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 keV
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
Wilhelm Röntgen , who usually is credited as its
discoverer, and who had 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 History
* 1.1 Discovery
* 1.2 Early research
* 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
* 3 Properties
* 4 Interaction with matter
* 4.1 Photoelectric absorption
* 5 Production
* 5.1 Production by electrons
* 5.2 Production by fast positive ions
* 6 Detectors
* 7 Medical uses
* 7.1 Projectional radiographs
* 8 Adverse effects
* 9 Other uses
* 10 Visibility
* 11 Units of measure and exposure
* 12 See also
* 13 References
* 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 presented to
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 detailed below.
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
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 .
Eugen Goldstein proved that they came from the cathode, and
named them cathode rays (Kathodenstrahlen). Both William Crookes (in
the 1880s) and German physicist
Johann Hittorf , a co-inventor and
early researcher of the Crookes tube, found that paper wrapped
photographic plates placed near the tube became unaccountably fogged
or flawed by shadows, although they had not been exposed to light.
Neither found the cause nor investigated this effect.
In 1877 Ukrainian -born
Ivan Pulyui , a lecturer in experimental
physics at the
University of Vienna
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 in the years 1873–75, his biographer Gaida (1997) asserts
that his subsequent research was conducted independently. Taking
X-ray image with early
Crookes tube apparatus, late 1800s. The
Crookes tube is visible in center. The standing man is viewing his
hand with a fluoroscope screen. The seated man is taking a radiograph
of his hand by placing it on a photographic plate . No precautions
against radiation exposure are taken; its hazards were not known at
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
Philipp Lenard . 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
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 associated with
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 Röntgen photography.
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
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.
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
England on 11 January 1896, when he
radiographed a needle stuck in the hand of an associate. On 14
February 1896 Hall-Edwards was also the first to use X-rays in a
surgical operation. In early 1896, several weeks after Röntgen's
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.
Morton , and
Nikola Tesla also reported burns. Elihu Thomson
deliberately exposed a finger to an x-ray tube over a period of time
and suffered pain, swelling, and blistering. 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
Elizabeth Fleischman , American woman X-ray
pioneer, died from complications as a result of her work with X-rays.
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
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 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
X-ray tubes, and hence heated-cathode
X-ray tubes, called "Coolidge
tubes", completely replaced the troublesome cold cathode tubes by
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.
Max von Laue
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
William Henry Bragg , and
William Lawrence Bragg
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
England . Then in 1908, he had to have his left arm
amputated because of the spread of
X-ray dermatitis on his arm.
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.
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. 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 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 using different
technologies). Dog hip xray posterior view Phase-contrast
x-ray image of spider
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
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
X-ray is 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 micrometer.
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.
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
X-ray spectroscopy .
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
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 or an Auger
electron . These effects can be used for elemental detection through
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
Klein–Nishina formula . The transferred energy can be
directly obtained from the scattering angle from the conservation of
energy and momentum .
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 materials.
number PHOTON ENERGY
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 tube .
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.
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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X-ray. A chest radiograph of a female, demonstrating a
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 worldwide.
Radiation exposure from medical imaging in 2006 made up about 50% of
total ionizing radiation exposure in the United States.
Projectional radiography An arm radiograph,
demonstrating broken ulna and radius with implanted internal fixation
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
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 medical radiography
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 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
the U.S. government. It is estimated that 0.4% of current cancers in
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 type of
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
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 risk.
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.
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. 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 the soft
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
be used to identify the composition of the sample.
Industrial radiography uses X-rays for inspection of industrial
parts, particularly welds .
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 of trucks.
X-ray fine art photography of needlefish by
X-ray art and fine art photography , artistic use of X-rays, for
example the works by
X-ray hair removal , a method popular in the 1920s but now banned
by the FDA.
* 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 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
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 intensity.
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
* 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 dose
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
* Medical portal
* Physics portal
Detective quantum efficiency
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
The X-Rays – 1897 British short silent comedy film
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* Röntgen's 1895 article, on line and analyzed on BibNum