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This article is about the nature, production, and uses of the radiation. X-ray_sentence_0

For the method of imaging, see Radiography. X-ray_sentence_1

For the medical specialty, see Radiology. X-ray_sentence_2

For other meanings, see X-ray (disambiguation). X-ray_sentence_3

Not to be confused with X-wave or X-band. X-ray_sentence_4

An X-ray, or X-radiation, is a penetrating form of high-energy electromagnetic radiation. X-ray_sentence_5

Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3×10 Hz to 3×10 Hz) and energies in the range 124 eV to 124 keV. X-ray_sentence_6

X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays. X-ray_sentence_7

In many languages, X-radiation is referred to as Röntgen radiation, after the German scientist Wilhelm Röntgen, who discovered it on November 8, 1895. X-ray_sentence_8

He named it X-radiation to signify an unknown type of radiation. X-ray_sentence_9

Spellings of X-ray(s) in English include the variants x-ray(s), xray(s), and X ray(s). X-ray_sentence_10

History X-ray_section_0

Pre-Röntgen observations and research X-ray_section_1

Before their discovery in 1895, X-rays were just a type of unidentified radiation emanating from experimental discharge tubes. X-ray_sentence_11

They were noticed by scientists investigating cathode rays produced by such tubes, which are energetic electron beams that were first observed in 1869. X-ray_sentence_12

Many of the early Crookes tubes (invented around 1875) undoubtedly radiated X-rays, because early researchers noticed effects that were attributable to them, as detailed below. X-ray_sentence_13

Crookes tubes created free electrons by ionization of the residual air in the tube by a high DC voltage of anywhere between a few kilovolts and 100 kV. X-ray_sentence_14

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. X-ray_sentence_15

The earliest experimenter thought to have (unknowingly) produced X-rays was actuary William Morgan. X-ray_sentence_16

In 1785 he presented a paper to the Royal Society of London describing the effects of passing electrical currents through a partially evacuated glass tube, producing a glow created by X-rays. X-ray_sentence_17

This work was further explored by Humphry Davy and his assistant Michael Faraday. X-ray_sentence_18

When Stanford University physics professor Fernando Sanford created his "electric photography" he also unknowingly generated and detected X-rays. X-ray_sentence_19

From 1886 to 1888 he had studied in the Hermann Helmholtz laboratory in Berlin, where he became familiar with the cathode rays generated in vacuum tubes when a voltage was applied across separate electrodes, as previously studied by Heinrich Hertz and Philipp Lenard. X-ray_sentence_20

His letter of January 6, 1893 (describing his discovery as "electric photography") to The Physical Review was duly published and an article entitled Without Lens or Light, Photographs Taken With Plate and Object in Darkness appeared in the San Francisco Examiner. X-ray_sentence_21

Starting in 1888, Philipp Lenard conducted experiments to see whether cathode rays could pass out of the Crookes tube into the air. X-ray_sentence_22

He built a Crookes tube with a "window" in the end made of thin aluminum, facing the cathode so the cathode rays would strike it (later called a "Lenard tube"). X-ray_sentence_23

He found that something came through, that would expose photographic plates and cause fluorescence. X-ray_sentence_24

He measured the penetrating power of these rays through various materials. X-ray_sentence_25

It has been suggested that at least some of these "Lenard rays" were actually X-rays. X-ray_sentence_26

In 1889 Ukrainian-born Ivan Puluj, a lecturer in experimental physics at the Prague Polytechnic who since 1877 had been constructing various designs of gas-filled tubes to investigate their properties, published a paper on how sealed photographic plates became dark when exposed to the emanations from the tubes. X-ray_sentence_27

Hermann von Helmholtz formulated mathematical equations for X-rays. X-ray_sentence_28

He postulated a dispersion theory before Röntgen made his discovery and announcement. X-ray_sentence_29

It was formed on the basis of the electromagnetic theory of light. X-ray_sentence_30

However, he did not work with actual X-rays. X-ray_sentence_31

In 1894 Nikola Tesla noticed damaged film in his lab that seemed to be associated with Crookes tube experiments and began investigating this radiant energy of "invisible" kinds. X-ray_sentence_32

After Röntgen identified the X-ray, Tesla began making X-ray images of his own using high voltages and tubes of his own design, as well as Crookes tubes. X-ray_sentence_33

Discovery by Röntgen X-ray_section_2

On November 8, 1895, German physics professor Wilhelm Röntgen stumbled on X-rays while experimenting with Lenard tubes and Crookes tubes and began studying them. X-ray_sentence_34

He wrote an initial report "On a new kind of ray: A preliminary communication" and on December 28, 1895 submitted it to Würzburg's Physical-Medical Society journal. X-ray_sentence_35

This was the first paper written on X-rays. X-ray_sentence_36

Röntgen referred to the radiation as "X", to indicate that it was an unknown type of radiation. X-ray_sentence_37

The name stuck, although (over Röntgen's great objections) many of his colleagues suggested calling them Röntgen rays. X-ray_sentence_38

They are still referred to as such in many languages, including German, Hungarian, Ukrainian, Danish, Polish, Bulgarian, Swedish, Finnish, Estonian, Turkish, Russian, Latvian, Japanese, Dutch, Georgian, Hebrew and Norwegian. X-ray_sentence_39

Röntgen received the first Nobel Prize in Physics for his discovery. X-ray_sentence_40

There are conflicting accounts of his discovery because Röntgen had his lab notes burned after his death, but this is a likely reconstruction by his biographers: 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. X-ray_sentence_41

He noticed a faint green glow from the screen, about 1 meter away. X-ray_sentence_42

Röntgen realized some invisible rays coming from the tube were passing through the cardboard to make the screen glow. X-ray_sentence_43

He found they could also pass through books and papers on his desk. X-ray_sentence_44

Röntgen threw himself into investigating these unknown rays systematically. X-ray_sentence_45

Two months after his initial discovery, he published his paper. X-ray_sentence_46

Röntgen discovered their medical use when he made a picture of his wife's hand on a photographic plate formed due to X-rays. X-ray_sentence_47

The photograph of his wife's hand was the first photograph of a human body part using X-rays. X-ray_sentence_48

When she saw the picture, she said "I have seen my death." X-ray_sentence_49

The discovery of X-rays stimulated a veritable sensation. X-ray_sentence_50

Röntgen's biographer Otto Glasser estimated that, in 1896 alone, as many as 49 essays and 1044 articles about the new rays were published. X-ray_sentence_51

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 dedicating as many as 23 articles to it in that year alone. X-ray_sentence_52

Sensationalist reactions to the new discovery included publications linking the new kind of rays to occult and paranormal theories, such as telepathy. X-ray_sentence_53

Advances in radiology X-ray_section_3

Röntgen immediately noticed X-rays could have medical applications. X-ray_sentence_54

Along with his 28 December Physical-Medical Society submission he sent a letter to physicians he knew around Europe (January 1, 1896). X-ray_sentence_55

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 (of a hand). X-ray_sentence_56

Through February there were 46 experimenters taking up the technique in North America alone. X-ray_sentence_57

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. X-ray_sentence_58

On February 14, 1896 Hall-Edwards was also the first to use X-rays in a surgical operation. X-ray_sentence_59

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". X-ray_sentence_60

The first medical X-ray made in the United States was obtained using a discharge tube of Pului's design. X-ray_sentence_61

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 Pului tube produced X-rays. X-ray_sentence_62

This was a result of Pului's inclusion of an oblique "target" of mica, used for holding samples of fluorescent material, within the tube. X-ray_sentence_63

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 gelatin photographic plates obtained from Howard Langill, a local photographer also interested in Röntgen's work. X-ray_sentence_64

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. X-ray_sentence_65

Röntgen used a screen coated with barium platinocyanide. X-ray_sentence_66

On February 5, 1896 live imaging devices were developed by both Italian scientist Enrico Salvioni (his "cryptoscope") and Professor McGie of Princeton University (his "Skiascope"), both using barium platinocyanide. X-ray_sentence_67

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. X-ray_sentence_68

In May 1896 he developed the first mass-produced live imaging device, his "Vitascope", later called the fluoroscope, which became the standard for medical X-ray examinations. X-ray_sentence_69

Edison dropped X-ray research around 1903, before the death of Clarence Madison Dally, one of his glassblowers. X-ray_sentence_70

Dally had a habit of testing X-ray tubes on his own hands, developing a cancer in them so tenacious that both arms were amputated in a futile attempt to save his life; in 1904, he became the first known death attributed to X-ray exposure. X-ray_sentence_71

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. X-ray_sentence_72

In 1901, U.S. X-ray_sentence_73 President William McKinley was shot twice in an assassination attempt. X-ray_sentence_74

While one bullet only grazed his sternum, another had lodged somewhere deep inside his abdomen and could not be found. X-ray_sentence_75

A worried McKinley aide sent word to inventor Thomas Edison to rush an X-ray machine to Buffalo to find the stray bullet. X-ray_sentence_76

It arrived but was not used. X-ray_sentence_77

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. X-ray_sentence_78

Hazards discovered X-ray_section_4

With the widespread experimentation with x‑rays after their discovery in 1895 by scientists, physicians, and inventors came many stories of burns, hair loss, and worse in technical journals of the time. X-ray_sentence_79

In February 1896, Professor John Daniel and Dr. William Lofland Dudley of Vanderbilt University reported hair loss after Dr. Dudley was X-rayed. X-ray_sentence_80

A child who had been shot in the head was brought to the Vanderbilt laboratory in 1896. X-ray_sentence_81

Before trying to find the bullet an experiment was attempted, for which Dudley "with his characteristic devotion to science" volunteered. X-ray_sentence_82

Daniel reported that 21 days after taking a picture of Dudley's skull (with an exposure time of one hour), he noticed a bald spot 2 inches (5.1 cm) 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. X-ray_sentence_83

The tube was fastened at the other side at a distance of one-half inch from the hair." X-ray_sentence_84

In August 1896 Dr. HD. X-ray_sentence_85

Hawks, a graduate of Columbia College, suffered severe hand and chest burns from an x-ray demonstration. X-ray_sentence_86

It was reported in Electrical Review and led to many other reports of problems associated with x-rays being sent in to the publication. X-ray_sentence_87

Many experimenters including Elihu Thomson at Edison's lab, William J. Morton, and Nikola Tesla also reported burns. X-ray_sentence_88

Elihu Thomson deliberately exposed a finger to an x-ray tube over a period of time and suffered pain, swelling, and blistering. X-ray_sentence_89

Other effects were sometimes blamed for the damage including ultraviolet rays and (according to Tesla) ozone. X-ray_sentence_90

Many physicians claimed there were no effects from X-ray exposure at all. X-ray_sentence_91

On August 3, 1905 in San Francisco, California, Elizabeth Fleischman, American X-ray pioneer, died from complications as a result of her work with X-rays. X-ray_sentence_92

20th century and beyond X-ray_section_5

The many applications of X-rays immediately generated enormous interest. X-ray_sentence_93

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. X-ray_sentence_94

A typical early 20th century medical x-ray system consisted of a Ruhmkorff coil connected to a cold cathode Crookes X-ray tube. X-ray_sentence_95

A spark gap was typically connected to the high voltage side in parallel to the tube and used for diagnostic purposes. X-ray_sentence_96

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. X-ray_sentence_97

To detect the hardness of the tube, the spark gap was initially opened to the widest setting. X-ray_sentence_98

While the coil was operating, the operator reduced the gap until sparks began to appear. X-ray_sentence_99

A tube in which the spark gap began to spark at around 2 1/2 inches was considered soft (low vacuum) and suitable for thin body parts such as hands and arms. X-ray_sentence_100

A 5-inch spark indicated the tube was suitable for shoulders and knees. X-ray_sentence_101

A 7-9 inch spark would indicate a higher vacuum suitable for imaging the abdomen of larger individuals. X-ray_sentence_102

Since the spark gap was connected in parallel to the tube, the spark gap had to be opened until the sparking ceased in order to operate the tube for imaging. X-ray_sentence_103

Exposure time for photographic plates was around half a minute for a hand to a couple of minutes for a thorax. X-ray_sentence_104

The plates may have a small addition of fluorescent salt to reduce exposure times. X-ray_sentence_105

Crookes tubes were unreliable. X-ray_sentence_106

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. X-ray_sentence_107

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. X-ray_sentence_108

Larger and more frequently used tubes were provided with devices for restoring the air, known as "softeners". X-ray_sentence_109

These often took the form of a small side tube which contained a small piece of mica, a mineral that traps relatively large quantities of air within its structure. X-ray_sentence_110

A small electrical heater heated the mica, causing it to release a small amount of air, thus restoring the tube's efficiency. X-ray_sentence_111

However, the mica had a limited life, and the restoration process was difficult to control. X-ray_sentence_112

In 1904, John Ambrose Fleming invented the thermionic diode, the first kind of vacuum tube. X-ray_sentence_113

This used a hot cathode that caused an electric current to flow in a vacuum. X-ray_sentence_114

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. X-ray_sentence_115

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. X-ray_sentence_116

He won the 1917 Nobel Prize in Physics for this discovery. X-ray_sentence_117

In 1912, Max von Laue, Paul Knipping, and Walter Friedrich first observed the diffraction of X-rays by crystals. X-ray_sentence_118

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. X-ray_sentence_119

In 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. X-ray_sentence_120

The Coolidge X-ray tube was invented the same year by William D. Coolidge. X-ray_sentence_121

It made possible the continuous emissions of X-rays. X-ray_sentence_122

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. X-ray_sentence_123

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. X-ray_sentence_124

Then in 1908, he had to have his left arm amputated because of the spread of X-ray dermatitis on his arm. X-ray_sentence_125

Medical science also used the motion picture to study human physiology. X-ray_sentence_126

In 1913, a motion picture was made in Detroit showing a hard-boiled egg inside a human stomach. X-ray_sentence_127

This early x-ray movie was recorded at a rate of one still image every four seconds. X-ray_sentence_128

Dr Lewis Gregory Cole of New York was a pioneer of the technique, which he called "serial radiography". X-ray_sentence_129

In 1918, x-rays were used in association with motion picture cameras to capture the human skeleton in motion. X-ray_sentence_130

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. X-ray_sentence_131

In 1914 Marie Curie developed radiological cars to support soldiers injured in World War I. X-ray_sentence_132

The cars would allow for rapid X-ray imaging of wounded soldiers so battlefield surgeons could quickly and more accurately operate. X-ray_sentence_133

From the early 1920s through to the 1950s, X-ray machines were developed to assist in the fitting of shoes and were sold to commercial shoe stores. X-ray_sentence_134

Concerns regarding the impact of frequent or poorly controlled use were expressed in the 1950s, leading to the practice's eventual end that decade. X-ray_sentence_135

The X-ray microscope was developed during the 1950s. X-ray_sentence_136

The Chandra X-ray Observatory, launched on July 23, 1999, has been allowing the exploration of the very violent processes in the universe which produce X-rays. X-ray_sentence_137

Unlike visible light, which gives a relatively stable view of the universe, the X-ray universe is unstable. X-ray_sentence_138

It features stars being torn apart by black holes, galactic collisions, and novae, and neutron stars that build up layers of plasma that then explode into space. X-ray_sentence_139

An X-ray laser device was proposed as part of the 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. X-ray_sentence_140

For technical and political reasons, the overall project (including the X-ray laser) was de-funded (though was later revived by the second Bush Administration as National Missile Defense using different technologies). X-ray_sentence_141

Phase-contrast X-ray imaging refers to a variety of techniques that use phase information of a coherent X-ray beam to image soft tissues. X-ray_sentence_142

It has become an important method for visualizing cellular and histological structures in a wide range of biological and medical studies. X-ray_sentence_143

There are several technologies being used for X-ray phase-contrast imaging, all utilizing different principles to convert phase variations in the X-rays emerging from an object into intensity variations. X-ray_sentence_144

These include propagation-based phase contrast, talbot interferometry, refraction-enhanced imaging, and X-ray interferometry. X-ray_sentence_145

These methods provide higher contrast compared to normal absorption-contrast X-ray imaging, making it possible to see smaller details. X-ray_sentence_146

A disadvantage is that these methods require more sophisticated equipment, such as synchrotron or microfocus X-ray sources, X-ray optics, and high resolution X-ray detectors. X-ray_sentence_147

Energy ranges X-ray_section_6

Soft and hard X-rays X-ray_section_7

X-rays with high photon energies (above 5–10 keV, below 0.2–0.1 nm wavelength) are called hard X-rays, while those with lower energy (and longer wavelength) are called soft X-rays. X-ray_sentence_148

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. X-ray_sentence_149

The term X-ray is metonymically used to refer to a radiographic image produced using this method, in addition to the method itself. X-ray_sentence_150

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. X-ray_sentence_151

By contrast, soft X-rays are easily absorbed in air; the attenuation length of 600 eV (~2 nm) X-rays in water is less than 1 micrometer. X-ray_sentence_152

Gamma rays X-ray_section_8

There is no consensus for a definition distinguishing between X-rays and gamma rays. X-ray_sentence_153

One common practice is to distinguish between the two types of radiation based on their source: X-rays are emitted by electrons, while gamma rays are emitted by the atomic nucleus. X-ray_sentence_154

This definition has several problems: other processes also can generate these high-energy photons, or sometimes the method of generation is not known. X-ray_sentence_155

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 m (0.1 Å), defined as gamma radiation. X-ray_sentence_156

This criterion assigns a photon to an unambiguous category, but is only possible if wavelength is known. X-ray_sentence_157

(Some measurement techniques do not distinguish between detected wavelengths.) X-ray_sentence_158

However, these two definitions often coincide since the electromagnetic radiation emitted by X-ray tubes generally has a longer wavelength and lower photon energy than the radiation emitted by radioactive nuclei. X-ray_sentence_159

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. X-ray_sentence_160

Thus, gamma-rays generated for medical and industrial uses, for example radiotherapy, in the ranges of 6–20 MeV, can in this context also be referred to as X-rays. X-ray_sentence_161

Properties X-ray_section_9

X-ray photons carry enough energy to ionize atoms and disrupt molecular bonds. X-ray_sentence_162

This makes it a type of ionizing radiation, and therefore harmful to living tissue. X-ray_sentence_163

A very high radiation dose over a short period of time causes radiation sickness, while lower doses can give an increased risk of radiation-induced cancer. X-ray_sentence_164

In medical imaging this increased cancer risk is generally greatly outweighed by the benefits of the examination. X-ray_sentence_165

The ionizing capability of X-rays can be utilized in cancer treatment to kill malignant cells using radiation therapy. X-ray_sentence_166

It is also used for material characterization using X-ray spectroscopy. X-ray_sentence_167

Hard X-rays can traverse relatively thick objects without being much absorbed or scattered. X-ray_sentence_168

For this reason, X-rays are widely used to image the inside of visually opaque objects. X-ray_sentence_169

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. small animal CT). X-ray_sentence_170

The penetration depth varies with several orders of magnitude over the X-ray spectrum. X-ray_sentence_171

This allows the photon energy to be adjusted for the application so as to give sufficient transmission through the object and at the same time provide good contrast in the image. X-ray_sentence_172

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. X-ray_sentence_173

This property is used in X-ray microscopy to acquire high resolution images, and also in X-ray crystallography to determine the positions of atoms in crystals. X-ray_sentence_174

Interaction with matter X-ray_section_10

X-rays interact with matter in three main ways, through photoabsorption, Compton scattering, and Rayleigh scattering. X-ray_sentence_175

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. X-ray_sentence_176

Photoabsorption or photoelectric absorption is the dominant interaction mechanism in the soft X-ray regime and for the lower hard X-ray energies. X-ray_sentence_177

At higher energies, Compton scattering dominates. X-ray_sentence_178

Photoelectric absorption X-ray_section_11

The probability of a photoelectric absorption per unit mass is approximately proportional to Z/E, where Z is the atomic number and E is the energy of the incident photon. X-ray_sentence_179

This rule is not valid close to inner shell electron binding energies where there are abrupt changes in interaction probability, so called absorption edges. X-ray_sentence_180

However, the general trend of high absorption coefficients and thus short penetration depths for low photon energies and high atomic numbers is very strong. X-ray_sentence_181

For soft tissue, photoabsorption dominates up to about 26 keV photon energy where Compton scattering takes over. X-ray_sentence_182

For higher atomic number substances this limit is higher. X-ray_sentence_183

The high amount of calcium (Z = 20) in bones, together with their high density, is what makes them show up so clearly on medical radiographs. X-ray_sentence_184

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. X-ray_sentence_185

An outer electron will fill the vacant electron position and produce either a characteristic X-ray or an Auger electron. X-ray_sentence_186

These effects can be used for elemental detection through X-ray spectroscopy or Auger electron spectroscopy. X-ray_sentence_187

Compton scattering X-ray_section_12

Compton scattering is the predominant interaction between X-rays and soft tissue in medical imaging. X-ray_sentence_188

Compton scattering is an inelastic scattering of the X-ray photon by an outer shell electron. X-ray_sentence_189

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. X-ray_sentence_190

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. X-ray_sentence_191

The probability for different scattering angles are described by the Klein–Nishina formula. X-ray_sentence_192

The transferred energy can be directly obtained from the scattering angle from the conservation of energy and momentum. X-ray_sentence_193

Rayleigh scattering X-ray_section_13

Rayleigh scattering is the dominant elastic scattering mechanism in the X-ray regime. X-ray_sentence_194

Inelastic forward scattering gives rise to the refractive index, which for X-rays is only slightly below 1. X-ray_sentence_195

Production X-ray_section_14

Whenever charged particles (electrons or ions) of sufficient energy hit a material, X-rays are produced. X-ray_sentence_196

Production by electrons X-ray_section_15


Characteristic X-ray emission lines for some common anode materials.X-ray_table_caption_0




Photon energy [keV]X-ray_header_cell_0_0_2 Wavelength [nm]X-ray_header_cell_0_0_4
Kα1X-ray_header_cell_0_1_0 Kβ1X-ray_header_cell_0_1_1 Kα1X-ray_header_cell_0_1_2 Kβ1X-ray_header_cell_0_1_3
WX-ray_header_cell_0_2_0 74X-ray_cell_0_2_1 59.3X-ray_cell_0_2_2 67.2X-ray_cell_0_2_3 0.0209X-ray_cell_0_2_4 0.0184X-ray_cell_0_2_5
MoX-ray_header_cell_0_3_0 42X-ray_cell_0_3_1 17.5X-ray_cell_0_3_2 19.6X-ray_cell_0_3_3 0.0709X-ray_cell_0_3_4 0.0632X-ray_cell_0_3_5
CuX-ray_header_cell_0_4_0 29X-ray_cell_0_4_1 8.05X-ray_cell_0_4_2 8.91X-ray_cell_0_4_3 0.154X-ray_cell_0_4_4 0.139X-ray_cell_0_4_5
AgX-ray_header_cell_0_5_0 47X-ray_cell_0_5_1 22.2X-ray_cell_0_5_2 24.9X-ray_cell_0_5_3 0.0559X-ray_cell_0_5_4 0.0497X-ray_cell_0_5_5
GaX-ray_header_cell_0_6_0 31X-ray_cell_0_6_1 9.25X-ray_cell_0_6_2 10.26X-ray_cell_0_6_3 0.134X-ray_cell_0_6_4 0.121X-ray_cell_0_6_5
InX-ray_header_cell_0_7_0 49X-ray_cell_0_7_1 24.2X-ray_cell_0_7_2 27.3X-ray_cell_0_7_3 0.0512X-ray_cell_0_7_4 0.455X-ray_cell_0_7_5

X-rays can be generated by an X-ray tube, a vacuum tube that uses a high voltage to accelerate the electrons released by a hot cathode to a high velocity. X-ray_sentence_197

The high velocity electrons collide with a metal target, the anode, creating the X-rays. X-ray_sentence_198

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. X-ray_sentence_199

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. X-ray_sentence_200

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 kV tube cannot create X-rays with an energy greater than 80 keV. X-ray_sentence_201

When the electrons hit the target, X-rays are created by two different atomic processes: X-ray_sentence_202


  1. 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).X-ray_item_0_0
  2. Bremsstrahlung: This is radiation given off by the electrons as they are scattered by the strong electric field near the high-Z (proton number) nuclei. These X-rays have a continuous spectrum. The frequency of bremsstrahlung is limited by the energy of incident electrons.X-ray_item_0_1

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. X-ray_sentence_203

The voltages used in diagnostic X-ray tubes range from roughly 20 kV to 150 kV and thus the highest energies of the X-ray photons range from roughly 20 keV to 150 keV. X-ray_sentence_204

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. X-ray_sentence_205

When producing a usable flux of X-rays, the X-ray tube must be designed to dissipate the excess heat. X-ray_sentence_206

A specialized source of X-rays which is becoming widely used in research is synchrotron radiation, which is generated by particle accelerators. X-ray_sentence_207

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. X-ray_sentence_208

Short nanosecond bursts of X-rays peaking at 15-keV in energy may be reliably produced by peeling pressure-sensitive adhesive tape from its backing in a moderate vacuum. X-ray_sentence_209

This is likely to be the result of recombination of electrical charges produced by triboelectric charging. X-ray_sentence_210

The intensity of X-ray triboluminescence is sufficient for it to be used as a source for X-ray imaging. X-ray_sentence_211

Production by fast positive ions X-ray_section_16

X-rays can also be produced by fast protons or other positive ions. X-ray_sentence_212

The proton-induced X-ray emission or particle-induced X-ray emission is widely used as an analytical procedure. X-ray_sentence_213

For high energies, the production cross section is proportional to Z1Z2, where Z1 refers to the atomic number of the ion, Z2 refers to that of the target atom. X-ray_sentence_214

An overview of these cross sections is given in the same reference. X-ray_sentence_215

Production in lightning and laboratory discharges X-ray_section_17

X-rays are also produced in lightning accompanying terrestrial gamma-ray flashes. X-ray_sentence_216

The underlying mechanism is the acceleration of electrons in lightning related electric fields and the subsequent production of photons through Bremsstrahlung. X-ray_sentence_217

This produces photons with energies of some few keV and several tens of MeV. X-ray_sentence_218

In laboratory discharges with a gap size of approximately 1 meter length and a peak voltage of 1 MV, X-rays with a characteristic energy of 160 keV are observed. X-ray_sentence_219

A possible explanation is the encounter of two streamers and the production of high-energy run-away electrons; however, microscopic simulations have shown that the duration of electric field enhancement between two streamers is too short to produce a significantly number of run-away electrons. X-ray_sentence_220

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. X-ray_sentence_221

Detectors X-ray_section_18

Main article: X-ray detector X-ray_sentence_222

X-ray detectors vary in shape and function depending on their purpose. X-ray_sentence_223

Imaging detectors such as those used for radiography were originally based on photographic plates and later photographic film, but are now mostly replaced by various digital detector types such as image plates and flat panel detectors. X-ray_sentence_224

For radiation protection direct exposure hazard is often evaluated using ionization chambers, while dosimeters are used to measure the radiation dose a person has been exposed to. X-ray_sentence_225

X-ray spectra can be measured either by energy dispersive or wavelength dispersive spectrometers. X-ray_sentence_226

For x-ray diffraction applications, such as x-ray crystallography, hybrid photon counting detectors are widely used. X-ray_sentence_227

Medical uses X-ray_section_19

Adverse effects X-ray_section_20

Diagnostic X-rays (primarily from CT scans due to the large dose used) increase the risk of developmental problems and cancer in those exposed. X-ray_sentence_228

X-rays are classified as a carcinogen by both the World Health Organization's International Agency for Research on Cancer and the U.S. government. X-ray_sentence_229

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. X-ray_sentence_230

Experimental and epidemiological data currently do not support the proposition that there is a threshold dose of radiation below which there is no increased risk of cancer. X-ray_sentence_231

However, this is under increasing doubt. X-ray_sentence_232

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%. X-ray_sentence_233

The amount of absorbed radiation depends upon the type of X-ray test and the body part involved. X-ray_sentence_234

CT and fluoroscopy entail higher doses of radiation than do plain X-rays. X-ray_sentence_235

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. X-ray_sentence_236

Each such X-ray would add less than 1 per 1,000,000 to the lifetime cancer risk. X-ray_sentence_237

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. X-ray_sentence_238

This is compared to the roughly 40% chance of a US citizen developing cancer during their lifetime. X-ray_sentence_239

For instance, the effective dose to the torso from a CT scan of the chest is about 5 mSv, and the absorbed dose is about 14 mGy. X-ray_sentence_240

A head CT scan (1.5mSv, 64mGy) that is performed once with and once without contrast agent, would be equivalent to 40 years of background radiation to the head. X-ray_sentence_241

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. X-ray_sentence_242

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. X-ray_sentence_243

In the US, there are an estimated 62 million CT scans performed annually, including more than 4 million on children. X-ray_sentence_244

Avoiding unnecessary X-rays (especially CT scans) reduces radiation dose and any associated cancer risk. X-ray_sentence_245

Medical X-rays are a significant source of man-made radiation exposure. X-ray_sentence_246

In 1987, they accounted for 58% of exposure from man-made sources in the United States. X-ray_sentence_247

Since man-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. X-ray_sentence_248

By 2006, however, medical procedures in the United States were contributing much more ionizing radiation than was the case in the early 1980s. X-ray_sentence_249

In 2006, medical exposure constituted nearly half of the total radiation exposure of the U.S. population from all sources. X-ray_sentence_250

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. X-ray_sentence_251

Dosage due to dental X-rays varies significantly depending on the procedure and the technology (film or digital). X-ray_sentence_252

Depending on the procedure and the technology, a single dental X-ray of a human results in an exposure of 0.5 to 4 mrem. X-ray_sentence_253

A full mouth series of X-rays may result in an exposure of up to 6 (digital) to 18 (film) mrem, for a yearly average of up to 40 mrem. X-ray_sentence_254

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. X-ray_sentence_255

Early photon tomography or EPT (as of 2015) along with other techniques are being researched as potential alternatives to X-rays for imaging applications. X-ray_sentence_256

Other uses X-ray_section_21

Other notable uses of X-rays include: X-ray_sentence_257


  • 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. In the early 1990s, experiments were done in which layers a few atoms thick of two different materials were deposited in a Thue-Morse sequence. The resulting object was found to yield X-ray diffraction patterns. A related technique, fiber diffraction, was used by Rosalind Franklin to discover the double helical structure of DNA.X-ray_item_1_2
  • X-ray astronomy, which is an observational branch of astronomy, which deals with the study of X-ray emission from celestial objects.X-ray_item_1_3
  • X-ray microscopic analysis, which uses electromagnetic radiation in the soft X-ray band to produce images of very small objects.X-ray_item_1_4
  • 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.X-ray_item_1_5
  • Industrial radiography uses X-rays for inspection of industrial parts, particularly welds.X-ray_item_1_6
  • 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 pigments such as lead white show well in radiographs.X-ray_item_1_7
  • X-ray spectromicroscopy has been used to analyse the reactions of pigments in paintings. For example, in analysing colour degradation in the paintings of van Gogh.X-ray_item_1_8


  • Authentication and quality control of packaged items.X-ray_item_2_9
  • Industrial CT (computed tomography), a process which 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.X-ray_item_2_10
  • Airport security luggage scanners use X-rays for inspecting the interior of luggage for security threats before loading on aircraft.X-ray_item_2_11
  • Border control truck scanners and domestic police departments use X-rays for inspecting the interior of trucks.X-ray_item_2_12


Visibility X-ray_section_22

While generally considered invisible to the human eye, in special circumstances X-rays can be visible. X-ray_sentence_258

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. X-ray_sentence_259

Upon hearing this, Röntgen reviewed his record books and found he too had seen the effect. X-ray_sentence_260

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. X-ray_sentence_261

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. X-ray_sentence_262

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. X-ray_sentence_263

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. X-ray_sentence_264

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. X-ray_sentence_265

The beamline from the wiggler at the at the European Synchrotron Radiation Facility is one example of such high intensity. X-ray_sentence_266

Units of measure and exposure X-ray_section_23

The measure of X-rays ionizing ability is called the exposure: X-ray_sentence_267


  • The coulomb per kilogram (C/kg) is the 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.X-ray_item_4_19
  • The 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 roentgen= 2.58×10 C/kg.X-ray_item_4_20

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 charge generated. X-ray_sentence_268

This measure of energy absorbed is called the absorbed dose: X-ray_sentence_269


  • The 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.X-ray_item_5_21
  • The rad is the (obsolete) corresponding traditional unit, equal to 10 millijoules of energy deposited per kilogram. 100 rad= 1 gray.X-ray_item_5_22

The equivalent dose is the measure of the biological effect of radiation on human tissue. X-ray_sentence_270

For X-rays it is equal to the absorbed dose. X-ray_sentence_271


  • The Roentgen equivalent man (rem) is the traditional unit of equivalent dose. For X-rays it is equal to the rad, or, in other words, 10 millijoules of energy deposited per kilogram. 100 rem = 1 Sv.X-ray_item_6_23
  • The sievert (Sv) is the SI unit of equivalent dose, and also of effective dose. For X-rays the "equivalent dose" is numerically equal to a Gray (Gy). 1 Sv= 1 Gy. For the "effective dose" of X-rays, it is usually not equal to the Gray (Gy).X-ray_item_6_24


Ionizing radiation related quantities view ‧ talk ‧X-ray_table_caption_1
QuantityX-ray_header_cell_1_0_0 UnitX-ray_header_cell_1_0_1 SymbolX-ray_header_cell_1_0_2 DerivationX-ray_header_cell_1_0_3 YearX-ray_header_cell_1_0_4 SI equivalenceX-ray_header_cell_1_0_5
Activity (A)X-ray_cell_1_1_0 becquerelX-ray_cell_1_1_1 BqX-ray_cell_1_1_2 sX-ray_cell_1_1_3 1974X-ray_cell_1_1_4 SI unitX-ray_cell_1_1_5
curieX-ray_cell_1_2_0 CiX-ray_cell_1_2_1 3.7 × 10 sX-ray_cell_1_2_2 1953X-ray_cell_1_2_3 3.7×10 BqX-ray_cell_1_2_4
rutherfordX-ray_cell_1_3_0 RdX-ray_cell_1_3_1 10 sX-ray_cell_1_3_2 1946X-ray_cell_1_3_3 1,000,000 BqX-ray_cell_1_3_4
Exposure (X)X-ray_cell_1_4_0 coulomb per kilogramX-ray_cell_1_4_1 C/kgX-ray_cell_1_4_2 C⋅kg of airX-ray_cell_1_4_3 1974X-ray_cell_1_4_4 SI unitX-ray_cell_1_4_5
röntgenX-ray_cell_1_5_0 RX-ray_cell_1_5_1 esu / 0.001293 g of airX-ray_cell_1_5_2 1928X-ray_cell_1_5_3 2.58 × 10 C/kgX-ray_cell_1_5_4
Absorbed dose (D)X-ray_cell_1_6_0 grayX-ray_cell_1_6_1 GyX-ray_cell_1_6_2 J⋅kgX-ray_cell_1_6_3 1974X-ray_cell_1_6_4 SI unitX-ray_cell_1_6_5
erg per gramX-ray_cell_1_7_0 erg/gX-ray_cell_1_7_1 erg⋅gX-ray_cell_1_7_2 1950X-ray_cell_1_7_3 1.0 × 10 GyX-ray_cell_1_7_4
radX-ray_cell_1_8_0 radX-ray_cell_1_8_1 100 erg⋅gX-ray_cell_1_8_2 1953X-ray_cell_1_8_3 0.010 GyX-ray_cell_1_8_4
Equivalent dose (H)X-ray_cell_1_9_0 sievertX-ray_cell_1_9_1 SvX-ray_cell_1_9_2 J⋅kg × WRX-ray_cell_1_9_3 1977X-ray_cell_1_9_4 SI unitX-ray_cell_1_9_5
röntgen equivalent manX-ray_cell_1_10_0 remX-ray_cell_1_10_1 100 erg⋅g x WRX-ray_cell_1_10_2 1971X-ray_cell_1_10_3 0.010 SvX-ray_cell_1_10_4
Effective dose (E)X-ray_cell_1_11_0 sievertX-ray_cell_1_11_1 SvX-ray_cell_1_11_2 J⋅kg × WR x WTX-ray_cell_1_11_3 1977X-ray_cell_1_11_4 SI unitX-ray_cell_1_11_5
röntgen equivalent manX-ray_cell_1_12_0 remX-ray_cell_1_12_1 100 erg⋅g x WR x WTX-ray_cell_1_12_2 1971X-ray_cell_1_12_3 0.010 SvX-ray_cell_1_12_4

See also X-ray_section_24

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/X-ray.