This article is about the nature, production, and uses of the radiation.
For the method of imaging, see Radiography.
For the medical specialty, see Radiology.
For other meanings, see X-ray (disambiguation).
An X-ray, or X-radiation, is a penetrating form of high-energy electromagnetic radiation.
Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3×10 Hz to 3×10 Hz) and energies in the range 124 eV to 124 keV.
In many languages, X-radiation is referred to as Röntgen radiation, after the German scientist Wilhelm Röntgen, who discovered it on November 8, 1895.
He named it X-radiation to signify an unknown type of radiation.
Spellings of X-ray(s) in English include the variants x-ray(s), xray(s), and X ray(s).
Pre-Röntgen observations and research
Before their discovery in 1895, X-rays were just a type of unidentified radiation emanating from experimental discharge tubes.
Many of the early Crookes tubes (invented around 1875) undoubtedly radiated X-rays, because early researchers noticed effects that were attributable to them, as detailed below.
The earliest experimenter thought to have (unknowingly) produced 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.
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") 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.
Starting in 1888, Philipp Lenard conducted experiments to see whether cathode rays could pass out of the Crookes tube into the air.
He built a Crookes tube with a "window" in the end made of thin aluminum, facing the cathode so the cathode rays would strike it (later called a "Lenard tube").
He found that something came through, that would expose photographic plates and cause fluorescence.
He measured the penetrating power of these rays through various materials.
It has been suggested that at least some of these "Lenard rays" were actually X-rays.
In 1889 Ukrainian-born Ivan Puluj, a lecturer in experimental physics at the Prague Polytechnic who since 1877 had been constructing various designs of gas-filled tubes to investigate their properties, published a paper on how sealed photographic plates became dark when exposed to the emanations from the tubes.
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.
In 1894 Nikola Tesla noticed damaged film in his lab that seemed to be associated with Crookes tube experiments and began investigating this radiant energy of "invisible" kinds.
After Röntgen identified the X-ray, Tesla began making X-ray images of his own using high voltages and tubes of his own design, as well as Crookes tubes.
Discovery by Röntgen
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, Ukrainian, Danish, Polish, Bulgarian, Swedish, Finnish, Estonian, Turkish, Russian, Latvian, Japanese, Dutch, Georgian, Hebrew and Norwegian.
Röntgen received the first Nobel Prize in Physics for his discovery.
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 death."
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.
Advances in radiology
Röntgen immediately noticed X-rays could have medical applications.
Along with his 28 December Physical-Medical Society submission he sent a letter to physicians he knew around Europe (January 1, 1896).
News (and the creation of "shadowgrams") spread rapidly with Scottish electrical engineer Alan Archibald Campbell-Swinton being the first after Röntgen to create an X-ray (of a hand).
Through February there were 46 experimenters taking up the technique in North America alone.
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 discovery, 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 Pului'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 Pului tube produced X-rays.
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.
Many experimenters, including Röntgen himself in his original experiments, came up with methods to view X-ray images "live" using some form of luminescent screen.
Röntgen used a screen coated with barium platinocyanide.
On February 5, 1896 live imaging devices were developed by both Italian scientist Enrico Salvioni (his "cryptoscope") and Professor McGie of Princeton University (his "Skiascope"), both using barium platinocyanide.
American inventor Thomas Edison started research soon after Röntgen's discovery and investigated materials' ability to fluoresce when exposed to X-rays, finding that calcium tungstate was the most effective substance.
In May 1896 he developed the first mass-produced live imaging device, his "Vitascope", later called the fluoroscope, which became the standard for medical X-ray examinations.
Edison dropped X-ray research around 1903, before the death of Clarence Madison Dally, one of his glassblowers.
Dally had a habit of testing X-ray tubes on his own hands, developing a cancer in them so tenacious that both arms were amputated in a futile attempt to save his life; in 1904, he became the first known death attributed to X-ray exposure.
During the time the fluoroscope was being developed, Serbian American physicist Mihajlo Pupin, using a calcium tungstate screen developed by Edison, found that using a fluorescent screen decreased the exposure time it took to create an X-ray for medical imaging from an hour to a few minutes.
A worried McKinley aide sent word to inventor Thomas Edison to rush an X-ray machine to Buffalo to find the stray bullet.
It arrived but was not used.
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.
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.
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.
20th century and beyond
The many applications of X-rays immediately generated enormous interest.
Workshops began making specialized versions of Crookes tubes for generating X-rays and these first-generation cold cathode or Crookes X-ray tubes were used until about 1920.
A spark gap was typically connected to the high voltage side in parallel to the tube and used for diagnostic purposes.
The spark gap allowed detecting the polarity of the sparks, measuring voltage by the length of the sparks thus determining the "hardness" of the vacuum of the tube, and it provided a load in the event the X-ray tube was disconnected.
To detect the hardness of the tube, the spark gap was initially opened to the widest setting.
While the coil was operating, the operator reduced the gap until sparks began to appear.
A tube in which the spark gap began to spark at around 2 1/2 inches was considered soft (low vacuum) and suitable for thin body parts such as hands and arms.
A 5-inch spark indicated the tube was suitable for shoulders and knees.
A 7-9 inch spark would indicate a higher vacuum suitable for imaging the abdomen of larger individuals.
Since the spark gap was connected in parallel to the tube, the spark gap had to be opened until the sparking ceased in order to operate the tube for imaging.
Exposure time for photographic plates was around half a minute for a hand to a couple of minutes for a thorax.
The plates may have a small addition of fluorescent salt to reduce exposure times.
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".
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.
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.
He won the 1917 Nobel Prize in Physics for this discovery.
In 1913, Henry Moseley performed crystallography experiments with X-rays emanating from various metals and formulated Moseley's law which relates the frequency of the X-rays to the atomic number of the metal.
It made possible the continuous emissions of X-rays.
Modern X-ray tubes are based on this design, often employing the use of rotating targets which allow for significantly higher heat dissipation than static targets, further allowing higher quantity X-ray output for use in high powered applications such as rotational CT scanners.
Then in 1908, he had to have his left arm amputated because of the spread of X-ray dermatitis on his arm.
Medical science also used the motion picture to study human physiology.
In 1913, a motion picture was made in Detroit showing a hard-boiled egg inside a human stomach.
This early x-ray movie was recorded at a rate of one still image every four seconds.
Dr Lewis Gregory Cole of New York was a pioneer of the technique, which he called "serial radiography".
In 1918, x-rays were used in association with motion picture cameras to capture the human skeleton in motion.
In 1920, it was used to record the movements of tongue and teeth in the study of languages by the Institute of Phonetics in England.
The cars would allow for rapid X-ray imaging of wounded soldiers so battlefield surgeons could quickly and more accurately operate.
From the early 1920s through to the 1950s, X-ray machines were developed to assist in the fitting of shoes and were sold to commercial shoe stores.
Concerns regarding the impact of frequent or poorly controlled use were expressed in the 1950s, leading to the practice's eventual end that decade.
The X-ray microscope was developed during the 1950s.
The Chandra X-ray Observatory, launched on July 23, 1999, has been allowing the exploration of the very violent processes in the universe which produce X-rays.
Unlike visible light, which gives a relatively stable view of the universe, the X-ray universe is unstable.
An X-ray laser device was proposed as part of the Reagan Administration's Strategic Defense Initiative in the 1980s, but the only test of the device (a sort of laser "blaster" or death ray, powered by a thermonuclear explosion) gave inconclusive results.
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).
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.
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 (and longer wavelength) are called soft X-rays.
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.
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 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.
In medical imaging this increased cancer risk is generally greatly outweighed by the benefits of the examination.
It is also used for material characterization using X-ray spectroscopy.
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).
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.
Interaction with matter
The strength of these interactions depends on the energy of the X-rays and the elemental composition of the material, but not much on chemical properties, since the X-ray photon energy is much higher than chemical binding energies.
Photoabsorption or photoelectric absorption is the dominant interaction mechanism in the soft X-ray regime and for the lower hard X-ray energies.
At higher energies, Compton scattering dominates.
The probability of a photoelectric absorption per unit mass is approximately proportional to Z/E, where Z is the atomic number and E is the energy of the incident photon.
This rule is not valid close to inner shell electron binding energies where there are abrupt changes in interaction probability, so called absorption edges.
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 X-ray or an Auger electron.
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.
Rayleigh scattering is the dominant elastic scattering mechanism in the 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
|Photon energy [keV]||Wavelength [nm]|
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.
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 electroluminescence): If the electron has enough energy, it can knock an orbital electron out of the inner electron shell of the target atom. After that, electrons from higher energy levels fill the vacancies, and X-ray photons are emitted. This process produces an emission spectrum of X-rays at a few discrete frequencies, sometimes referred to as spectral lines. Usually these are transitions from the upper shells to the K shell (called K lines), to the L shell (called L lines) and so on. If the transition is from 2p to 1s, it is called Kα, while if it is from 3p to 1s it is Kβ. The frequencies of these lines depend on the material of the target and are therefore called characteristic lines. The Kα line usually has greater intensity than the Kβ one and is more desirable in diffraction experiments. Thus the Kβ line is filtered out by a filter. The filter is usually made of a metal having one proton less than the anode material (e.g., Ni filter for Cu anode or Nb filter for Mo anode).
- Bremsstrahlung: This is radiation given off by the electrons as they are scattered by the strong electric field near the high-Z (proton number) nuclei. These X-rays have a continuous spectrum. The frequency of bremsstrahlung is limited by the energy of incident electrons.
So, the resulting output of a tube consists of a continuous bremsstrahlung spectrum falling off to zero at the tube voltage, plus several spikes at the characteristic lines.
The voltages used in diagnostic X-ray tubes range from roughly 20 kV to 150 kV and thus the highest energies of the X-ray photons range from roughly 20 keV to 150 keV.
Both of these X-ray production processes are inefficient, with only about one percent of the electrical energy used by the tube converted into X-rays, and thus most of the electric power consumed by the tube is released as waste heat.
When producing a usable flux of X-rays, the X-ray tube must be designed to dissipate the excess heat.
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 X-ray imaging.
Production by fast positive ions
X-rays can also be produced by fast protons or other positive ions.
The proton-induced X-ray emission or particle-induced X-ray emission is widely used as an analytical procedure.
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 characteristic 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.
Recently, it has been proposed that air perturbations in the vicinity of streamers can facilitate the production of run-away electrons and hence of X-rays from discharges.
Main article: X-ray detector
X-ray detectors vary in shape and function depending on their purpose.
Imaging detectors such as those used for radiography were originally based on photographic plates and later photographic film, but are now mostly replaced by various digital detector types such as image plates and flat panel detectors.
Diagnostic X-rays (primarily from CT scans due to the large dose used) increase the risk of developmental problems and cancer in those exposed.
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 from diagnostic X-rays will increase the average person's cumulative risk of getting cancer by age 75 by 0.6–3.0%.
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 X-ray will expose a person to the same amount from background radiation that people are exposed to (depending upon location) every day over 10 days, while exposure from a dental X-ray is approximately equivalent to 1 day of environmental background radiation.
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 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.
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.
Early photon tomography or EPT (as of 2015) along with other techniques are being researched as potential alternatives to X-rays for imaging applications.
Other notable uses of X-rays include:
- X-ray crystallography in which the pattern produced by the diffraction of X-rays through the closely spaced lattice of atoms in a crystal is recorded and then analysed to reveal the nature of that lattice. In the early 1990s, experiments were done in which layers a few atoms thick of two different materials were deposited in a Thue-Morse sequence. The resulting object was found to yield X-ray diffraction patterns. A related technique, fiber diffraction, was used by Rosalind Franklin to discover the double helical structure of DNA.
- X-ray 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 X-ray can be used to identify the composition of the sample.
- Industrial radiography uses X-rays for inspection of industrial parts, particularly welds.
- Radiography of cultural objects, most often x-rays of paintings to reveal underdrawing, pentimenti alterations in the course of painting or by later restorers, and sometimes previous paintings on the support. Many pigments such as lead white show well in radiographs.
- X-ray spectromicroscopy has been used to analyse the reactions of pigments in paintings. For example, in analysing colour degradation in the paintings of van Gogh.
- Authentication and quality control of packaged items.
- Industrial CT (computed tomography), a process which uses X-ray equipment to produce three-dimensional representations of components both externally and internally. This is accomplished through computer processing of projection images of the scanned object in many directions.
- Airport security luggage scanners use X-rays for inspecting the interior of luggage for security threats before loading on aircraft.
- Border control truck scanners and domestic police departments use X-rays for inspecting the interior of trucks.
- 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 the FDA.
- Shoe-fitting fluoroscopes were popularized in the 1920s, banned in the US in the 1960s, in the UK in the 1970s, and later in continental Europe.
- 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 science.
- 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.
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 C/kg.
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.
- The Roentgen equivalent man (rem) is the traditional unit of equivalent dose. For X-rays it is equal to the rad, or, in other words, 10 millijoules of energy deposited per kilogram. 100 rem = 1 Sv.
- 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).
|Activity (A)||becquerel||Bq||s||1974||SI unit|
|curie||Ci||3.7 × 10 s||1953||3.7×10 Bq|
|rutherford||Rd||10 s||1946||1,000,000 Bq|
|Exposure (X)||coulomb per kilogram||C/kg||C⋅kg of air||1974||SI unit|
|röntgen||R||esu / 0.001293 g of air||1928||2.58 × 10 C/kg|
|Absorbed dose (D)||gray||Gy||J⋅kg||1974||SI unit|
|erg per gram||erg/g||erg⋅g||1950||1.0 × 10 Gy|
|rad||rad||100 erg⋅g||1953||0.010 Gy|
|Equivalent dose (H)||sievert||Sv||J⋅kg × WR||1977||SI unit|
|röntgen equivalent man||rem||100 erg⋅g x WR||1971||0.010 Sv|
|Effective dose (E)||sievert||Sv||J⋅kg × WR x WT||1977||SI unit|
|röntgen equivalent man||rem||100 erg⋅g x WR x WT||1971||0.010 Sv|
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