-boiled-egg/|access-date=2020-11-26}}</ref> Dr Lewis Gregory Cole of New York was a pioneer of the technique, which he called "serial radiography".<ref>{{Cite news|date=22 June 1913|title=X-ray moving pictures latest (1913)|pages=32|work=Chicago Tribune|url=https://www.newspapers.com/clip/64031812/x-ray-moving-pictures-latest-1913/|access-date=2020-11-26}}</ref><ref>{{Cite news|date=10 May 1915|title=Homeopaths to show movies of body's organs at work (1915)|pages=6|work=The Central New Jersey Home News|url=https://www.newspapers.com/clip/64031868/homeopaths-to-show-movies-of-bodys/|access-date=2020-11-26}}</ref> In 1918, X-rays were used in association with [[Movie camera|motion picture cameras]] to capture the human skeleton in motion.<ref>{{Cite news|date=15 March 1918|title=How X-Ray Movies Are Taken (1918)|pages=2|work=Davis County Clipper|url=https://www.newspapers.com/clip/64031520/how-x-ray-movies-are-taken-1918/|access-date=2020-11-26}}</ref><ref>{{Cite news|date=12 January 1919|title=X-ray movies (1919)|pages=16|work=Tampa Bay Times|url=https://www.newspapers.com/clip/64031338/x-ray-movies-1919/|access-date=2020-11-26}}</ref><ref>{{Cite news|date=7 January 1918|title=X-ray movies perfected. Will show motions of bones and joints of human body. (1918)|pages=7|work=The Sun|url=https://www.newspapers.com/clip/64031941/x-ray-movies-perfected-will-show/|access-date=2020-11-26}}</ref> In 1920, it was used to record the movements of tongue and teeth in the study of languages by the Institute of Phonetics in England.<ref>{{Cite news|date=2 January 1920|title=Talk is cheap? X-ray used by Institute of Phonetics (1920)|pages=13|work=New Castle Herald|url=https://www.newspapers.com/clip/64031597/talk-is-cheap-x-ray-used-by-institute/|access-date=2020-11-26}}</ref>

In [[1914 in science|1914]], [[Marie Curie]] developed radiological cars to support soldiers injured in [[World War I]]. The cars would allow for rapid X-ray imaging of wounded soldiers so battlefield surgeons could quickly and more accurately operate.<ref>{{Cite web|url=http://theconversation.com/marie-curie-and-her-x-ray-vehicles-contribution-to-world-war-i-battlefield-medicine-83941|title=Marie Curie and her X-ray vehicles' contribution to World War I battlefield medicine| vauthors = Jorgensen TJ |date=10 October 2017|website=The Conversation|access-date=23 February 2018}}</ref>

From the early 1920s through to the 1950s, X-ray machines were developed to assist in the fitting of shoes<ref>{{Cite news|date=25 August 1921|title=X-Rays for Fitting Boots.|pages=4|work=Warwick Daily News (Qld.: 1919–1954)|url=http://nla.gov.au/nla.news-article177254793|access-date=2020-11-27}}</ref> and were sold to commercial shoe stores.<ref>{{Cite news|url=http://nla.gov.au/nla.news-article177100333|title=T. C. BEIRNE'S X-RAY SHOE FITTING|date=17 July 1925|work=Telegraph (Brisbane, Qld. : 1872–1947)|access-date=2017-11-05|pages=8}}</ref><ref>{{Cite news|url=http://nla.gov.au/nla.news-article58359293|title=THE PEDOSCOPE|date=15 July 1928|work=Sunday Times (Perth, WA : 1902–1954)|access-date=2017-11-05|pages=5}}</ref><ref>{{Cite news|url=http://nla.gov.au/nla.news-article195854195|title=X-RAY SHOE FITTINGS|date=27 July 1955|work=Biz (Fairfield, NSW : 1928–1972)|access-date=2017-11-05|pages=10}}</ref> Concerns regarding the impact of frequent or poorly controlled use were expressed in the 1950s,<ref>{{Cite news|url=http://nla.gov.au/nla.news-article212595591|title=SHOE X-RAY DANGERS|date=28 February 1951|work=Brisbane Telegraph (Qld. : 1948–1954)|access-date=2017-11-05|pages=7}}</ref><ref>{{Cite news|url=http://nla.gov.au/nla.news-article130371085|title=X-ray shoe sets in S.A. 'controlled'|date=27 April 1951|work=News (Adelaide, SA : 1923–1954)|access-date=2017-11-05|pages=12}}</ref> leading to the practice's eventual end that decade.<ref>{{Cite news|url=http://nla.gov.au/nla.news-article91592036|title=Ban On Shoe X-ray Machines Resented|date=26 June 1957|work=Canberra Times (ACT : 1926–1995)|access-date=2017-11-05|pages=4}}</ref>

The [[X-ray microscope]] was developed during the late 1940s and early 1950s.<ref>{{Cite web |title=X-ray microscope {{!}} High-Resolution, Non-Destructive, Imaging {{!}} Britannica |url=https://www.britannica.com/technology/X-ray-microscope |access-date=2025-02-17 |website=www.britannica.com |language=en}}</ref><ref>{{Cite web |date=2020-10-17 |title=Albert Baez Page of the NSHP |url=http://www.hispanicphysicists.org/recognition/bio(baez).html |access-date=2025-02-17 |archive-url=https://web.archive.org/web/20201017111846/http://www.hispanicphysicists.org/recognition/bio(baez).html |archive-date=17 October 2020 }}</ref><ref>{{Cite web |last=Aksnes |first=Ingrid |date=2020-11-08 |title=History of X-rays - 125 years in the making (pt 1) |url=https://www.excillum.com/history-of-x-rays-early-years/#:~:text=Outside%20of%20medical%20applications,%20another,the%20object%20at%20the%20detector. |access-date=2025-02-17 |website=Excillum |language=en-US}}</ref>

The [[Chandra X-ray Observatory]], launched on [[1999#July|23 July 1999]], has been allowing the exploration of the very violent processes in the [[universe]] that produce X-rays. Unlike [[Light|visible light]], which gives a relatively stable view of the universe, the X-ray universe is unstable. It features [[star]]s being torn apart by [[black hole]]s, [[Interacting galaxy|galactic collisions]], and [[nova]]e, and [[neutron star]]s that build up layers of [[Plasma (physics)|plasma]] that then [[Explosion|explode]] into [[Outer space|space]].{{cn|date=December 2024}}

[[File:Phase-contrast x-ray image of spider.jpg|thumb|upright=0.8|Phase-contrast X-ray image of a spider]]
An [[X-ray laser]] device was proposed as part of the [[presidency of Ronald Reagan|Reagan Administration]]'s [[Strategic Defense Initiative]] in the 1980s, but the only test of the device (a sort of laser "blaster" or [[death ray]], powered by a thermonuclear explosion) gave inconclusive results. For technical and political reasons, the overall project (including the X-ray laser) was defunded (though was later revived by the second [[presidency of George W. Bush|Bush Administration]] as [[National Missile Defense]] using different technologies).{{cn|date=December 2024}}

[[Phase-contrast X-ray imaging]] refers to a variety of techniques that use phase information of an X-ray beam to form the image. Due to its good sensitivity to density differences, it is especially useful for imaging soft tissues. It has become an important method for visualizing cellular and histological structures in a wide range of biological and medical studies. There are several technologies being used for X-ray phase-contrast imaging, all using different principles to convert phase variations in the X-rays emerging from an object into intensity variations.<ref>{{Cite journal | vauthors = Fitzgerald R |title= Phase-sensitive x-ray imaging |date= 2000 |journal= Physics Today |volume= 53 |issue= 7 |pages= 23–26 |doi= 10.1063/1.1292471 |bibcode= 2000PhT....53g..23F|s2cid= 121322301 |doi-access= free }}</ref><ref name=David>{{Cite journal |vauthors = David C, Nöhammer B, Solak H, Ziegler |title= Differential x-ray phase contrast imaging using a shearing interferometer |journal= Applied Physics Letters |date= 2002 |volume= 81 |issue= 17 |pages= 3287–3289 |doi= 10.1063/1.1516611 |bibcode= 2002ApPhL..81.3287D|doi-access= free }}</ref> These include propagation-based phase contrast,<ref>{{Cite journal | vauthors =Wilkins SW, Gureyev TE, Gao D, Pogany A, Stevenson AW |date= 1996 |title= Phase-contrast imaging using polychromatic hard X-rays |journal= Nature |volume= 384 |pages= 335–338 |doi= 10.1038/384335a0 |bibcode= 1996Natur.384..335W |issue= 6607|s2cid= 4273199 }}</ref> [[Talbot effect|Talbot]] interferometry,<ref name=David/> refraction-enhanced imaging,<ref>{{Cite journal | vauthors = Davis TJ, Gao D, Gureyev TE, Stevenson AW, Wilkins SW |date= 1995 |title= Phase-contrast imaging of weakly absorbing materials using hard X-rays |journal= Nature |volume= 373 |pages= 595–598 |doi= 10.1038/373595a0 |bibcode= 1995Natur.373..595D |issue= 6515|s2cid= 4287341 }}</ref> and X-ray interferometry.<ref>{{cite journal | vauthors = Momose A, Takeda T, Itai Y, Hirano K | title = Phase-contrast X-ray computed tomography for observing biological soft tissues | journal = Nature Medicine | volume = 2 | issue = 4 | pages = 473–475 | date = April 1996 | pmid = 8597962 | doi = 10.1038/nm0496-473 | s2cid = 23523144 }}</ref> These methods provide higher contrast compared to normal absorption-based X-ray imaging, making it possible to distinguish from each other details that have almost similar density. A disadvantage is that these methods require more sophisticated equipment, such as [[synchrotron]] or [[X-ray tube#Microfocus X-ray tube|microfocus]] X-ray sources, [[X-ray optics]], and high resolution X-ray detectors.{{cn|date=December 2024}}

==Energy ranges==
[[File:X-ray applications.svg|thumb|upright=1.8|X-rays are part of the [[electromagnetic spectrum]], with wavelengths shorter than [[Ultraviolet|UV light]]. Different applications use different parts of the X-ray spectrum.]]

===Soft and hard X-rays===
X-rays with high [[photon energy|photon energies]] above 5–10&nbsp;keV (below 0.2–0.1&nbsp;nm wavelength) are called ''hard X-rays'', while those with lower energy (and longer wavelength) are called ''soft X-rays''.<ref name="Attwood">{{Cite book |author= Attwood, David |title= Soft X-rays and extreme ultraviolet radiation |publisher= Cambridge University |date= 1999 |isbn= 978-0-521-65214-8 |url= http://ast.coe.berkeley.edu/sxreuv/ |page= 2 |access-date= 4 November 2012 |archive-url= https://web.archive.org/web/20121111141255/http://ast.coe.berkeley.edu/sxreuv/ |archive-date= 2012-11-11 }}</ref> The intermediate range with photon energies of several keV is often referred to as ''tender X-rays''. Due to their penetrating ability, hard X-rays are widely used to image the inside of objects (e.g. in [[medical radiography]] and [[airport security]]). The term ''X-ray'' is [[metonymy|metonymically]] used to refer to a [[radiographic]] image produced using this method, in addition to the method itself. Since the wavelengths of hard X-rays are similar to the size of atoms, they are also useful for determining crystal structures by [[X-ray crystallography]]. By contrast, soft X-rays are easily absorbed in air; the [[attenuation length]] of 600&nbsp;eV (~2&nbsp;nm) X-rays in water is less than 1&nbsp;micrometer.<ref>{{Cite web |url=http://physics.nist.gov/cgi-bin/ffast/ffast.pl?Formula=H2O&gtype=5&range=S&lower=0.300&upper=2.00&density=1.00 |title=Physics.nist.gov |publisher=Physics.nist.gov |access-date=2011-11-08}}</ref>

===Gamma rays===
There is no consensus for a definition distinguishing between X-rays and [[gamma ray]]s. One common practice is to distinguish between the two types of radiation based on their source: X-rays are emitted by [[electron]]s, while gamma rays are emitted by the [[atomic nucleus]].<ref name="Dendy">{{Cite book | vauthors = Denny PP, Heaton B |title= Physics for Diagnostic Radiology |publisher= CRC Press |date= 1999 |location= US |page= 12 |url= https://books.google.com/books?id=1BTQvsQIs4wC&pg=PA12 |isbn= 978-0-7503-0591-4}}</ref><ref>{{Cite book | vauthors = Feynman R, Leighton R, Sands M |title= The Feynman Lectures on Physics | volume = 1 |publisher= Addison-Wesley |date= 1963 |location= US |pages= 2–8 |isbn= 978-0-201-02116-5}}</ref><ref>{{Cite book | vauthors = L'Annunziata M, Abrade M |title= Handbook of Radioactivity Analysis |publisher= Academic Press |date= 2003 |page= 58 |url= https://books.google.com/books?id=b519e10OPT0C&q=gamma+x-ray&pg=PA58 |isbn= 978-0-12-436603-9}}</ref><ref>{{Cite book | vauthors = Grupen C, Cowan G, Eidelman SD, Stroh T |title= Astroparticle Physics |publisher= Springer |date= 2005 |page= 109 |isbn= 978-3-540-25312-9}}</ref> This definition has several problems: other processes can also generate these high-energy [[photon]]s, or sometimes the method of generation is not known. One common alternative is to distinguish X- and gamma radiation on the basis of wavelength (or, equivalently, frequency or photon energy), with radiation shorter than some arbitrary wavelength, such as 10<sup>−11</sup>&nbsp;m (0.1&nbsp;[[Angstrom|Å]]), defined as gamma radiation.<ref>{{Cite book |editor= Hodgman, Charles |title= CRC Handbook of Chemistry and Physics, 44th Ed |publisher= Chemical Rubber Co. |date= 1961 |location= US |page= 2850}}</ref> This criterion assigns a photon to an unambiguous category, but is only possible if wavelength is known. (Some measurement techniques do not distinguish between detected wavelengths.) However, these two definitions often coincide since the electromagnetic radiation emitted by [[X-ray tube]]s generally has a longer wavelength and lower photon energy than the radiation emitted by [[radioactive]] [[atomic nucleus|nuclei]].<ref name="Dendy" /> Occasionally, one term or the other is used in specific contexts due to historical precedent, based on measurement (detection) technique, or based on their intended use rather than their wavelength or source.
Thus, gamma-rays generated for medical and industrial uses, for example [[radiotherapy]], in the ranges of 6–20&nbsp;[[MeV]], can in this context also be referred to as X-rays.<ref>{{Cite web |url=http://www.ccohs.ca/ |title=Radiation – Quantities and Units of Ionizing Radiation: OSH Answers |last=Government of Canada |first=Canadian Centre for Occupational Health and Safety |date=9 May 2019 |website=CCOHS.ca |access-date=2019-05-09 }}</ref>

==Properties==
[[File:Radioactive.svg|thumb|upright=0.55|Ionizing radiation hazard symbol]]

X-ray [[photon]]s carry enough energy to [[Ionization|ionize]] atoms and disrupt [[molecular bond]]s.<ref>{{Citation |last1=Paul |first1=Sudip |title=Chapter 4 - Radiological devices |date=2022-01-01 |work=Introduction to Biomedical Instrumentation and Its Applications |pages=169–211 |editor-last=Paul |editor-first=Sudip |url=https://linkinghub.elsevier.com/retrieve/pii/B9780128216743000048 |access-date=2025-02-19 |publisher=Academic Press |doi=10.1016/b978-0-12-821674-3.00004-8 |isbn=978-0-12-821674-3 |last2=Saikia |first2=Angana |last3=Majhi |first3=Vinayak |last4=Pandey |first4=Vinay Kumar |editor2-last=Saikia |editor2-first=Angana |editor3-last=Majhi |editor3-first=Vinayak |editor4-last=Pandey |editor4-first=Vinay Kumar}}</ref> This makes it a type of [[ionizing radiation]], and therefore harmful to living [[Tissue (biology)|tissue]]. A very high [[radiation dose]] over a short period of time causes burns and [[radiation sickness]], while lower doses can give an increased risk of [[radiation-induced cancer]]. In medical imaging, this increased cancer risk is generally greatly outweighed by the benefits of the examination.<ref>{{Cite journal |last=Lin |first=Eugene C. |date=2010 |title=Radiation Risk From Medical Imaging |journal=Mayo Clinic Proceedings |language=en |volume=85 |issue=12 |pages=1142–1146 |doi=10.4065/mcp.2010.0260 |pmc=2996147 |pmid=21123642}}</ref> The ionizing capability of X-rays can be used in [[Oncology|cancer treatment]] to kill [[malignant]] [[Cell (biology)|cells]] using [[radiation therapy]]. It is also used for material characterization using [[X-ray spectroscopy]].{{cn|date=December 2024}}

Hard X-rays can traverse relatively thick objects without being much [[Absorption (electromagnetic radiation)|absorbed]] or [[Scattering|scattered]]. For this reason, X-rays are widely used to [[Imaging science|image]] the inside of visually opaque objects. The most often seen applications are in medical [[radiography]] and [[airport security]] scanners, but similar techniques are also important in industry (e.g. [[industrial radiography]] and [[industrial CT scanning]]) and research (e.g. [[Preclinical imaging#Micro-CT|small animal CT]]). The [[penetration depth]] varies with several [[orders of magnitude]] over the X-ray spectrum. This allows the photon energy to be adjusted for the application so as to give sufficient [[Transmittance|transmission]] through the object and at the same time provide good [[contrast (vision)|contrast]] in the image.{{cn|date=December 2024}}

X-rays have much shorter wavelengths than visible light, which makes it possible to probe structures much smaller than can be seen using a normal [[microscope]]. This property is used in [[X-ray microscopy]] to acquire high-resolution images, and also in [[X-ray crystallography]] to determine the positions of [[atom]]s in [[crystal]]s.{{cn|date=December 2024}}

==Interaction with matter==
[[File:Attenuation.svg|thumb|Attenuation length of X-rays in water showing the oxygen [[absorption edge]] at 540&nbsp;eV, the energy<sup>−3</sup> dependence of [[Photoelectric effect|photoabsorption]], as well as a leveling off at higher photon energies due to [[Compton scattering]]. The attenuation length is about four orders of magnitude longer for hard X-rays (right half) compared to soft X-rays (left half).]]

X-rays interact with matter in three main ways, through [[Photoelectric effect|photoabsorption]], [[Compton scattering]], and [[Rayleigh scattering]]. The strength of these interactions depends on the energy of the X-rays and the elemental composition of the material, but not much on chemical properties, since the X-ray photon energy is much higher than chemical binding energies. Photoabsorption or photoelectric absorption is the dominant interaction mechanism in the soft X-ray regime and for the lower hard X-ray energies. At higher energies, Compton scattering dominates.{{cn|date=December 2024}}

===Photoelectric absorption===
The probability of a photoelectric absorption per unit mass is approximately proportional to <math display="inline">Z^3/E^3</math>, where <math display="inline">Z</math> is the [[atomic number]] and <math display="inline">E</math> is the energy of the incident photon.<ref>{{Cite book |author1=Bushberg, Jerrold T. |author2=Seibert, J. Anthony |author3=Leidholdt, Edwin M. |author4=Boone, John M. |title= The essential physics of medical imaging |publisher= Lippincott Williams & Wilkins |date= 2002 |isbn= 978-0-683-30118-2 |page= 42}}</ref> This rule is not valid close to inner shell electron binding energies where there are abrupt changes in interaction probability, so called [[absorption edges]]. However, the general trend of high [[absorption coefficient]]s and thus short [[penetration depth]]s for low photon energies and high atomic numbers is very strong. For soft tissue, photoabsorption dominates up to about 26&nbsp;keV photon energy where Compton scattering takes over. For higher atomic number substances, this limit is higher. The high amount of [[calcium]] (<math display="inline">Z=20</math>) in bones, together with their high density, is what makes them show up so clearly on medical radiographs.{{cn|date=December 2024}}

A photoabsorbed photon transfers all its energy to the electron with which it interacts, thus ionizing the atom to which the electron was bound and producing a photoelectron that is likely to ionize more atoms in its path. An outer electron will fill the vacant electron position and produce either a characteristic X-ray or an [[Auger electron]]. These effects can be used for elemental detection through [[X-ray spectroscopy]] or [[Auger electron spectroscopy]].{{cn|date=December 2024}}

===Compton scattering===
Compton scattering is the predominant interaction between X-rays and soft tissue in medical imaging.<ref>{{Cite book |author1=Bushberg, Jerrold T. |author2=Seibert, J. Anthony |author3=Leidholdt, Edwin M. |author4=Boone, John M. |title= The essential physics of medical imaging |publisher= Lippincott Williams & Wilkins |date= 2002 |isbn= 978-0-683-30118-2 |page= 38}}</ref> Compton scattering is an [[inelastic scattering]] of the X-ray photon by an outer shell electron. Part of the energy of the photon is transferred to the scattering electron, thereby ionizing the atom and increasing the wavelength of the X-ray. The scattered photon can go in any direction, but a direction similar to the original direction is more likely, especially for [[high-energy X-rays]]. The probability for different scattering angles is described by the [[Klein–Nishina formula]]. The transferred energy can be directly obtained from the scattering angle from the [[conservation of energy]] and [[conservation of momentum|momentum]].{{cn|date=December 2024}}

===Rayleigh scattering===
Rayleigh scattering is the dominant [[elastic scattering]] mechanism in the X-ray regime.<ref>{{Cite journal |url=http://adg.llnl.gov/Research/scattering/RTAB.html |title=RTAB: the Rayleigh scattering database |journal=Radiation Physics and Chemistry |volume=59 |issue=2 |pages=185–200 |publisher=Lynn Kissel |date=2 September 2000 |access-date=2012-11-08 |archive-url=https://web.archive.org/web/20111212054732/http://adg.llnl.gov/Research/scattering/RTAB.html |archive-date=2011-12-12 |bibcode=2000RaPC...59..185K | vauthors = Kissel L |doi=10.1016/S0969-806X(00)00290-5 }}</ref> Inelastic forward scattering gives rise to the refractive index, which for X-rays is only slightly below 1.<ref>{{Cite book |author= Attwood, David |title= Soft X-rays and extreme ultraviolet radiation |publisher= Cambridge University Press |date= 1999 |isbn= 978-0-521-65214-8 |chapter-url= http://ast.coe.berkeley.edu/sxreuv/ |chapter= 3 |access-date= 4 November 2012 |archive-url= https://web.archive.org/web/20121111141255/http://ast.coe.berkeley.edu/sxreuv/ |archive-date= 2012-11-11 }}</ref>

==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 | vauthors = Camara CG, Escobar JV, Hird JR, Putterman SJ |title=Correlation between nanosecond X-ray flashes and stick–slip friction in peeling tape |journal=Nature |date=2008 |volume=455 |pages=1089–1092 |doi=10.1038/nature07378 |url=http://homepage.usask.ca/~jrm011/X-ray_tape.pdf |access-date=2 February 2013 |bibcode= 2008Natur.455.1089C |issue=7216|s2cid=4372536 }}</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 |doi = 10.1016/j.atmosres.2013.03.012|title = Angular distribution of Bremsstrahlung photons and of positrons for calculations of terrestrial gamma-ray flashes and positron beams|journal = Atmospheric Research|volume = 135–136|pages = 432–465|date= 2014| vauthors = Köhn C, Ebert U |author2-link= Ute Ebert |url = https://ir.cwi.nl/pub/21633|arxiv = 1202.4879|bibcode = 2014AtmRe.135..432K|s2cid = 10679475}}</ref> This produces photons with energies of some few [[electronvolt|keV]] and several tens of MeV.<ref>{{Cite journal |doi = 10.1002/2014JD022229 |title = Calculation of beams of positrons, neutrons, and protons associated with terrestrial gamma ray flashes |journal = Journal of Geophysical Research: Atmospheres |volume = 120 |issue = 4 |pages = 1620–1635 |date= 2015 |vauthors = Köhn C, Ebert U |author2-link= Ute Ebert |url = https://ir.cwi.nl/pub/23845|bibcode = 2015JGRD..120.1620K|doi-access = free}}</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 |vauthors = Kochkin P, Köhn C, Ebert U, Van Deursen L |title = Analyzing x-ray emissions from meter-scale negative discharges in ambient air. |journal = Plasma Sources Science and Technology |date = May 2016 |volume = 25 |issue = 4 |pages = 044002 |doi = 10.1088/0963-0252/25/4/044002 |author3-link= Ute Ebert |bibcode = 2016PSST...25d4002K |s2cid=43609721 |url = https://ir.cwi.nl/pub/25086}}</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>

==Detectors==
{{Main|X-ray detector}}
X-ray detectors vary in shape and function depending on their purpose. Imaging detectors such as those used for [[radiography]] were originally based on [[photographic plate]]s and later [[photographic film]], but are now mostly replaced by various [[digital data|digital]] detector types such as [[image plate]]s and [[flat panel detectors]]. For [[radiation protection]] direct exposure hazard is often evaluated using [[ionization chamber]]s, while [[dosimeters]] are used to measure the [[radiation dose]] the person has been exposed to. X-ray [[energy spectrum|spectra]] can be measured either by energy dispersive or wavelength dispersive [[spectrometer]]s. For [[X-ray diffraction]] applications, such as [[X-ray crystallography]], [[hybrid pixel detector|hybrid photon counting detectors]] are widely used.<ref>{{cite journal | vauthors = Förster A, Brandstetter S, Schulze-Briese C | title = Transforming X-ray detection with hybrid photon counting detectors | journal = Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences | volume = 377 | issue = 2147 | pages = 20180241 | date = June 2019 | pmid = 31030653 | pmc = 6501887 | doi = 10.1098/rsta.2018.0241 | bibcode = 2019RSPTA.37780241F }}</ref>

==Medical uses==
{{More citations needed|section|date=November 2017}}<!--"Projectional radiographs" and "Fluoroscopy" sections have no citations-->
[[File:Hospital Radiology Room Philips DigitalDiagnost Digital Radiography System.jpg|thumb|Patient undergoing an X-ray exam in a hospital radiology room]]
[[File:Radiografía pulmones Francisca Lorca.cropped.jpg|thumb|A [[chest radiograph]] of a female patient, demonstrating a [[hiatal hernia]]]]

Since Röntgen's discovery that X-rays can identify bone structures, X-rays have been used for [[medical imaging]].<ref name=Thomas2007>{{cite journal |last1=Thomas |first1=Adrian M.K. |title=The first 50 years of military radiology 1895–1945 |journal=European Journal of Radiology |date=August 2007 |volume=63 |issue=2 |pages=214–219 |doi=10.1016/j.ejrad.2007.05.024 |pmid=17629432 |url=https://www.ejradiology.com/article/S0720-048X(07)00274-4/pdf|url-access=subscription}}</ref> The first medical use was less than a month after his paper on the subject.<ref name="PKS" /> Up to 2010, five&nbsp;billion medical imaging examinations had been conducted worldwide.<ref name="Roobottom CA, Mitchell G, Morgan-Hughes G 2010 859–67">{{cite journal | vauthors = Roobottom CA, Mitchell G, Morgan-Hughes G | title = Radiation-reduction strategies in cardiac computed tomographic angiography | journal = Clinical Radiology | volume = 65 | issue = 11 | pages = 859–867 | date = November 2010 | pmid = 20933639 | doi = 10.1016/j.crad.2010.04.021 | quote = Of the 5 billion imaging investigations performed worldwide... | doi-access = free }}</ref> Radiation exposure from medical imaging in 2006 made up about 50% of total ionizing radiation exposure in the United States.<ref name=Science09/>

===Projectional radiographs===
{{Main|Projectional radiography}}
[[File:Knee plain X-ray.jpg|thumb|upright|Plain radiograph of the right knee]]

[[Projectional radiography]] is the practice of producing two-dimensional images using X-ray radiation. Bones contain a high concentration of [[calcium]], which, due to its relatively high [[atomic number]], absorbs X-rays efficiently. This reduces the amount of X-rays reaching the detector in the shadow of the bones, making them clearly visible on the radiograph. The lungs and trapped gas also show up clearly because of lower absorption compared to tissue, while differences between tissue types are harder to see.<ref>{{Cite journal |last=Rhinehart |first=D. A. |date=December 1931 |title=Air and Gas in the Soft Tissues: A Radiologic Study |url=http://pubs.rsna.org/doi/10.1148/17.6.1158 |journal=Radiology |volume=17 |issue=6 |pages=1158–1170 |doi=10.1148/17.6.1158 |issn=0033-8419}}</ref>

Projectional radiographs are useful in the detection of [[pathology]] of the [[bone|skeletal system]] as well as for detecting some disease processes in [[soft tissue]]. Some notable examples are the very common [[chest radiograph|chest X-ray]], which can be used to identify lung diseases such as [[pneumonia]], lung cancer, or [[pulmonary edema]], and the [[abdominal x-ray]], which can detect [[bowel obstruction|bowel (or intestinal) obstruction]], free air (from visceral perforations), and free fluid (in [[ascites]]). X-rays may also be used to detect pathology such as [[gallstone]]s (which are rarely [[radiodensity|radiopaque]]) or [[kidney stone]]s which are often (but not always) visible. Traditional plain X-rays are less useful in the imaging of soft tissues such as the brain or [[muscle]]. One area where projectional radiographs are used extensively is in evaluating how an orthopedic [[Implant (medicine)|implant]], such as a knee, hip or shoulder replacement, is situated in the body with respect to the surrounding bone. This can be assessed in two dimensions from plain radiographs, or it can be assessed in three dimensions if a technique called '2D to 3D registration' is used. This technique purportedly negates projection errors associated with evaluating implant position from plain radiographs.<ref>{{cite journal | vauthors = Van Haver A, Kolk S, DeBoodt S, Valkering K, Verdonk P |title=Accuracy of total knee implant position assessment based on postoperative X-rays, registered to pre-operative CT-based 3D models |journal=Orthopaedic Proceedings |url=http://bjjprocs.boneandjoint.org.uk/content/99-B/SUPP_4/80 |volume=99-B |number=Supp 4 |date=2018}}</ref>

[[Dental radiography]] is commonly used in the diagnoses of common oral problems, such as [[dental caries|cavities]].

In medical diagnostic applications, the low energy (soft) X-rays are unwanted, since they are totally absorbed by the body, increasing the radiation dose without contributing to the image. Hence, a thin metal sheet, often of aluminium, called an [[X-ray filter]], is usually placed over the window of the X-ray tube, absorbing the low energy part in the spectrum. This is called ''hardening'' the beam since it shifts the center of the spectrum towards higher energy (or harder) X-rays.

To generate an image of the [[Cardiovascular system#Human cardiovascular system|cardiovascular system]], including the arteries and veins ([[angiography]]) an initial image is taken of the anatomical region of interest. A second image is then taken of the same region after an iodinated [[Radiocontrast agent|contrast agent]] has been injected into the blood vessels within this area. These two images are then digitally subtracted, leaving an image of only the iodinated contrast outlining the blood vessels. The [[Radiology|radiologist]] or surgeon then compares the image obtained to normal anatomical images to determine whether there is any damage or blockage of the vessel.

===Computed tomography===
{{Main|CT scan}}
[[File:Brain CT scan.jpg|thumb|Head [[X-ray computed tomography|CT scan]] ([[transverse plane]]) slice – a modern application of [[medical radiography]]]]

[[Computed tomography]] (CT scanning) is a medical imaging modality where [[tomography|tomographic images]] or slices of specific areas of the body are obtained from a large series of two-dimensional X-ray images taken in different directions.<ref name="ref1">{{Cite book | vauthors = Herman GT |author-link=Gabor Herman |title= Fundamentals of Computerized Tomography: Image Reconstruction from Projections |date= 2009 |publisher= Springer |edition=2nd |isbn=978-1-85233-617-2 }}</ref> These cross-sectional images can be combined into a [[three-dimensional space|three-dimensional]] image of the inside of the body.<ref name=":0">{{Citation |last1=Hermena |first1=Shady |title=CT-scan Image Production Procedures |date=2024 |work=StatPearls |url=http://www.ncbi.nlm.nih.gov/books/NBK574548/ |access-date=2024-04-20 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=34662062 |last2=Young |first2=Michael}}</ref> CT scans are a quicker and more cost effective imaging modality that can be used for diagnostic and therapeutic purposes in various medical disciplines.<ref name=":0" />

===Fluoroscopy===
{{Main|Fluoroscopy}}

[[Fluoroscopy]] is an imaging technique commonly used by physicians or [[radiation therapist]]s to obtain real-time moving images of the internal structures of a patient through the use of a fluoroscope.<ref name=":1">{{Cite journal |last=Davros |first=William J. |date=2007-04-01 |title=Fluoroscopy: basic science, optimal use, and patient/operator protection |url=https://www.sciencedirect.com/science/article/pii/S1084208X07000067 |journal=Techniques in Regional Anesthesia and Pain Management |series=Imaging for Interventional Management of Chronic Pain |volume=11 |issue=2 |pages=44–54 |doi=10.1053/j.trap.2007.02.005 |issn=1084-208X}}</ref> In its simplest form, a fluoroscope consists of an X-ray source and a fluorescent screen, between which a patient is placed. However, modern fluoroscopes couple the screen to an [[X-ray image intensifier]] and [[charge-coupled device|CCD]] [[video camera]] allowing the images to be recorded and played on a monitor. This method may use a contrast material. Examples include cardiac catheterization (to examine for [[coronary circulation|coronary artery blockages]]), embolization procedures (to stop bleeding during [[hemorrhoidal artery embolization]]), and barium swallow (to examine for [[esophageal disorder]]s and swallowing disorders). As of recent, modern fluoroscopy utilizes short bursts of x-rays, rather than a continuous beam, to effectively lower radiation exposure for both the patient and operator.<ref name=":1" />

===Radiotherapy===
The use of X-rays as a treatment is known as [[radiation therapy]] and is largely used for the management (including [[palliation]]) of cancer; it requires higher radiation doses than those received for imaging alone. X-rays beams are used for treating skin cancers using lower energy X-ray beams while higher energy beams are used for treating cancers within the body such as brain, lung, prostate, and breast.<ref>Advances in kilovoltage x-ray beam dosimetry in {{cite journal | vauthors = Hill R, Healy B, Holloway L, Kuncic Z, Thwaites D, Baldock C | title = Advances in kilovoltage x-ray beam dosimetry | journal = Physics in Medicine and Biology | volume = 59 | issue = 6 | pages = R183–R231 | date = March 2014 | pmid = 24584183 | doi = 10.1088/0031-9155/59/6/r183 | s2cid = 18082594 | bibcode = 2014PMB....59R.183H }}</ref><ref>{{cite journal | vauthors = Thwaites DI, Tuohy JB | title = Back to the future: the history and development of the clinical linear accelerator | journal = Physics in Medicine and Biology | volume = 51 | issue = 13 | pages = R343–R362 | date = July 2006 | pmid = 16790912 | doi = 10.1088/0031-9155/51/13/R20 | s2cid = 7672187 | bibcode = 2006PMB....51R.343T }}</ref>

==Adverse effects==
{{Globalize|section|US|date=September 2024}}
[[File:BabyXray.png|thumb|upright|left|[[Kidneys, ureters, and bladder x-ray|Abdominal radiograph]] of a pregnant woman]]

X-rays are a form of [[ionizing radiation]], and are classified as a [[carcinogen]] by both the World Health Organization's [[International Agency for Research on Cancer]] and the U.S. government.<ref name="Roobottom CA, Mitchell G, Morgan-Hughes G 2010 859–67" /><ref>{{Cite web |url=https://ntp.niehs.nih.gov/go/roc |archive-url=https://web.archive.org/web/20101209082016/https://ntp.niehs.nih.gov/ntp/roc/toc11.html|archive-date=2010-12-09|title=11th Report on Carcinogens |website=Ntp.niehs.nih.gov |access-date=2010-11-08}}</ref>  Diagnostic X-rays (primarily from CT scans due to the large dose used) increase the risk of developmental problems and cancer in those exposed.<ref>{{cite journal | vauthors = Hall EJ, Brenner DJ | title = Cancer risks from diagnostic radiology | journal = The British Journal of Radiology | volume = 81 | issue = 965 | pages = 362–378 | date = May 2008 | pmid = 18440940 | doi = 10.1259/bjr/01948454 }}</ref><ref>{{cite journal | vauthors = Brenner DJ | title = Should we be concerned about the rapid increase in CT usage? | journal = Reviews on Environmental Health | volume = 25 | issue = 1 | pages = 63–68 |date= 2010 | pmid = 20429161 | doi = 10.1515/REVEH.2010.25.1.63 | s2cid = 17264651 }}</ref><ref>{{cite journal | vauthors = De Santis M, Cesari E, Nobili E, Straface G, Cavaliere AF, Caruso A | title = Radiation effects on development | journal = Birth Defects Research. Part C, Embryo Today | volume = 81 | issue = 3 | pages = 177–182 | date = September 2007 | pmid = 17963274 | doi = 10.1002/bdrc.20099 }}</ref>  It is estimated that 0.4% of current cancers in the United States are due to [[computed tomography]] (CT scans) performed in the past and that this may increase to as high as 1.5–2% with 2007 rates of CT usage.<ref name="Brenner 2007" />

Experimental and epidemiological data currently do not support the proposition that there is a [[linear no-threshold model|threshold dose of radiation]] below which there is no increased risk of cancer.<ref>{{cite journal | vauthors = Upton AC | title = The state of the art in the 1990's: NCRP Report No. 136 on the scientific bases for linearity in the dose-response relationship for ionizing radiation | journal = Health Physics | volume = 85 | issue = 1 | pages = 15–22 | date = July 2003 | pmid = 12852466 | doi = 10.1097/00004032-200307000-00005 | bibcode = 2003HeaPh..85...15U | s2cid = 13301920 }}</ref> However, this is under increasing doubt.<ref>{{cite journal | vauthors = Calabrese EJ, Baldwin LA | title = Toxicology rethinks its central belief | journal = Nature | volume = 421 | issue = 6924 | pages = 691–692 | date = February 2003 | pmid = 12610596 | doi = 10.1038/421691a | url = http://www.cerrie.org/committee_papers/INFO_9-F.pdf | s2cid = 4419048 | bibcode = 2003Natur.421..691C | archive-url = https://web.archive.org/web/20110912031712/http://www.cerrie.org/committee_papers/INFO_9-F.pdf | archive-date = 12 September 2011 }}</ref> Cancer risk can start at the exposure of 1100 mGy.<ref>{{cite journal | vauthors = Oakley PA, Ehsani NN, Harrison DE | title = The Scoliosis Quandary: Are Radiation Exposures From Repeated X-Rays Harmful? | journal = Dose-Response | volume = 17 | issue = 2 | pages = 1559325819852810 | date = 1 April 2019 | pmid = 31217755 | pmc = 6560808 | doi = 10.1177/1559325819852810 }}</ref> It is estimated that the additional radiation from diagnostic X-rays will increase the average person's cumulative risk of getting cancer by age 75 by 0.6–3.0%.<ref>{{cite journal | vauthors = Berrington de González A, Darby S | title = Risk of cancer from diagnostic X-rays: estimates for the UK and 14 other countries | journal = Lancet | volume = 363 | issue = 9406 | pages = 345–351 | date = January 2004 | pmid = 15070562 | doi = 10.1016/S0140-6736(04)15433-0 | s2cid = 8516754 }}</ref> The amount of absorbed radiation depends upon the type of X-ray test and the body part involved.<ref name="Brenner 2007">{{cite journal | vauthors = Brenner DJ, Hall EJ | title = Computed tomography—an increasing source of radiation exposure | journal = The New England Journal of Medicine | volume = 357 | issue = 22 | pages = 2277–2284 | date = November 2007 | pmid = 18046031 | doi = 10.1056/NEJMra072149 | s2cid = 2760372 | url = https://repositorio.unal.edu.co/handle/unal/79492 }}</ref> CT and fluoroscopy entail higher doses of radiation than do plain X-rays.

To place the increased risk in perspective, a plain chest X-ray will expose a person to the same amount from [[background radiation]] that people are exposed to (depending upon location) every day over 10 days, while exposure from a dental X-ray is approximately equivalent to 1 day of environmental background radiation.<ref name="ACR">{{Cite web|publisher=Radiological Society of North America (RSNA) and American College of Radiology (ACR)|title=Radiation Dose in X-Ray and CT Exams|url=https://www.radiologyinfo.org/en/info/safety-xray|access-date=2022-01-24|website=RadiologyInfo.org }}</ref> Each such X-ray would add less than 1 per 1,000,000 to the lifetime cancer risk. An abdominal or chest CT would be the equivalent to 2–3 years of background radiation to the whole body, or 4–5 years to the abdomen or chest, increasing the lifetime cancer risk between 1 per 1,000 to 1 per 10,000.<ref name="ACR" /> This is compared to the roughly 40% chance of a US citizen developing cancer during their lifetime.<ref>{{Cite web |url=http://seer.cancer.gov/csr/1975_2006/browse_csr.php?section=2&page=sect_02_table.11.html#table1 |title=National Cancer Institute: Surveillance Epidemiology and End Results (SEER) data |publisher=Seer.cancer.gov |date=30 June 2010 |access-date=2011-11-08}}</ref> For instance, the effective dose to the torso from a CT scan of the chest is about 5&nbsp;mSv, and the absorbed dose is about 14&nbsp;mGy.<ref>{{Cite journal | vauthors = Caon M, Bibbo G, Pattison J |date=2000 |title= Monte Carlo calculated effective dose to teenage girls from computed tomography examinations |journal=Radiation Protection Dosimetry |volume=90 |issue=4 |pages=445–448 |doi=10.1093/oxfordjournals.rpd.a033172}}</ref> A head CT scan (1.5&nbsp;mSv, 64&nbsp;mGy)<ref name="nrpb2005">Shrimpton, P.C; Miller, H.C; Lewis, M.A; Dunn, M. [http://www.hpa.org.uk/web/HPAwebFile/HPAweb_C/1194947420292 Doses from Computed Tomography (CT) examinations in the UK – 2003 Review] {{webarchive |url=https://web.archive.org/web/20110922122151/http://www.hpa.org.uk/web/HPAwebFile/HPAweb_C/1194947420292 |date=22 September 2011 }}</ref> that is performed once with and once without contrast agent, would be equivalent to 40 years of background radiation to the head. Accurate estimation of effective doses due to CT is difficult with the estimation uncertainty range of about ±19% to ±32% for adult head scans depending upon the method used.<ref>{{cite journal | vauthors = Gregory KJ, Bibbo G, Pattison JE | title = On the Uncertainties in Effective Dose Estimates of Adult CT Head Scans | journal = Medical Physics | volume = 35 | issue = 8 | pages = 3501–3510 | date = August 2008 | pmid = 18777910 | doi = 10.1118/1.2952359 | bibcode = 2008MedPh..35.3501G }}</ref>

The risk of radiation is greater to a fetus, so in pregnant patients, the benefits of the investigation (X-ray) should be balanced with the potential hazards to the fetus.<ref>{{cite journal | vauthors = Giles D, Hewitt D, Stewart A, Webb J | title = Malignant disease in childhood and diagnostic irradiation in utero | journal = Lancet | volume = 271 | issue = 6940 | pages = 447 | date = September 1956 | pmid = 13358242 | doi = 10.1016/S0140-6736(56)91923-7 }}</ref><ref>{{Cite web |url=http://emedicinelive.com/index.php/Women-s-Health/pregnant-women-and-radiation-exposure.html |title=Pregnant Women and Radiation Exposure |date=28 December 2008 |website=eMedicine Live online medical consultation |publisher=[[Medscape]] |access-date=2009-01-16 |archive-url=https://web.archive.org/web/20090123034228/http://emedicinelive.com/index.php/Women-s-Health/pregnant-women-and-radiation-exposure.html |archive-date=23 January 2009}}</ref> If there is 1 scan in 9 months, it can be harmful to the fetus.<ref name="Doctor, will that x-ray harm my un">{{cite journal | vauthors = Ratnapalan S, Bentur Y, Koren G | title = "Doctor, will that x-ray harm my unborn child?" | journal = CMAJ | volume = 179 | issue = 12 | pages = 1293–1296 | date = December 2008 | pmid = 19047611 | pmc = 2585137 | doi = 10.1503/cmaj.080247 }}</ref> Therefore, women who are pregnant get ultrasounds as their diagnostic imaging because this does not use radiation.<ref name="Doctor, will that x-ray harm my un" /> If there is too much radiation exposure there could be harmful effects on the fetus or the reproductive organs of the mother.<ref name="Doctor, will that x-ray harm my un" /> In the US, there are an estimated 62&nbsp;million CT scans performed annually, including more than 4&nbsp;million on children.<ref name="Brenner 2007" /> Avoiding unnecessary X-rays (especially CT scans) reduces radiation dose and any associated cancer risk.<ref>{{cite journal | vauthors = Donnelly LF | title = Reducing radiation dose associated with pediatric CT by decreasing unnecessary examinations | journal = AJR. American Journal of Roentgenology | volume = 184 | issue = 2 | pages = 655–657 | date = February 2005 | pmid = 15671393 | doi = 10.2214/ajr.184.2.01840655 }}</ref>

Medical X-rays are a significant source of human-made radiation exposure. In 1987, they accounted for 58% of exposure from human-made sources in the United States. Since human-made sources accounted for only 18% of the total radiation exposure, most of which came from natural sources (82%), medical X-rays only accounted for 10% of ''total'' American radiation exposure; medical procedures as a whole (including [[nuclear medicine]]) accounted for 14% of total radiation exposure. By 2006, however, medical procedures in the United States were contributing much more ionizing radiation than was the case in the early 1980s. In 2006, medical exposure constituted nearly half of the total radiation exposure of the U.S. population from all sources. The increase is traceable to the growth in the use of medical imaging procedures, in particular [[computed tomography]] (CT), and to the growth in the use of nuclear medicine.<ref name=Science09>{{Cite web|title=Medical Radiation Exposure Of The U.S. Population Greatly Increased Since The Early 1980s|url=https://www.sciencedaily.com/releases/2009/03/090303125809.htm|access-date=2022-01-24|website=ScienceDaily }}</ref><ref>{{Cite book |last= US National Research Council |title= Health Risks from Low Levels of Ionizing Radiation, BEIR 7 phase 2 |publisher= National Academies Press |date= 2006 |pages= 5, fig.PS–2 |url= https://books.google.com/books?id=Uqj4OzBKlHwC&pg=PA5 |isbn= 978-0-309-09156-5}}, data credited to NCRP (US National Committee on Radiation Protection) 1987</ref>

[[File:Raybloc X-Ray Protective Viewing Window - 2023-10-03 - Andy Mabbett.jpg|thumb|An X-ray protective window at [[Birmingham Dental Hospital]], England. The maker's sticker states that it is equivalent to 2.24mm of lead at 150Kv. ]]

Dosage due to dental X-rays varies significantly depending on the procedure and the technology (film or digital). Depending on the procedure and the technology, a single dental X-ray of a human results in an exposure of 5 to 40&nbsp;μSv. A full mouth series of X-rays may result in an exposure of up to 60 (digital) to 180 (film) μSv, for a yearly average of up to 400&nbsp;μSv.<ref>{{cite web|url=http://www.new.ans.org/pi/resources/dosechart/|title=ANS / Public Information / Resources / Radiation Dose Calculator|access-date=2012-05-16|archive-date=2012-05-16|archive-url=https://web.archive.org/web/20120516085010/http://www.new.ans.org/pi/resources/dosechart/}}</ref><ref>{{Cite web|title=HOW DANGEROUS IS RADIATION?|url=http://www.phyast.pitt.edu/~blc/book/chapter5.html|access-date=2022-01-24|website=PhyAst.Pitt.edu}}</ref><ref>Muller, Richard. '' Physics for Future Presidents'', Princeton University Press, 2010</ref><ref>[http://www.doctorspiller.com/Dental%20_X-Rays.htm X-Rays] {{webarchive|url=https://web.archive.org/web/20070315211141/http://www.doctorspiller.com/Dental%20_X-Rays.htm |date=15 March 2007 }}. Doctorspiller.com (9 May 2007). Retrieved on 2011-05-05.</ref><ref>[http://www.dentalgentlecare.com/x-ray_safety.htm X-Ray Safety] {{webarchive |url=https://web.archive.org/web/20070404222105/http://www.dentalgentlecare.com/x-ray_safety.htm |date=4 April 2007 }}. Dentalgentlecare.com (6 February 2008). Retrieved on 2011-05-05.</ref><ref>{{Cite web |url=http://www.physics.isu.edu/radinf/dental.htm |title=Dental X-Rays |publisher=Idaho State University |access-date=7 November 2012 |archive-date=7 November 2012 |archive-url=https://web.archive.org/web/20121107120350/http://www.physics.isu.edu/radinf/dental.htm |url-status=dead }}</ref><ref>[http://www.oakridge.doe.gov/external/PublicActivities/EmergencyPublicInformation/AboutRadiation/tabid/319/Default.aspx D.O.E. – About Radiation] {{webarchive |url=https://web.archive.org/web/20120427175013/http://www.oakridge.doe.gov/external/PublicActivities/EmergencyPublicInformation/AboutRadiation/tabid/319/Default.aspx |date=27 April 2012 }}</ref>

Financial incentives have been shown to have a significant impact on X-ray use with doctors who are paid a separate fee for each X-ray providing more X-rays.<ref>{{cite journal | vauthors = Chalkley M, Listl S | title = First do no harm – The impact of financial incentives on dental X-rays | journal = Journal of Health Economics | volume = 58 | issue = March 2018 | pages = 1–9 | date = March 2018 | pmid = 29408150 | doi = 10.1016/j.jhealeco.2017.12.005 | doi-access = free | hdl = 2066/190628 | hdl-access = free }}</ref>

Early photon tomography or EPT<ref>{{cite web |url=https://www.open.edu/openlearn/body-mind/using-lasers-instead-x-rays |title=Using lasers instead of x-rays |publisher=Open University |date=24 February 2011 |access-date=28 July 2021}}</ref> (as of 2015) along with other techniques<ref>{{cite web |url=https://www.engadget.com/2015/02/12/visible-light-super-vision/ |title=Scientists achieve X-ray vision with safe, visible light |work=Engadget | vauthors = Dent S |date=12 February 2015 |access-date=28 July 2021}}</ref> are being researched as potential alternatives to X-rays for imaging applications.

==Other uses==
Other notable uses of X-rays include:
[[File:X-ray diffraction pattern 3clpro.jpg|thumb|upright|Each dot, called a reflection, in this diffraction pattern forms from the constructive interference of scattered X-rays passing through a crystal. The data can be used to determine the crystalline structure.]]{{More citations needed|date=January 2025}}
* [[X-ray crystallography]] in which the pattern produced by the [[diffraction]] of X-rays through the closely spaced lattice of atoms in a crystal is recorded and then analysed to reveal the nature of that lattice. A related technique, [[fiber diffraction]], was used by [[Rosalind Franklin]] to discover the [[double helix|double helical]] structure of [[DNA]].<ref>{{Cite book | vauthors = Kasai N, Kakudo, M |title= X-ray diffraction by macromolecules |publisher= Kodansha |date= 2005 |location= Tokyo |pages= 291–2 |url= https://books.google.com/books?id=_y5hM5_vx0EC&pg=PA291 |isbn= 978-3-540-25317-4}}</ref>
* [[X-ray astronomy]], which is an observational branch of [[astronomy]], which deals with the study of X-ray emission from celestial objects.{{cn|date=December 2024}}
* [[X-ray microscope|X-ray microscopic]] analysis, which uses [[electromagnetic radiation]] in the soft X-ray band to produce images of very small objects.{{cn|date=December 2024}}
* [[X-ray fluorescence]], a technique in which X-rays are generated within a specimen and detected. The outgoing energy of the X-ray can be used to identify the composition of the sample.{{cn|date=December 2024}}
* [[Industrial radiography]] uses X-rays for inspection of industrial parts, particularly [[welding|welds]].{{cn|date=December 2024}}
* [[Radiography of cultural objects]], most often X-rays of paintings to reveal [[underdrawing]], [[pentimenti]] alterations in the course of painting or by later restorers, and sometimes previous paintings on the support. Many [[pigment]]s such as [[lead white]] show well in radiographs.{{cn|date=December 2024}}
* X-ray spectromicroscopy has been used to analyse the reactions of pigments in paintings. For example, in analysing colour degradation in the paintings of [[Vincent van Gogh|van Gogh]].<ref name="Analytical Chemistry">{{cite journal | vauthors = Monico L, Van der Snickt G, Janssens K, De Nolf W, Miliani C, Verbeeck J, Tian H, Tan H, Dik J, Radepont M, Cotte M | display-authors = 6 | title = Degradation process of lead chromate in paintings by Vincent van Gogh studied by means of synchrotron X-ray spectromicroscopy and related methods. 1. Artificially aged model samples | journal = Analytical Chemistry | volume = 83 | issue = 4 | pages = 1214–1223 | date = February 2011 | pmid = 21314201 | doi = 10.1021/ac102424h }} {{cite journal | vauthors = Monico L, Van der Snickt G, Janssens K, De Nolf W, Miliani C, Dik J, Radepont M, Hendriks E, Geldof M, Cotte M | display-authors = 6 | title = Degradation process of lead chromate in paintings by Vincent van Gogh studied by means of synchrotron X-ray spectromicroscopy and related methods. 2. Original paint layer samples | journal = Analytical Chemistry | volume = 83 | issue = 4 | pages = 1224–1231 | date = February 2011 | pmid = 21314202 | doi = 10.1021/ac1025122 }}</ref>
[[File:Using X-ray for authentication and quality control in electronics industry.jpg|thumb|Using X-ray for inspection and quality control: the differences in the structures of the die and bond wires reveal the left chip to be counterfeit.<ref>{{cite conference |vauthors=Ahi K, Anwar M |title=Advanced terahertz techniques for quality control and counterfeit detection |veditors=Anwar MF, Crowe TW, Manzur T |book-title=Terahertz Physics, Devices, and Systems X: Advanced Applications in Industry and Defense | date = May 2016 | volume = 9856 |pages=31–44 |publisher=Society of Photographic Instrumentation Engineers}}</ref>]]
* Authentication and quality control of packaged items.
* [[Industrial CT]] (computed tomography), a process that uses X-ray equipment to produce three-dimensional representations of components both externally and internally. This is accomplished through computer processing of projection images of the scanned object in many directions.{{cn|date=December 2024}}
* [[Airport security]] luggage scanners use X-rays for inspecting the interior of luggage for security threats before loading on aircraft.{{cn|date=December 2024}}
* {{visible anchor|Border control|text=[[Border control]]}} truck scanners and [[NYPD X-ray vans|domestic police departments]] use X-rays for inspecting the interior of trucks.{{cn|date=December 2024}}
[[File:X-RayOfNeedlefish-1.jpg|thumb|upright|X-ray fine art photography of [[needlefish]] by [[Peter Dazeley]]]]
* X-ray art and [[fine art photography]], artistic use of X-rays, for example the works by [[Stane Jagodič]]{{cn|date=December 2024}}
* X-ray [[hair removal]], a method popular in the 1920s but now banned by the FDA.<ref name="milday">{{Cite book |url=https://books.google.com/books?id=oAFSBBLU_UUC&pg=PT250 |title=Milady's Hair Removal Techniques: A Comprehensive Manual |author=Bickmore, Helen |isbn=978-1401815554 |date=2003 |publisher=Thomson Delmar Learning }}</ref>
* [[Shoe-fitting fluoroscope]]s were popularized in the 1920s, banned in the US in the 1960s, in the UK in the 1970s, and later in continental Europe.{{cn|date=December 2024}}
* [[Roentgen stereophotogrammetry]] is used to track movement of bones based on the implantation of markers{{cn|date=December 2024}}
* [[X-ray photoelectron spectroscopy]] is a chemical analysis technique relying on the [[photoelectric effect]], usually employed in [[surface science]]{{cn|date=December 2024}}.
* [[Radiation implosion]] is the use of high energy X-rays generated from a fission explosion (an [[A-bomb]]) to compress nuclear fuel to the point of fusion ignition (an [[H-bomb]]).{{Citation needed|date=February 2025}}
* In 2024, Saw Wai Hla of the U.S. Department of Energy (DOE) Argonne National Laboratory was selected by the Falling Walls Foundation as the Science Breakthrough of the Year award winner in the physical sciences category for his team's work on the use of X-rays in single atom-molecule manipulation.<ref>{{Cite web |date=2024-09-25 |title=Argonne physicist honored with 2024 Science Breakthrough of the Year |url=https://www.anl.gov/article/argonne-physicist-honored-with-2024-science-breakthrough-of-the-year |access-date=2025-02-22 |website=Argonne National Laboratory |language=en}}</ref>

==Visibility==
While generally considered invisible to the human eye, in special circumstances X-rays can be visible. Brandes, in an experiment a short time after Röntgen's landmark 1895 paper, reported after dark adaptation and placing his eye close to an X-ray tube, seeing a faint "blue-gray" glow which seemed to originate within the eye itself.<ref>{{Cite web | vauthors = Frame P |title= Wilhelm Röntgen and the Invisible Light |website= Tales from the Atomic Age |publisher= Oak Ridge Associated Universities |url= https://www.orau.org/health-physics-museum/articles/wilhelm-rontgen-invisible-light.html |access-date= 11 October 2021}}</ref> Upon hearing this, Röntgen reviewed his record books and found he too had seen the effect. When placing an X-ray tube on the opposite side of a wooden door Röntgen had noted the same blue glow, seeming to emanate from the eye itself, but thought his observations to be spurious because he only saw the effect when he used one type of tube. Later he realized that the tube which had created the effect was the only one powerful enough to make the glow plainly visible and the experiment was thereafter readily repeatable. The knowledge that X-rays are actually faintly visible to the dark-adapted naked eye has largely been forgotten today; this is probably due to the desire not to repeat what would now be seen as a recklessly dangerous and potentially harmful experiment with [[ionizing radiation]]. It is not known what exact mechanism in the eye produces the visibility: it could be due to conventional detection (excitation of [[rhodopsin]] molecules in the retina), direct excitation of retinal nerve cells, or secondary detection via, for instance, X-ray induction of [[phosphorescence]] in the eyeball with conventional retinal detection of the secondarily produced visible light.{{cn|date=December 2024}}

Though X-rays are otherwise invisible, it is possible to see the [[ionization]] of the air molecules if the intensity of the X-ray beam is high enough. The beamline from the [[Wiggler (synchrotron)|wiggler]] at the  [[European Synchrotron Radiation Facility]]<ref>[https://web.archive.org/web/20081114111144/http://www.esrf.eu/UsersAndScience/Experiments/MaterialsScience/faisceau European Synchrotron Radiation Facility ID11]</ref> is one example of such high intensity.<ref>{{Cite book |author1=Als-Nielsen, Jens  |author2=Mcmorrow, Des |title= Elements of Modern X-Ray Physics |publisher= John Wiley & Sons Ltd |date= 2001 |pages= 40–41 |isbn= 978-0-471-49858-2}}</ref>

==Units of measure and exposure==
The measure of X-rays [[ionization|ionizing]] ability is called the exposure:{{cn|date=December 2024}}
* The [[coulomb]] per kilogram (C/kg) is the [[Systeme International|SI]] unit of [[ionizing radiation]] exposure, and it is the amount of radiation required to create one coulomb of charge of each polarity in one kilogram of matter.
* The [[Roentgen (unit)|roentgen]] (R) is an obsolete traditional unit of exposure, which represented the amount of radiation required to create one [[electrostatic unit]] of charge of each polarity in one cubic centimeter of dry air. 1&nbsp;roentgen&nbsp;=&nbsp;{{val|2.58|e=−4|u=C/kg}}.

However, the effect of ionizing radiation on matter (especially living tissue) is more closely related to the amount of energy deposited into them rather than the [[electric charge|charge]] generated. This measure of energy absorbed is called the [[absorbed dose]]:{{cn|date=December 2024}}
* The [[gray (unit)|gray]] (Gy), which has units of (joules/kilogram), is the SI unit of [[absorbed dose]], and it is the amount of radiation required to deposit one [[joule]] of energy in one kilogram of any kind of matter.
* The [[rad (unit)|rad]] is the (obsolete) corresponding traditional unit, equal to 10&nbsp;millijoules of energy deposited per kilogram. 100&nbsp;rad&nbsp;=&nbsp;1&nbsp;gray.

The [[equivalent dose]] is the measure of the biological effect of radiation on human tissue. For X-rays it is equal to the [[absorbed dose]].{{cn|date=December 2024}}
* The [[Roentgen equivalent man]] (rem) is the traditional unit of equivalent dose. For X-rays it is equal to the [[Rad (unit)|rad]], or, in other words, 10&nbsp;millijoules of energy deposited per kilogram. 100&nbsp;rem&nbsp;=&nbsp;1&nbsp;Sv.
* The [[sievert]] (Sv) is the SI unit of [[equivalent dose]], and also of [[Effective dose (radiation)|effective dose]]. For X-rays the "equivalent dose" is numerically equal to a [[Gray (unit)|Gray]] (Gy). 1&nbsp;Sv&nbsp;=&nbsp;1&nbsp;Gy. For the "effective dose" of X-rays, it is usually not equal to the Gray (Gy).
{{Radiation related quantities}}

== See also ==
{{Portal|Medical|Physics}}
* {{annotated link|Backscatter X-ray}}
* {{annotated link|Neutron radiation}}
* {{annotated link|NuSTAR}}
* {{annotated link|Radiographer}}
* {{annotated link|Resonant inelastic X-ray scattering}}
* {{annotated link|Small-angle X-ray scattering}}
* {{annotated link|X-ray absorption spectroscopy}}
* {{annotated link|X-ray reflectivity}}
* {{annotated link|X-ray vision}}
* {{annotated link|X-ray welding}}

== References ==
{{Reflist}}

== External links ==
{{Sister project links|auto=1|wikt=x-ray}}
* {{cite journal |title=On a New Kind of Rays |journal=Nature |date=January 1896 |volume=53 |issue=1369 |pages=274–276 |doi=10.1038/053274b0|bibcode=1896Natur..53R.274. |doi-access=free}}
* {{cite web |title=Ion X-Ray tubes |url=http://www.crtsite.com/page5.html |website=The Cathode Ray Tube site}}
* {{cite web |title=Index of Early Bremsstrahlung Articles |url=http://shadetreephysics.com/bremindx.htm |website=Shade Tree Physics |date=12 April 2010}}
* {{cite web | vauthors = Samuel JJ |title=La découverte des rayons X par Röntgen |url= http://www.bibnum.education.fr/physique/electricite-electromagnetisme/la-decouverte-des-rayons-x-par-roentgen |website=Bibnum Education |language=fr |date=20 October 2013}} [http://www.bibnum.education.fr/sites/default/files/rontgen-analysis.pdf Röntgen's discovery of X-rays] (PDF; English translation)
* Oakley, P. A., Navid Ehsani, N., & Harrison, D. E. (2020). 5 Reasons Why Scoliosis X-Rays Are Not Harmful. Dose-Response. https://doi.org/10.1177/1559325820957797
* {{cite web |title=X-ray Crystallography |url=https://www.xtal.iqf.csic.es/Cristalografia/index-en.html |website=A web for learning how X-rays can "see" inside the crystals}}

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[[Category:X-rays| ]]
[[Category:1895 in Germany]]
[[Category:1895 in science]]
[[Category:Electromagnetic spectrum]]
[[Category:IARC Group 1 carcinogens]]
[[Category:Ionizing radiation]]
[[Category:Medical physics]]
[[Category:Radiography]]
[[Category:Wilhelm Röntgen]]