X-rays make up X-radiation, a form of electromagnetic radiation. Most
X-rays have a wavelength ranging from 0.01 to 10 nanometers,
corresponding to frequencies in the range 30 petahertz to 30 exahertz
(3×1016 Hz to 3×1019 Hz) and energies in the range 100 eV to 100
X-ray wavelengths are shorter than those of UV rays and typically
longer than those of gamma rays. In many languages, X-radiation is
referred to with terms meaning Röntgen radiation, after the German
scientist Wilhelm Röntgen, who usually is credited as its
discoverer, and who named it X-radiation to signify an unknown type of
radiation. Spelling of X-ray(s) in the English language includes
the variants x-ray(s), xray(s), and X ray(s).
1.2 Early research
1.3 Wilhelm Röntgen
1.4 Advances in radiology
1.5 Hazards discovered
1.6 20th century and beyond
2.1 Soft and hard X-rays
2.2 Gamma rays
4 Interaction with matter
4.1 Photoelectric absorption
4.2 Compton scattering
4.3 Rayleigh scattering
5.1 Production by electrons
5.2 Production by fast positive ions
5.3 Production in lightning and laboratory discharges
7 Medical uses
7.1 Projectional radiographs
7.2 Computed tomography
8 Adverse effects
9 Other uses
11 Units of measure and exposure
12 See also
14 External links
Wilhelm Röntgen is usually credited as the
discoverer of X-rays in 1895, because he was the first to
systematically study them, though he is not the first to have observed
their effects. He is also the one who gave them the name "X-rays"
(signifying an unknown quantity) though many others referred to
these as "Röntgen rays" (and the associated
X-ray radiograms as,
"Röntgenograms") for several decades after their discovery and even
to this day in some languages, including Röntgen's native German.
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
Ludwig Zehnder of the Physik Institut, University of
Freiburg, on 1 January 1896
X-rays were found emanating from Crookes tubes, experimental discharge
tubes invented around 1875, by scientists investigating the cathode
rays, that is energetic electron beams, that were first created in the
tubes. 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. 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. Many
of the early Crookes tubes undoubtedly radiated X-rays, because early
researchers noticed effects that were attributable to them, as
Wilhelm Röntgen was the first to systematically study
them, in 1895.
The discovery of X-rays stimulated a veritable sensation. Röntgen's
biographer Otto Glasser estimated that, in 1896 alone, as many as 49
essays and 1044 articles about the new rays were published. 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. Sensationalist reactions to the
new discovery included publications linking the new kind of rays to
occult and paranormal theories, such as telepathy.
The earliest experimenter thought to have (unknowingly) produce X-rays
was actuary William Morgan. 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. This work was further explored by Humphry Davy
and his assistant Michael Faraday.
In 1877 Ukrainian-born Ivan Pulyui, a lecturer in experimental physics
at the University of Vienna, constructed various designs of vacuum
discharge tube to investigate their properties. He continued his
investigations when appointed professor at the Prague Polytechnic and
in 1886 he found that sealed photographic plates became dark when
exposed to the emanations from the tubes. Early in 1896, just a few
weeks after Röntgen published his first
X-ray photograph, Pulyui
X-ray images in journals in Paris and
London. Although Pulyui had studied with Röntgen at the
University of Strasbourg
University of Strasbourg in the years 1873–75, his biographer Gaida
(1997) asserts that his subsequent research was conducted
X-ray image with early
Crookes tube apparatus, late 1800s.
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
X-rays were generated and detected by
Fernando Sanford (1854–1948),
the foundation Professor of Physics at Stanford University, in 1891.
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. His letter of
January 6, 1893 (describing his discovery as "electric photography")
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.
Starting in 1888, Philipp Lenard, a student of Heinrich Hertz,
conducted experiments to see whether cathode rays could pass out of
Crookes tube into the air. He built a
Crookes tube (later called a
"Lenard tube") with a "window" in the end made of thin aluminum,
facing the cathode so the cathode rays would strike it. 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.
Hermann von Helmholtz
Hermann von Helmholtz formulated mathematical equations for X-rays. He
postulated a dispersion theory before Röntgen made his discovery and
announcement. It was formed on the basis of the electromagnetic theory
of light. However, he did not work with actual X-rays.
Nikola Tesla noticed damaged film in his lab that seemed to be
Crookes tube experiments and began investigating this
radiant energy of "invisible" kinds. After Röntgen identified
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.
In November 1896, the inventor Dr. Robert D'Unger proposed a X-Ray
telephot, supposed able to make transmission of pictures by telegraph
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 designated the technique as
On November 8, 1895, German physics professor Wilhelm Röntgen
stumbled on X-rays while experimenting with Lenard and Crookes tubes
and began studying them. 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. 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. The name
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, Hungarian,
Danish, Polish, Swedish, Finnish, Estonian, Russian, Japanese, Dutch,
and Norwegian. Röntgen received the first
Nobel Prize in Physics
Nobel Prize in Physics for
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. He
noticed a faint green glow from the screen, about 1 meter 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.[citation
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
Advances in radiology
A simplified diagram of a water-cooled
There was immediate interest from researchers for the X-Ray.
Nikola Tesla were amongst the firsts to test the
new discovery. In 1896,
Thomas Edison investigated materials' ability
to fluoresce when exposed to X-rays, and found that calcium tungstate
was the most effective substance. Around March 1896, the fluoroscope
he developed became the standard for medical
Nevertheless, Edison dropped
X-ray research around 1903, even before
the death of Clarence Madison Dally, one of his glassblowers. Dally
had a habit of testing
X-ray tubes on his hands, and acquired a cancer
in them so tenacious that both arms were amputated in a futile attempt
to save his life.
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
February 14, 1896 Hall-Edwards was also the first to use X-rays in a
surgical operation. In early 1896, several weeks after Röntgen's
Ivan Romanovich Tarkhanov
Ivan Romanovich Tarkhanov irradiated frogs and insects with
X-rays, concluding that the rays "not only photograph, but also affect
the living function".
The first medical
X-ray made in the
United States was obtained using a
discharge tube of Pulyui's design. 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
Pulyui tube produced X-rays. This was a result of Pulyui'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 gelatin
photographic plates obtained from Howard Langill, a local photographer
also interested in Röntgen's work.
In 1901, U.S. President William McKinley was shot twice in an
assassination attempt. 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
X-ray machine to Buffalo to find the stray bullet. It arrived
but wasn't 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.
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. In February 1896, Professor John Daniel and Dr. William Lofland
Vanderbilt University reported hair loss after Dr. 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" volunteered. 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. The tube was fastened at
the other side at a distance of one-half inch from the hair."
In August 1896 Dr. HD. 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.
Many experimenters including
Elihu Thomson at Edison's lab, William J.
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. Other effects were
sometimes blamed for the damage including ultraviolet rays and
(according to Tesla) ozone. Many physicians claimed there were no
effects from x-ray exposure at all. On August 3, 1905 at San
Francisco, California, Elizabeth Fleischman, American woman X-ray
pioneer, died from complications as a result of her work with
20th century and beyond
A patient being examined with a thoracic fluoroscope in 1940, which
displayed continuous moving images. This image was used to argue that
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
X-ray tubes were used until about 1920.
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 which
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.
John Ambrose Fleming
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.
In about 1906, the physicist
Charles Barkla discovered that X-rays
could be scattered by gases, and that each element had a
X-ray spectrum. He won the 1917 Nobel Prize in Physics
for this discovery.
In 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
X-ray tube was invented during the following year by
William D. Coolidge. It made possible the continuous emissions of
X-ray tubes similar to this are still in use in 2012.
Chandra's image of the galaxy cluster Abell 2125 reveals a complex of
several massive multimillion-degree-Celsius gas clouds in the process
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. Then in 1908, he had to have his left arm
amputated because of the spread of
X-ray dermatitis on his arm.
Marie Curie developed radiological cars to support soldiers
injured in World War I. The cars would allow for rapid
of wounded soldiers so battlefield surgeons could quickly and more
From the 1920s through to the 1950s, x-ray machines were developed to
assist in the fitting of shoes and were sold to commercial shoe
stores. Concerns regarding the impact of frequent or
poorly controlled use were expressed in the 1950s, leading to
the practise's eventual end that decade.
X-ray microscope was developed during the 1950s.
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. Unlike visible light, which gives a relatively
stable view of the universe, the
X-ray universe is unstable. 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 laser device was proposed as part of the Reagan
Strategic Defense Initiative
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 de-funded (though was later revived by the second
Bush Administration as
National Missile Defense
National Missile Defense using different
Dog hip xray posterior view
Phase-contrast x-ray image of spider
Phase-contrast X-ray imaging
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.
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 utilizing different principles to convert
phase variations in the x-rays emerging from an object into intensity
variations. These include propagation-based phase
contrast, talbot interferometry, refraction-enhanced
imaging, and x-ray interferometry. These methods provide
higher contrast compared to normal absorption-contrast x-ray imaging,
making it possible to see smaller details. 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.
Soft and hard X-rays
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 are called soft 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
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 eV (~2 nm) X-rays in water is less than 1
There is no consensus for a definition distinguishing between X-rays
and gamma rays. 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. This definition has several problems: other
processes also can generate these high-energy photons, 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 10−11 m (0.1 Å), defined as
gamma radiation. 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 tubes generally has a
longer wavelength and lower photon energy than the radiation emitted
by radioactive nuclei. 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
MeV, can in this context also be referred to as X-rays.[citation
Ionizing radiation hazard symbol
X-ray photons carry enough energy to ionize atoms and disrupt
molecular bonds. This makes it a type of ionizing radiation, and
therefore harmful to living tissue. 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. In medical imaging
this increased cancer risk is generally greatly outweighed by the
benefits of the examination. The ionizing capability of X-rays can be
utilized in cancer treatment to kill malignant cells using radiation
therapy. It is also used for material characterization using X-ray
Attenuation length of X-rays in water showing the oxygen absorption
edge at 540 eV, the energy−3 dependence of 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).
Hard X-rays can traverse relatively thick objects without being much
absorbed or scattered. For this reason, X-rays are widely used to
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. 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 transmission
through the object and at the same time provide good contrast in the
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 atoms in crystals.
Interaction with matter
X-rays interact with matter in three main ways, through
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
X-ray energies. At higher energies, Compton scattering
The probability of a photoelectric absorption per unit mass is
approximately proportional to Z3/E3, where Z is the atomic number and
E is the energy of the incident photon. 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 coefficients and thus
short penetration depths for low photon energies and high atomic
numbers is very strong. For soft tissue, photoabsorption dominates up
to about 26 keV photon energy where
Compton scattering takes over. For
higher atomic number substances this limit is higher. 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.
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 photon[clarification
needed] or an Auger electron. These effects can be used for elemental
X-ray spectroscopy or
Auger electron spectroscopy.
Compton scattering is the predominant interaction between X-rays and
soft tissue in medical imaging.
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 are described
by the Klein–Nishina formula. The transferred energy can be directly
obtained from the scattering angle from the conservation of energy and
Rayleigh scattering is the dominant elastic scattering mechanism in
X-ray regime. Inelastic forward scattering gives rise to the
refractive index, which for X-rays is only slightly below 1.
Whenever charged particles (electrons or ions) of sufficient energy
hit a material, X-rays are produced.
Production by electrons
Characteristic X-ray emission lines for some common anode
Photon energy [keV]
Spectrum of the X-rays emitted by an
X-ray tube with a rhodium target,
operated at 60 kV. The smooth, continuous curve is due to
bremsstrahlung, and the spikes are characteristic K lines for rhodium
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. The high velocity electrons collide with a metal
target, the anode, creating the X-rays. 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.
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. When the electrons hit
the target, X-rays are created by two different atomic processes:
Characteristic X-ray emission (
X-ray fluorescence): If the electron
has enough energy it can knock an orbital electron out of the inner
electron shell of a metal atom, and as a result electrons from higher
energy levels then fill up the vacancy and
X-ray photons are emitted.
This process produces an emission spectrum of X-rays at a few discrete
frequencies, sometimes referred to as the spectral lines. The spectral
lines generated depend on the target (anode) element used and thus are
called characteristic lines. Usually these are transitions from upper
shells into K shell (called K lines), into L shell (called L lines)
and so on.
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 intensity
of the X-rays increases linearly with decreasing frequency, from zero
at the energy of the incident electrons, the voltage on the X-ray
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
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
Both of these
X-ray production processes are inefficient, with a
production efficiency of only about one percent, 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.
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. This is likely to be the result of
recombination of electrical charges produced by triboelectric
charging. The intensity of
X-ray triboluminescence is sufficient for
it to be used as a source for
A specialized source of X-rays which is becoming widely used in
research is synchrotron radiation, which is generated by particle
accelerators. Its unique features are
X-ray outputs many orders of
magnitude greater than those of
X-ray tubes, wide
excellent collimation, and linear polarization.
Production by fast positive ions
X-rays can also be produced by fast protons or other positive ions.
X-ray emission or particle-induced
is widely used as an analytical procedure. For high energies, the
production cross section is proportional to Z12Z2−4, where Z1 refers
to the atomic number of the ion, Z2 to that of the target atom. 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 flashes. The underlying mechanism is the acceleration of
electrons in lightning related electric fields and the subsequent
production of photons through
Bremsstrahlung . This produces
photons with energies of some few keV and several tens of
MeV . In
laboratory discharges with a gap size of approximately 1 meter length
and a peak voltage of 1 MV, X-rays with a caracteristic energy of 160
keV are observed . 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 detectors vary in shape and function depending on their purpose.
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. 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 spectra can be measured either by energy dispersive or
wavelength dispersive spectrometers.
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A chest radiograph of a female, demonstrating a hiatus hernia
Since Röntgen's discovery that X-rays can identify bone structures,
X-rays have been used for medical imaging. The first medical use was
less than a month after his paper on the subject. Up to 2010,
5 billion medical imaging examinations had been conducted
Radiation exposure from medical imaging in 2006 made up
about 50% of total ionizing radiation exposure in the United
Main article: Projectional radiography
An arm radiograph, demonstrating broken ulna and radius with implanted
Projectional radiography is the practice of producing two-dimensional
images using x-ray radiation. Bones contain much 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
Projectional radiographs are useful in the detection of pathology of
the skeletal system as well as for detecting some disease processes in
soft tissue. Some notable examples are the very common 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 (or intestinal) obstruction, free air (from visceral
perforations) and free fluid (in ascites). X-rays may also be used to
detect pathology such as gallstones (which are rarely radiopaque) or
kidney stones 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, 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.
Dental radiography is commonly used in the diagnoses of common oral
problems, such as 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, 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 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 radiologist or surgeon then compares
the image obtained to normal anatomical images to determine whether
there is any damage or blockage of the vessel.
Head CT scan (transverse plane) slice -– a modern application of
Computed tomography (CT scanning) is a medical imaging modality where
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. These cross-sectional images can be combined
into a three-dimensional image of the inside of the body and used for
diagnostic and therapeutic purposes in various medical disciplines.
Fluoroscopy is an imaging technique commonly used by physicians or
radiation therapists to obtain real-time moving images of the internal
structures of a patient through the use of a fluoroscope. 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
X-ray image intensifier and 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 artery blockages) and
barium swallow (to examine for esophageal disorders).
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.
Abdominal radiograph of a pregnant woman, a procedure that should be
performed only after proper assessment of benefit versus risk
Deformity of hand due to an
X-ray burn. These burns are accidents.
X-rays were not shielded when they were first discovered and used, and
people received radiation burns.
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-rays are classified as a carcinogen by both the
World Health Organization's International Agency for Research on
Cancer and the U.S. government. 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.
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. However, this is under
increasing doubt. It is estimated that the additional radiation
will increase a person's cumulative risk of getting cancer by age 75
by 0.6–1.8%. The amount of absorbed radiation depends upon the
X-ray test and the body part involved. CT and fluoroscopy
entail higher doses of radiation than do plain X-rays.
To place the increased risk in perspective, a plain chest
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. 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. This is compared to the roughly 40% chance of a US
citizen developing cancer during their lifetime. 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. 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. 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.
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. In the US, there are an
estimated 62 million CT scans performed annually, including more
than 4 million on children. Avoiding unnecessary X-rays
(especially CT scans) reduces radiation dose and any associated cancer
Medical X-rays are a significant source of man-made radiation
exposure. In 1987, they accounted for 58% of exposure from man-made
sources in the United States. 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. 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.
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 0.5 to 4 mrem. 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.
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. 
Other notable uses of X-rays include
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.
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
patterns. A related technique, fiber diffraction, was used by
Rosalind Franklin to discover the double helical structure of
X-ray astronomy, which is an observational branch of astronomy, which
deals with the study of
X-ray emission from celestial objects.
X-ray microscopic analysis, which uses electromagnetic radiation in
X-ray band to produce images of very small objects.
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.
Industrial radiography uses X-rays for inspection of industrial parts,
X-ray for inspection and quality control: the differences in the
structures of the die and bond wires reveal the left chip to be
Authentication and quality control,
X-ray is used for authentication
and quality control of packaged items.
Industrial CT (computed tomography) is 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
Paintings are often X-rayed to reveal underdrawings and pentimenti,
alterations in the course of painting or by later restorers. Many
pigments such as lead white show well in radiographs.
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
Airport security luggage scanners use X-rays for inspecting the
interior of luggage for security threats before loading on aircraft.
Border control truck scanners use X-rays for inspecting the interior
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
X-ray hair removal, a method popular in the 1920s but now banned by
Shoe-fitting fluoroscopes were popularized in the 1920s, banned in the
US in the 1960s, banned in the UK in the 1970s, and even later in
Roentgen stereophotogrammetry is used to track movement of bones based
on the implantation of markers
X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy is a chemical analysis technique
relying on the photoelectric effect, usually employed in surface
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).
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.
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.
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 at the ID11 at the European
Radiation Facility is one example of such high
Units of measure and exposure
The measure of X-rays ionizing ability is called the exposure:
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.
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= 2.58×10−4 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 charge generated. This measure of energy absorbed
is called the absorbed dose:
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.
The rad is the (obsolete) corresponding traditional unit, equal to 10
millijoules of energy deposited per kilogram. 100 rad= 1 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
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
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).
Radiation related quantities
view ‧ talk ‧ edit
3.7 × 1010 s−1
esu / 0.001293 g of air
2.58 × 10−4 C/kg
Absorbed dose (D)
1.0 × 10−4 Gy
Dose equivalent (H)
röntgen equivalent man
J⋅kg−1 × WR
Detective quantum efficiency
Resonant inelastic X-ray scattering
Resonant inelastic X-ray scattering (RIXS)
Small-angle X-ray scattering (SAXS)
X-ray absorption spectroscopy
Macintyre's X-Ray Film – 1896 documentary radiography film
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1996 Costa Rica accident
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See also: the categories
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