Difference between revisions of "X-ray" - New World Encyclopedia

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:''In the [[NATO phonetic alphabet]], X-ray represents the letter X.''
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[[Image:Roentgen-x-ray-von-kollikers-hand.jpg|thumb|right|An X-ray picture (radiograph), taken by [[Wilhelm Conrad Röntgen|Wilhelm Röntgen]] in 1896, of Albert von Kölliker's hand.]]
  
[[Image:Chest x-ray.jpg|right|thumb|[[Chest X-ray]]]]
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'''X-rays''' (or '''Röntgen rays''') are a form of [[electromagnetic radiation]] with a [[wavelength]] in the range of 10 to 0.01 [[nanometer]]s, corresponding to [[frequency|frequencies]] in the range 30 to 30,000 [[Hertz|PHz]] (10<sup>15</sup> hertz). X-rays are a form of [[ionizing radiation]] and as such can be dangerous. In many languages it is called ''Röntgen radiation'' after one of the investigators of the radiation, [[Wilhelm Röntgen]].
[[Image:Laprascopy-Roentgen.jpg|right|thumb|250px|X-Ray during [[Cholecystectomy]]]]
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{{toc}}
An '''X-ray''' or '''Röntgen ray''' is a form of [[electromagnetic radiation]] with a [[wavelength]] in the range of 10 [[nanometre|nanometers]] to 100 [[picometre|picometers]] (corresponding to frequencies in the range 30 [[SI prefix|PHz]] to 3 [[SI prefix|EHz]]). X-rays are primarily used for diagnostic [[medical imaging|medical imaging]] and [[X-ray crystallography|crystallography]]. X-rays are a form of [[ionizing radiation]] and as such can be dangerous.
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X-rays are valuable in the field of [[medicine]] and for [[science|scientific]] research. In medicine, they are primarily used for diagnostic [[radiography]]. In science, they are used for the determination of [[crystal]] structures by the technique known as [[X-ray crystallography]]. In addition, X-ray [[telescope]]s are important for astronomical observations. As [[technology]] develops at a steady pace, many other applications of X-rays may emerge as well.
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==History==
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Among the important early researchers in X-rays were Professor [[Ivan Pului]], Sir [[William Crookes]], [[Johann Wilhelm Hittorf]], [[Eugen Goldstein]], [[Heinrich Hertz]], [[Philipp Lenard]], [[Hermann von Helmholtz]], [[Nikola Tesla]], [[Thomas Edison]], [[Charles Glover Barkla]], [[Max von Laue]], and [[Wilhelm Conrad Röntgen]].
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[[Image:Roentgen-Roehre.png|thumb|left|222px|Diagram of a water cooled X-ray tube. (simplified/outdated)]]
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Physicist Johann Hittorf (1824-1914) observed [[vacuum tube|tubes]] with energy rays extending from a negative [[electrode]]. These rays produced a [[fluorescence]] when they hit the glass walls of the tubes. In 1876 the effect was named "[[cathode ray]]s" by [[Eugen Goldstein]], and today are known to be streams of [[electron]]s. Later, English [[physics|physicist]] [[William Crookes]] investigated the effects of [[electric current]]s in gases at low pressure, and constructed what is called the Crookes tube. It is a glass vacuum cylinder, containing electrodes for discharges of a high voltage electric current. He found, when he placed unexposed photographic plates near the tube, that some of them were flawed by shadows, though he did not investigate this effect.
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===Tesla===
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In April 1887, [[Nikola Tesla]] began to investigate X-rays using high [[volt]]ages and [[vacuum]] tubes of his own design, as well as [[Crookes tube]]s. From his technical publications, it is indicated that he invented and developed a special single-[[electrode]] X-ray tube, which differed from other X-ray tubes in having no target electrode. He stated these facts in his 1897 X-ray lecture before the [[New York Academy of Sciences]].
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The principle behind Tesla's device is nowadays called the [[Bremsstrahlung]] process, in which a high-energy secondary X-ray emission is produced when charged particles (such as electrons) pass through matter. By 1892, Tesla performed several such experiments, but he did not categorize the emissions as what were later called X-rays, instead generalizing the phenomenon as [[radiant energy]]. He did not publicly declare his findings nor did he make them widely known. His subsequent X-ray experimentation by vacuum high field emissions led him to alert the scientific community to the biological hazards associated with X-ray exposure.
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===Hertz===
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In 1892, [[Heinrich Hertz]] began experimenting and demonstrated that cathode rays could penetrate very thin metal foil (such as [[aluminum]]). [[Philipp Lenard]], a student of Heinrich Hertz, further researched this effect. He developed a version of the cathode tube and studied the penetration by X-rays of various materials. Philipp Lenard, though, did not realize that he was producing X-rays. [[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 (''Wiedmann's Annalen'', Vol. XLVIII). However, he did not work with actual X-rays.
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===Röntgen===
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On November 8, 1895, [[Wilhelm Conrad Röntgen]], a [[Germany|German]] scientist, began observing and further documenting X-rays while experimenting with vacuum tubes. Röntgen, on December 28, 1895, wrote a preliminary report "''On a new kind of ray: A preliminary communication''." He submitted it to the [[Würzburg]]'s Physical-Medical Society journal. This was the first formal and public recognition of the categorization of 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. Röntgen received the first [[Nobel Prize in Physics]] for his discovery.
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Röntgen was working on a primitive [[cathode]] ray generator that was projected through a glass vacuum tube. Suddenly he noticed a faint green light against the wall. The odd thing he had noticed, was that the light from the cathode ray generator was traveling through a bunch of the materials in its way (paper, wood, and books). He then started to put various objects in front of the generator, and as he was doing this, he noticed that the outline of the bones from his hand were displayed on the wall. Röntgen said he did not know what to think and kept experimenting. Two months after his initial discovery, he published his paper translated "On a New Kind of Radiation" and gave a demonstration in 1896.
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Rontgen discovered its medical use when he saw picture of his wife's hand on a photographic plate formed due to X-Rays. His wife's hand's photograph was the first ever photograph of a human body part using the X-Rays.
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===Edison===
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In 1895, [[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 X-ray examinations. Nevertheless, Edison dropped X-ray research around 1903 after 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 [[amputation|amputated]] in a futile attempt to save his life.
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===The twentieth century and beyond===
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In 1906, [[physics|physicist]] [[Charles Glover Barkla|Charles Barkla]] discovered that X-rays could be scattered by gases, and that each element had a characteristic X-ray. He won the 1917 [[Nobel Prize in Physics]] for this discovery.
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In 1912, [[Max von Laue]], Paul Knipping and Walter Friedrich observed for the first time the [[diffraction]] of X-rays by crystals. This discovery, along with the early works of [[Paul Peter Ewald]], [[William Henry Bragg]], and [[William Lawrence Bragg]] gave birth to the field of X-ray [[crystallography]].
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The use of X-rays for medical purposes (to develop into the field of [[radiation therapy]]) was pioneered by Major [[John Hall-Edwards]] in [[Birmingham]], [[England]]. In 1908, he had to have his left arm amputated owing to the spread of [[X-ray dermatitis]].<ref>[http://www.birmingham.gov.uk/xray Major John Hall-Edwards], Birmingham City Council. Retrieved April 16, 2008.</ref> In the 1950s, X-rays were used to produce an [[X-ray microscope]].
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In the 1980s, an [[X-ray laser]] device was proposed as part of the [[Ronald Reagan|Reagan]] administration's [[Strategic Defense Initiative]], but the first and 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 [[George W. Bush|Bush]] administration as [[National Missile Defense]] using different technologies).
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In the 1990s, the [[Chandra X-ray Observatory]] was launched, allowing the exploration of the very violent processes in the universe which produce X-rays. Unlike visible light, which is a relatively stable view of the universe, the X-ray universe is unstable, it features stars being torn apart by [[black hole]]s, galactic collisions, and novas, [[neutron star]]s that build up layers of plasma that then explode into space.
  
 
==Physics==
 
==Physics==
X-rays with a wavelength approximately longer than 0.1 nm are called ''soft X-rays''. At wavelengths shorter than this, they are called ''hard X-rays''.  Hard X-rays overlap the range of "long"-wavelength (lower energy) [[gamma rays]], however the distinction between the two terms depends on the source of the radiation, not its wavelength: X-ray [[photon]]s are generated by energetic [[electron]] processes, gamma rays by transitions within [[atomic nucleus|atomic nuclei]].
 
  
{| align=right
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X-rays are a type of [[electromagnetic radiation]] with wavelengths of around 10<sup>-10</sup> meters. When medical X-rays are being produced, a thin metallic sheet is placed between the emitter and the target, effectively filtering out the lower energy (soft) X-rays. This is often placed close to the window of the [[X-ray tube]]. The resultant X-ray is said to be ''hard.'' Soft X-rays overlap the range of [[extreme ultraviolet]]. The frequency of hard X-rays is higher than that of soft X-rays, and the wavelength is shorter. Hard X-rays overlap the range of "long"-wavelength (lower energy) [[gamma rays]], however the distinction between the two terms depends on the source of the radiation, not its wavelength; X-ray [[photon]]s are generated by energetic [[electron]] processes, [[gamma ray]]s by transitions within [[atomic nucleus|atomic nuclei]].
|+ X-ray K-series spectral line wavelengths (Å) for some common target materials
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|+ ''Unverified: if a reliable reference exists, please check the values and cite the reference.''
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{| align=right class="wikitable"
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|+ X-ray K-series spectral line wavelengths (nm) for some common target materials
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|+ {{citebook | first= | last=| year=| title=CRC Handbook of Chemistry and Physics 75th edition | chapter= | editor=David R. Lide | others= | pages=10-227 | publisher=CRC Press | id=ISBN 0-8493-0475-X | url= | authorlink= }}
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! Target
 
! Target
! K&beta;&#8321;
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! Kβ₁
! K&beta;&#8322;
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! Kβ₂
! K&alpha;&#8321;
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! Kα₁
! K&alpha;&#8322;
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! Kα₂
 
|-
 
|-
 
! Fe
 
! Fe
| 17.566
+
| 0.17566
| 17.442
+
| 0.17442
| 19.3604
+
| 0.193604
| 19.3998
+
| 0.193998
 
|-
 
|-
 
! Ni
 
! Ni
| 15.001
+
| 0.15001
| 14.886
+
| 0.14886
| 16.5791
+
| 0.165791
| 16.6175
+
| 0.166175
 
|-
 
|-
 
! Cu
 
! Cu
| 13.9222
+
| 0.139222
| 13.8109
+
| 0.138109
| 15.4056
+
| 0.154056
| 15.4439
+
| 0.154439
 
|-
 
|-
 
! Zr
 
! Zr
| 7.0173
+
| 0.070173
| 6.8993
+
| 0.068993
| 7.8593
+
| 0.078593
| 7.9015
+
| 0.079015
 
|-
 
|-
 
! Mo
 
! Mo
| 6.3229
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| 0.063229
| 6.2099
+
| 0.062099
| 7.0930
+
| 0.070930
| 7.1359
+
| 0.071359
 
|}
 
|}
  
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The basic production of X-rays is by accelerating [[electron]]s in order to collide with a metal target (usually [[tungsten]] or a more crack resistant alloy of rhenium (5 percent) and tungsten (95 percent), but sometimes [[molybdenum]] for more specialized applications). Molybdenum targets are often used when soft X-rays are needed as in mammography. Here the electrons suddenly decelerate upon colliding with the metal target and if enough energy is contained within the electron it is able to knock out an electron from the inner shell of the metal atom and as a result electrons from higher energy levels then fill up the vacancy and X-ray [[photon]]s are emitted. This process is extremely inefficient and requires a way of removing the heat generated.
  
The basic production of X-rays is by accelerating electrons in order to collide with a [[metal]] target ([[tungsten]] usually). Here the electrons suddenly decelerate upon colliding with the metal target and if enough energy is contained within the electron it is able to knock out an electron from the inner shell of the metal atom and as a result electrons from higher energy levels then fill up the vacancy and X-ray photons are emitted.
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This causes the spectral line part of the wavelength distribution. There is also a continuum [[bremsstrahlung]] component given off by the electrons as they are scattered by the strong electric field near the high Z ([[proton]] number) nuclei.
This causes the spectral line part of the wavelength distribution. There is also a continuum [[bremsstrahlung]] component given off by the electrons as they are scattered by the strong electric field near the high Z ([[proton]] number) nuclei.
 
  
Nowadays, for many applications, X-ray production is achieved by [[synchrotron]]s (see [[synchrotron light]]).
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Nowadays, for many (non medical) applications, X-ray production is achieved by [[synchrotron]]s (see [[synchrotron light]]).
  
 
==Detectors==
 
==Detectors==
===Photographic plates===
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===Photographic Plate===
The detection of X-rays is based on various methods. The most commonly known method are a [[photographic plate]] and a fluorescent screen.
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The detection of X-rays is based on various methods. The most commonly known method are a [[photographic plate]], X-ray [[photographic film|film]] in a cassette, and [[rare earth]] screens.
  
The X-ray [[photographic plate]] is frequently used in [[hospital]]s to produce images of the internal organs and bones of a patient. The part of the patient to be X-rayed is placed between the X-ray source and the photographic plate to produce what is a shadow of all the internal structure of that particular part of the body being X-rayed. The X-rays are blocked by dense tissues such as bone and pass through soft tissues. Where the X-rays strike the photographic plate it turns black when it is developed. So where the X-rays go through "soft" parts of the body like organs and skin the plate turns black. Contrast compounds containing [[barium]] or [[iodine]] can be injected in the artery of a particular organ. The contrast compounds strongly block the X-rays and hence the circulation of the organ can be more readily seen.
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The X-ray [[photographic plate]] or film is used in [[hospital]]s to produce images of the internal organs and bones of a patient. Since photographic plates are not generally sensitive to X-rays, phosphorescent screens are usually placed in contact with the emulsion of the plate or film. The X-rays strike the phosphor screen, which emits visible light, which exposes the film. The emulsion still needs to be heavily doped with silver compounds and can be coated on both sides of the film or plate. The part of the patient to be X-rayed is placed between the X-ray source and the photographic receptor to produce what is a shadow of all the internal structure of that particular part of the body being X-rayed. The X-rays are blocked by dense tissues such as bone and pass through soft tissues. Those areas where the X-rays strike the photographic receptor turn black when it is developed. So where the X-rays pass through "soft" parts of the body such as organs, muscle, and skin, the plate or film turns black. Contrast compounds containing [[barium]] or [[iodine]], which are [[radiopaque]], can be injected in the artery of a particular organ, or given intravenously. The contrast compounds essentially block the X-rays and hence the circulation of the organ can be more readily seen. Many years ago thorium was used as a contrast medium (Thorotrast)—this caused many people to be injured or even die from the effects of the radiation from the thorium.
  
Another method of detecting X-rays is a fluorescent plate. In modern hospitals a special plastic sheet is used in place of the photographic plate. The plastic sheet is read by a scanning laser beam. The resultant image is then stored in a computer.
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===Photostimulable Phosphors (PSPs)===
The plastic sheet can be used over and over again.
 
  
===Geiger counters===
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An increasingly common method of detecting X-rays is the use of Photostimulable Luminescence (PSL), pioneered by Fuji in the 1980s. In modern hospitals a PSP plate is used in place of the photographic plate. After the plate is X-rayed, excited electrons in the phosphor material remain 'trapped' in 'colour centres' in the crystal lattice until stimulated by a laser beam passed over the plate surface. The light given off during laser stimulation is collected by a photomultiplier tube and the resulting signal is converted into a digital image by computer technology, which gives this process its common name, [[computed radiography]]. The PSP plate can be used over and over again.
Initially, most common detection methods were based on the ionisation of gases, as in the [[Geiger counter|Geiger-Müller counter]]: a sealed cylinder with a polymer window contains a gas, and a wire, and a high voltage is applied between the cylinder ([[cathode]]) and the wire ([[anode]]). When an X-ray photon enters the cylinder, it ionizes the gas which becomes conducting, creating a current flow (a kind of flash); this peak of current is detected and is called a "count".
 
  
When the high voltage between anode and cathode is decreased, the detector is no longer saturated, and the height of the current peak is proportional to the energy of the photon; it is thus called a "proportional counter". Most of time, the cylinder is not sealed but is constantly fed with "fresh gas", is thus called a "flow counter". This proportionality property allows filtering the "interesting" peaks from the noise and other photons, but the resolution in energy is not enough to determine the [[energy spectrum]]; such a feature requires a [[diffraction|diffracting]] crystal to first separate the different photons, the method is called [[wavelength dispersive X-ray spectroscopy]] ([[WDX]] or WDS).
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===Geiger counter===
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Initially, most common detection methods were based on the ionisation of gases, as in the [[Geiger counter|Geiger-Müller counter]]: a sealed volume, usually a cylinder, with a polymer or thin metal window contains a gas, and a wire, and a high voltage is applied between the cylinder ([[cathode]]) and the wire ([[anode]]). When an X-ray photon enters the cylinder, it [[ion]]izes the gas. These ions accelerate toward the anode, in the process causing further ionisation along their trajectory. This process, known as an avalanche, is detected as a sudden flow of current, called a "count" or "event."
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Ultimately, the electrons form a virtual cathode around the anode wire drastically reducing the electric field in the outer portions of the tube. This halts the collisional ionizations and limits further growth of avalanches. As a result, all "counts" on a [[Geiger counter]] are the same size and it can give no indication as to the particle energy of the radiation, unlike the [[proportional counter]]. The intensity of the radiation is measurable by the Geiger counter as the counting-rate of the system.
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In order to gain energy spectrum information a [[diffraction|diffracting]] crystal may be used to first separate the different photons, the method is called [[wavelength dispersive X-ray spectroscopy]] ([[WDX]] or WDS). Position-sensitive detectors are often used in conjunction with dispersive elements. Other detection equipment may be used which are inherently energy-resolving, such as the aforementioned [[proportional counter]]s. In either case, use of suitable pulse-processing (MCA) equipment allows digital spectra to be created for later analysis.
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For many applications, counters are not sealed but are constantly fed with purified gas (thus reducing problems of contamination or gas aging). These are called "flow counter."
  
 
===Scintillators===
 
===Scintillators===
Some materials such as NaI can "convert" an X photon to a visible photon; an electronic detector can be built by adding a [[photomultiplier]]. These detectors are called "[[scintillator|scintillators]]" or "[[scintillation counter|scintillation counters]]".
 
  
===Direct semiconductor detectors===
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Some materials such as sodium iodide (NaI) can "convert" an X-ray photon to a visible photon; an electronic detector can be built by adding a [[photomultiplier]]. These detectors are called "[[scintillator]]s," filmscreens or "[[scintillation counter]]s." The main advantage of using these is that an adequate image can be obtained while subjecting the patient to a much lower dose of X-rays.
Since the [[1970s]], new [[semiconductor diode detector]]s have been developed ([[silicon]] or [[germanium]] doped with [[lithium]], Si(Li) or Ge(Li)). X-ray photons are converted to electron-hole pairs in the semiconductor, and are collected to detect the X-rays. When the temperature is low enough (the detector is cooled by [[Peltier effect]] or best by liquid [[nitrogen]]), it is possible to directly determine the X-ray energy spectrum; this method is called [[energy dispersive X-ray spectroscopy]] ([[EDX]] or EDS); it is often used in small [[X-ray fluorescence]] spectrometers. These detectors are often called "solid detectors". More recently, [[cadmium telluride]] ([[cadmium|Cd]]Te) and its alloy with [[zinc]], [[cadmium zinc telluride]] detectors have been developed with an increased sensitivity. This allows lower doses of X-rays to be used.
 
  
===Scintillator + Semiconductor detectors===
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===Image Intensification===
With the advent of large semiconductor array detectors it has become possible to design detector systems using a scintillator screen to convert from X-rays to visible light which is then converted to electrical signals in an array detector.
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[[Image:Laprascopy-Roentgen.jpg|right|thumb|250px|X-ray during [[Cholecystectomy]].]]
  
===Visibility to the Human Eye===
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X-rays are also used in "real-time" procedures such as [[angiography]] or contrast studies of the hollow organs (e.g. [[barium enema]] of the small or large intestine) using [[fluoroscopy]] acquired using an [[X-ray image intensifier]]. [[Angioplasty]], medical interventions of the arterial system, rely heavily on X-ray-sensitive contrast to identify potentially treatable lesions.
It is commonly thought that X-rays are invisible to the human eye, and for almost all everyday uses of X-rays this may seem true; however, very strictly speaking, it is actually false. In special circumstances, X-rays are in fact visible to the "naked [[eye]]". An effect first discovered by Brandes in experimentation a short time after [[Wilhelm Röntgen|Röntgen's]] landmark 1895 paper; he 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.[http://www.orau.org/ptp/articlesstories/invisiblelight.htm] Upon hearing this, Röntgen reviewed his record books and found he in fact, also saw the effect. When placing an X-ray tube on the opposite side of a wooden door Röntgen saw the same blue glow seeming to emanate from the eye itself, but thought his observations were spurious due to the fact that he only saw the effect when he used one type of tube. Later he realized that the tube which
 
created the effect was the only one which produced X-rays powerful enough to make the glow plainly visible and the [[experiment]] was thereafter repeated readily. The fact that X-rays are actually faintly visible to the dark-adapted naked eye has largely been forgotten today is probably due to the lack of desire to repeat what we would now see as a recklessly dangerous and harmful experiment with [[ionizing radiation]]. It is not known what the exact mechanism in the eye is which produces the visibility and it could be due to either 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 and then conventional retinal detection of the secondarily produced visible light.
 
  
==Medical uses==
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===Direct Semiconductor Detectors===
[[Image:Dental x-ray.jpg|right|thumb|222px||[[X-ray]]s can reveal the details of bones and teeth]]
 
[[Image:X-Ray Skull.jpg|right|thumb|150px||]]
 
Since Röntgen's discovery that X-rays can identify bony structures, X-rays have been developed for their use in [[medical imaging]]. [[Radiology]] is a specialised field of [[medicine]] that employs [[radiography]] and other techniques for [[diagnostic imaging]]. Indeed, this is probably the most common use of X-ray technology.
 
  
The use of X-rays are especially useful in the detection of pathology of the [[bone|skeletal system]], but are also useful 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 [[ileus]] (blockage of the [[intestine]]), free air (from visceral perforations) and free fluid (in [[ascites]]). In some cases, the use of X-rays is debatable, such as [[gallstone]]s (which are rarely [[radiopaque]]) or [[kidney stone]]s (which are often visible, but not always). Also, Traditional plain X-rays pose very little use in the imaging of soft tissues such as the [[brain]] or [[muscle]]. Imaging alternatives for soft tissues are [[computed axial tomography]] (CAT or CT scanning), [[magnetic resonance imaging]] (MRI) or [[medical ultrasonography|ultrasound]].
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Since the 1970s, new [[semiconductor detector]]s have been developed ([[silicon]] or [[germanium]] doped with [[lithium]], Si(Li) or Ge(Li)). X-ray photons are converted to electron-hole pairs in the semiconductor and are collected to detect the X-rays. When the temperature is low enough (the detector is cooled by [[Peltier effect]] or best by liquid [[nitrogen]]), it is possible to directly determine the X-ray energy spectrum; this method is called [[energy dispersive X-ray spectroscopy]] ([[EDX]] or EDS); it is often used in small [[X-ray fluorescence]] spectrometers. These detectors are sometimes called "solid detectors." [[Cadmium telluride]] ([[cadmium|Cd]]Te) and its alloy with [[zinc]], [[cadmium zinc telluride]] detectors have an increased sensitivity, which allows lower doses of X-rays to be used.
  
X-rays are also used in "real-time" procedures such as [[angiography]] or contrast studies of the hollow organs (e.g. [[barium enema]] of the small or large intestine) using [[fluoroscopy]]. [[Angioplasty]], medical interventions of the arterial system, rely heavily on X-ray-sensitive contrast to identify potentially treatable lesions.
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Practical application in Medical Imaging didn't start taking place until the 1990s. Currently amorphous [[selenium]] is used in commercial large area flat panel x-ray detectors for chest [[radiography]] and [[mammography]].
  
[[Radiotherapy]], a curative medical intervention, now used almost exclusively for [[cancer]], employs higher energies of radiation.
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Note: A standard [[semiconductor]] [[diode]], such as a 1N4007, will produce a small amount of current when placed in an X-ray beam. A test device once used by Medical Imaging Service personnel was a small project box that contained several diodes of this type in [[series and parallel circuits#Series circuits|series]], which could be connected to an [[oscilloscope]] as a quick diagnostic.  
  
==History==
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Silicon drift detectors<ref>[http://www.ketek.biz/x-ray.detectors.php Silicon drift detectors], KETEK GmbH. Retrieved April 16, 2008.</ref> (SDDs), produced by conventional [[semiconductor fabrication]], now provide a cost-effective and high resolving radiation measurement. They replace conventional X-ray detectors, such as Si(Li)s, as they do not need to be cooled with liquid [[nitrogen]].
Among the important early researchers in X-rays were Professor [[Ivan Pului]], Sir [[William Crookes]], [[Johann Wilhelm Hittorf]], [[Eugen Goldstein]], [[Heinrich Hertz]], [[Philipp Lenard]], [[Hermann von Helmholtz]], [[Nikola Tesla]], [[Thomas Edison]], [[Charles Glover Barkla]], [[Max von Laue]], and [[Wilhelm Conrad Röntgen]].
 
  
[[Image:Roentgen-Roehre.png|thumb|left|222px|Diagram of a water cooled X-ray tube. (simplified/outdated)]]
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===Scintillator and semiconductor detectors (indirect detection)===
  
Physicist Johann Hittorf (1824 - 1914) observed [[vacuum tube|tubes]] with energy rays extending from a negative electrode. These rays produced a fluorescence when they hit the glass walls of the tubes. In [[1876]] the effect was named "[[Cathode ray|cathode rays]]" by [[Eugen Goldstein]]. Later, English physicist [[William Crookes]] investigated the effects of energy discharges on rare gases, and constructed what is called the Crookes tube. It is a glass vacuum cylinder, containing electrodes for discharges of a high voltage electric current. He found, when he placed unexposed photographic plates near the tube, that some of them were flawed by shadows, though he did not investigate this effect.
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With the advent of large [[semiconductor]] array [[detectors]] it has become possible to design detector systems using a scintillator screen to convert from X-rays to visible light which is then converted to [[electrical]] signals in an array detector. Indirect [[Flat Panel]] Detectors (FPDs) are in widespread use today in [[medicine|medical]], [[dentistry|dental]], [[veterinary]], and industrial applications. A common form of these detectors is based on amorphous silicon TFT/photodiode arrays.  
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The array technology is a variant on the amorphous [[silicon]] TFT arrays used in many flat panel displays, like the ones in [[computer]] laptops. The array consists of a sheet of [[glass]] covered with a thin layer of silicon that is in an amorphous or disordered state. At a [[microscopic]] scale, the silicon has been imprinted with millions of transistors arranged in a highly ordered array, like the grid on a sheet of graph paper. Each of these thin film transistors (TFT’s) are attached to a [[light]]-absorbing photodiode making up an individual pixel (picture element). Photons striking the photodiode are converted into two carriers of electrical charge, called electron-hole pairs. Since the number of charge carriers produced will vary with the intensity of incoming light photons, an electrical pattern is created that can be swiftly converted to a voltage and then a digital signal, which is interpreted by a computer to produce a digital image. Although silicon has outstanding electronic properties, it is not a particularly good absorber of X-ray [[photons]]. For this reason, X-rays first impinge upon scintillators made from either gadolinium oxysulfide or cesium iodide. The scintillator absorbs the X-rays and converts them into visible light photons that then pass onto the photodiode array.
  
In [[April]] [[1887]], [[Nikola Tesla]] began to investigate X-rays using high voltages and vacuum tubes of his own design, as well as Crookes tubes. From his technical publications, it is indicated that he invented and developed a special single-electrode X-ray tube, which differed from other X-ray tubes in having no target electrode. He stated these facts in his [[1897]] X-ray lecture before the [[New York Academy of Sciences]].
+
===Visibility to the human eye===
  
The principle behind Tesla's device is nowadays called the [[Bremsstrahlung]] process, in which a high-energy secondary X-ray emission is produced when charged particles (such as electrons) pass through matter. By [[1892]], Tesla performed several such experiments, but he did not categorize the emissions as what were later called X-rays, instead generalizing the phenomenon as [[radiant energy]]. He did not publicly declare his findings nor did he make them widely known. His subsequent X-ray experimentation by vacuum high field emissions led him to alert the scientific community to the biological hazards associated with X-ray exposure.
+
While generally considered invisible to the human eye, in special circumstances X-rays can be visible. Brandes, in an experiment a short time after [[Wilhelm Röntgen|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 that seemed to originate within the eye itself.<ref>[http://www.orau.org/ptp/articlesstories/invisiblelight.htm Tales from the Atomic Age], Thomas W. Frame. Retrieved April 16, 2008.</ref> Upon hearing this, Röntgen reviewed his record books and found he too had seen the effect. When placing an X-ray tube on the opposite side of a wooden door Röntgen had noted the same blue glow, seeming to emanate from the eye itself, but thought his observations to be spurious because he only saw the effect when he used one type of tube. Later he realized that the tube which had created the effect was the only one powerful enough to make the glow plainly visible and the [[experiment]] was thereafter readily repeatable.
  
In 1892, [[Heinrich Hertz]] began experimenting and demonstrated that cathode rays could penetrate very thin metal foil (such as [[aluminium]]). [[Philipp Lenard]], a student of Heinrich Hertz, further researched this effect. He developed a version of the cathode tube and studied the penetration by X-rays of various materials. Philipp Lenard, though, did not realize that he was producing X-rays. [[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 (''Wiedmann's Annalen'', Vol. XLVIII).  However, he did not work with actual X-rays.
+
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 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); or 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.
  
[[Image:Roentgen-x-ray-von-kollikers-hand.jpg|thumb|right|An X-ray picture (radiograph) taken by [[Wilhelm Conrad Röntgen|Wilhelm Röntgen]]]]
+
==Medical uses==
On [[November 8]] [[1895]], [[Wilhelm Röntgen]], a [[Germany|German]] scientist, began observing and further documenting X-rays while experimenting with vacuum tubes. Röntgen, on [[December 28]], [[1895]], wrote a preliminary report "''On a new kind of ray: A preliminary communication''". He submitted it to the [[Würzburg]]'s Physical-Medical Society journal. This was the first formal and public recognition of the categorization of 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, where available see  the list of titles for versions of this article in other languages. Röntgen received the first [[Nobel Prize in Physics]] for his discovery.
+
[[Image:Dental x-ray.jpg|right|thumb|222px||X-rays can reveal the details of bones and teeth.]]
 +
[[Image:X-Ray Skull.jpg|right|thumb|150px||X-Ray Image of a male skull.]]
  
[[Wilhelm Conrad Röntgen]] was working on a primitive [[cathode]] ray generator that was projected through a glass vacuum tube. All of a sudden he noticed a faint green light against the wall. The odd thing he had noticed, was that the light from the cathode ray generator was traveling through a bunch of the materials in its way (paper,wood, and books). He then started to put various objects in front of the generator,and as he was doing this, he noticed that the outline of the bones from his hand were displayed on the wall. He then studied this phenomenon in [[seclusion]].
+
Since Röntgen's discovery that X-rays can identify bony structures, X-rays have been developed for their use in [[medical imaging]]. [[Radiology]] is a specialized field of [[medicine]]. Radiographers employ [[radiography]] and other techniques for [[diagnostic imaging]]. Indeed, this is probably the most common use of X-ray technology.
  
In [[1895]], [[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 X-ray examinations. Nevertheless, Edison dropped X-ray research around [[1903]] after 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 [[amputation|amputated]] in a futile attempt to save his life[http://www.ratical.org/radiation/KillingOurOwn/KOO6.html]. In [[1906]], physicist [[Charles Glover Barkla|Charles Barkla]] discovered that X-rays could be scattered by gases, and that each element had a characteristic X-ray. He won the [[1917]] [[Nobel Prize in Physics]] for this discovery.
+
X-rays are especially useful in the detection of pathology of the [[bone|skeletal system]], but are also useful 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 [[ileus]] (blockage of the [[intestine]]), free air (from visceral perforations) and free fluid (in [[ascites]]). In some cases, the use of X-rays is debatable, such as [[gallstone]]s (which are rarely [[radiopaque]]) or [[kidney stone]]s (which are often visible, but not always). Also, traditional plain X-rays pose very little use in the imaging of soft tissues such as the [[brain]] or [[muscle]]. Imaging alternatives for soft tissues are [[computed axial tomography]] (CAT or CT scanning), [[magnetic resonance imaging]] (MRI) or [[medical ultrasonography|ultrasound]]. Since 2005, X-rays are listed as a carcinogen by the U.S. government.
  
The use of X-rays for medical purposes (to develop into the field of [[radiation therapy]]) was pioneered by Major [[John Hall-Edwards]] in [[Birmingham]], [[England]]. In [[1908]], he had to have his left arm amputated owing to the spread of [[X-ray dermatitis]][http://www.birmingham.gov.uk/xray]. In the [[1950s]] X-rays were first harnessed to produce an [[X-ray microscope]].
+
[[Radiotherapy]], a curative medical intervention, now used almost exclusively for [[cancer]], employs higher energies of radiation.
  
[[Image:Moon in x-rays.gif|thumb|right|222px|[[ROSAT]] image of X-ray [[fluorescence]] of, and [[occultation]] of the [[X-ray background]] by, the [[Moon]].]]
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The efficiency of X-ray tubes is less than 2 percent. Most of the energy is used to heat up the [[anode]].
  
In the [[1980s]] an X-ray laser device was proposed as part of the [[Ronald Reagan|Reagan]] administration's [[Strategic Defense Initiative]], but the first and 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 [[George W. Bush|Bush]] administration as [[National Missile Defense]] using different technologies).
+
==Other uses==
  
In the [[1990s]] the [[Chandra X-Ray Observatory]] was launched, allowing the exploration of the very violent processes in the universe which produce X-Rays. Unlike visible light, which is 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 novas, neutron stars that build up layers of plasma that then explode into space.
+
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 [[atom]]s in a crystal is recorded and then analyzed to reveal the nature of that lattice.
 +
*[[X-ray astronomy]], which is an observational branch of [[astronomy]], which deals with the study of X-ray emission from celestial objects.
 +
*[[X-ray microscope|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.
 +
*Paintings are often X-rayed to reveal the underdrawing and [[pentimento|pentimenti]] or alterations in the course of painting, or by later restorers. Many [[pigment]]s such as [[lead white]] show well in X-ray photographs.
  
 
==See also==
 
==See also==
 +
 +
* [[Electromagnetic spectrum]]
 
* [[X-ray crystallography]]
 
* [[X-ray crystallography]]
 
* [[X-ray astronomy]]
 
* [[X-ray astronomy]]
* [[X-ray machine]]
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* [[X-ray microscope]]
* [[X-ray microscopy]]
+
* [[X-ray optics]]
 
* [[Geiger counter]]
 
* [[Geiger counter]]
* [[N ray|N-ray]]
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* [[Radiography]]
* [[X-ray vision]]
+
 
 +
==Notes==
 +
<references/>
 +
 
 +
==References==
 +
*Guinier, A. 1994. ''X-Ray Diffraction: In Crystals, Imperfect Crystals, and Amorphous Bodies''. Mineola, NY: Dover Publications. ISBN 0486680118.
 +
*Fitzgerald, Frances. 2001. ''Way Out There in the Blue: Reagan, Star Wars and the End of the Cold War''. New York, NY: Simon & Schuster. ISBN 0743200233.
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*Rothenberg, Mikel A. 1998. ''Understanding X-Rays: A Plain English Approach''. Eau Claire, WI: PESI HealthCare. ISBN 1559579994.
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*Sample, Sharron. [http://imagers.gsfc.nasa.gov/ems/xrays.html The Electromagnetic Spectrum: X-rays.] ''NASA''. Retrieved October 14, 2007.
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==External links==
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*[http://www.bigs.de/en/shop/htm/roentgen01.html Design of an x-ray tube (Animation)] – ''BIGS''.
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*[http://www.bbc.co.uk/dna/h2g2/A767225 X-Rays.] – ''h2g2''.
  
 
{{EMSpectrum}}
 
{{EMSpectrum}}
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{{Nuclear Technology}}
  
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Revision as of 14:36, 1 August 2013

An X-ray picture (radiograph), taken by Wilhelm Röntgen in 1896, of Albert von Kölliker's hand.

X-rays (or Röntgen rays) are a form of electromagnetic radiation with a wavelength in the range of 10 to 0.01 nanometers, corresponding to frequencies in the range 30 to 30,000 PHz (1015 hertz). X-rays are a form of ionizing radiation and as such can be dangerous. In many languages it is called Röntgen radiation after one of the investigators of the radiation, Wilhelm Röntgen.

X-rays are valuable in the field of medicine and for scientific research. In medicine, they are primarily used for diagnostic radiography. In science, they are used for the determination of crystal structures by the technique known as X-ray crystallography. In addition, X-ray telescopes are important for astronomical observations. As technology develops at a steady pace, many other applications of X-rays may emerge as well.

History

Among the important early researchers in X-rays were Professor Ivan Pului, Sir William Crookes, Johann Wilhelm Hittorf, Eugen Goldstein, Heinrich Hertz, Philipp Lenard, Hermann von Helmholtz, Nikola Tesla, Thomas Edison, Charles Glover Barkla, Max von Laue, and Wilhelm Conrad Röntgen.

Diagram of a water cooled X-ray tube. (simplified/outdated)

Physicist Johann Hittorf (1824-1914) observed tubes with energy rays extending from a negative electrode. These rays produced a fluorescence when they hit the glass walls of the tubes. In 1876 the effect was named "cathode rays" by Eugen Goldstein, and today are known to be streams of electrons. Later, English physicist William Crookes investigated the effects of electric currents in gases at low pressure, and constructed what is called the Crookes tube. It is a glass vacuum cylinder, containing electrodes for discharges of a high voltage electric current. He found, when he placed unexposed photographic plates near the tube, that some of them were flawed by shadows, though he did not investigate this effect.

Tesla

In April 1887, Nikola Tesla began to investigate X-rays using high voltages and vacuum tubes of his own design, as well as Crookes tubes. From his technical publications, it is indicated that he invented and developed a special single-electrode X-ray tube, which differed from other X-ray tubes in having no target electrode. He stated these facts in his 1897 X-ray lecture before the New York Academy of Sciences.

The principle behind Tesla's device is nowadays called the Bremsstrahlung process, in which a high-energy secondary X-ray emission is produced when charged particles (such as electrons) pass through matter. By 1892, Tesla performed several such experiments, but he did not categorize the emissions as what were later called X-rays, instead generalizing the phenomenon as radiant energy. He did not publicly declare his findings nor did he make them widely known. His subsequent X-ray experimentation by vacuum high field emissions led him to alert the scientific community to the biological hazards associated with X-ray exposure.

Hertz

In 1892, Heinrich Hertz began experimenting and demonstrated that cathode rays could penetrate very thin metal foil (such as aluminum). Philipp Lenard, a student of Heinrich Hertz, further researched this effect. He developed a version of the cathode tube and studied the penetration by X-rays of various materials. Philipp Lenard, though, did not realize that he was producing X-rays. 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 (Wiedmann's Annalen, Vol. XLVIII). However, he did not work with actual X-rays.

Röntgen

On November 8, 1895, Wilhelm Conrad Röntgen, a German scientist, began observing and further documenting X-rays while experimenting with vacuum tubes. Röntgen, on December 28, 1895, wrote a preliminary report "On a new kind of ray: A preliminary communication." He submitted it to the Würzburg's Physical-Medical Society journal. This was the first formal and public recognition of the categorization of 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. Röntgen received the first Nobel Prize in Physics for his discovery.

Röntgen was working on a primitive cathode ray generator that was projected through a glass vacuum tube. Suddenly he noticed a faint green light against the wall. The odd thing he had noticed, was that the light from the cathode ray generator was traveling through a bunch of the materials in its way (paper, wood, and books). He then started to put various objects in front of the generator, and as he was doing this, he noticed that the outline of the bones from his hand were displayed on the wall. Röntgen said he did not know what to think and kept experimenting. Two months after his initial discovery, he published his paper translated "On a New Kind of Radiation" and gave a demonstration in 1896.

Rontgen discovered its medical use when he saw picture of his wife's hand on a photographic plate formed due to X-Rays. His wife's hand's photograph was the first ever photograph of a human body part using the X-Rays.

Edison

In 1895, 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 X-ray examinations. Nevertheless, Edison dropped X-ray research around 1903 after 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 twentieth century and beyond

In 1906, physicist Charles Barkla discovered that X-rays could be scattered by gases, and that each element had a characteristic X-ray. He won the 1917 Nobel Prize in Physics for this discovery.

In 1912, Max von Laue, Paul Knipping and Walter Friedrich observed for the first time the diffraction of X-rays by crystals. This discovery, along with the early works of Paul Peter Ewald, William Henry Bragg, and William Lawrence Bragg gave birth to the field of X-ray crystallography.

The use of X-rays for medical purposes (to develop into the field of radiation therapy) was pioneered by Major John Hall-Edwards in Birmingham, England. In 1908, he had to have his left arm amputated owing to the spread of X-ray dermatitis.[1] In the 1950s, X-rays were used to produce an X-ray microscope.

In the 1980s, an X-ray laser device was proposed as part of the Reagan administration's Strategic Defense Initiative, but the first and 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).

In the 1990s, the Chandra X-ray Observatory was launched, allowing the exploration of the very violent processes in the universe which produce X-rays. Unlike visible light, which is 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 novas, neutron stars that build up layers of plasma that then explode into space.

Physics

X-rays are a type of electromagnetic radiation with wavelengths of around 10-10 meters. When medical X-rays are being produced, a thin metallic sheet is placed between the emitter and the target, effectively filtering out the lower energy (soft) X-rays. This is often placed close to the window of the X-ray tube. The resultant X-ray is said to be hard. Soft X-rays overlap the range of extreme ultraviolet. The frequency of hard X-rays is higher than that of soft X-rays, and the wavelength is shorter. Hard X-rays overlap the range of "long"-wavelength (lower energy) gamma rays, however the distinction between the two terms depends on the source of the radiation, not its wavelength; X-ray photons are generated by energetic electron processes, gamma rays by transitions within atomic nuclei.

X-ray K-series spectral line wavelengths (nm) for some common target materials in David R. Lide: CRC Handbook of Chemistry and Physics 75th edition. CRC Press, 10-227. ISBN 0-8493-0475-X. 
Target Kβ₁ Kβ₂ Kα₁ Kα₂
Fe 0.17566 0.17442 0.193604 0.193998
Ni 0.15001 0.14886 0.165791 0.166175
Cu 0.139222 0.138109 0.154056 0.154439
Zr 0.070173 0.068993 0.078593 0.079015
Mo 0.063229 0.062099 0.070930 0.071359

The basic production of X-rays is by accelerating electrons in order to collide with a metal target (usually tungsten or a more crack resistant alloy of rhenium (5 percent) and tungsten (95 percent), but sometimes molybdenum for more specialized applications). Molybdenum targets are often used when soft X-rays are needed as in mammography. Here the electrons suddenly decelerate upon colliding with the metal target and if enough energy is contained within the electron it is able to knock out an electron from the inner shell of the metal atom and as a result electrons from higher energy levels then fill up the vacancy and X-ray photons are emitted. This process is extremely inefficient and requires a way of removing the heat generated.

This causes the spectral line part of the wavelength distribution. There is also a continuum bremsstrahlung component given off by the electrons as they are scattered by the strong electric field near the high Z (proton number) nuclei.

Nowadays, for many (non medical) applications, X-ray production is achieved by synchrotrons (see synchrotron light).

Detectors

Photographic Plate

The detection of X-rays is based on various methods. The most commonly known method are a photographic plate, X-ray film in a cassette, and rare earth screens.

The X-ray photographic plate or film is used in hospitals to produce images of the internal organs and bones of a patient. Since photographic plates are not generally sensitive to X-rays, phosphorescent screens are usually placed in contact with the emulsion of the plate or film. The X-rays strike the phosphor screen, which emits visible light, which exposes the film. The emulsion still needs to be heavily doped with silver compounds and can be coated on both sides of the film or plate. The part of the patient to be X-rayed is placed between the X-ray source and the photographic receptor to produce what is a shadow of all the internal structure of that particular part of the body being X-rayed. The X-rays are blocked by dense tissues such as bone and pass through soft tissues. Those areas where the X-rays strike the photographic receptor turn black when it is developed. So where the X-rays pass through "soft" parts of the body such as organs, muscle, and skin, the plate or film turns black. Contrast compounds containing barium or iodine, which are radiopaque, can be injected in the artery of a particular organ, or given intravenously. The contrast compounds essentially block the X-rays and hence the circulation of the organ can be more readily seen. Many years ago thorium was used as a contrast medium (Thorotrast)—this caused many people to be injured or even die from the effects of the radiation from the thorium.

Photostimulable Phosphors (PSPs)

An increasingly common method of detecting X-rays is the use of Photostimulable Luminescence (PSL), pioneered by Fuji in the 1980s. In modern hospitals a PSP plate is used in place of the photographic plate. After the plate is X-rayed, excited electrons in the phosphor material remain 'trapped' in 'colour centres' in the crystal lattice until stimulated by a laser beam passed over the plate surface. The light given off during laser stimulation is collected by a photomultiplier tube and the resulting signal is converted into a digital image by computer technology, which gives this process its common name, computed radiography. The PSP plate can be used over and over again.

Geiger counter

Initially, most common detection methods were based on the ionisation of gases, as in the Geiger-Müller counter: a sealed volume, usually a cylinder, with a polymer or thin metal window contains a gas, and a wire, and a high voltage is applied between the cylinder (cathode) and the wire (anode). When an X-ray photon enters the cylinder, it ionizes the gas. These ions accelerate toward the anode, in the process causing further ionisation along their trajectory. This process, known as an avalanche, is detected as a sudden flow of current, called a "count" or "event."

Ultimately, the electrons form a virtual cathode around the anode wire drastically reducing the electric field in the outer portions of the tube. This halts the collisional ionizations and limits further growth of avalanches. As a result, all "counts" on a Geiger counter are the same size and it can give no indication as to the particle energy of the radiation, unlike the proportional counter. The intensity of the radiation is measurable by the Geiger counter as the counting-rate of the system.

In order to gain energy spectrum information a diffracting crystal may be used to first separate the different photons, the method is called wavelength dispersive X-ray spectroscopy (WDX or WDS). Position-sensitive detectors are often used in conjunction with dispersive elements. Other detection equipment may be used which are inherently energy-resolving, such as the aforementioned proportional counters. In either case, use of suitable pulse-processing (MCA) equipment allows digital spectra to be created for later analysis.

For many applications, counters are not sealed but are constantly fed with purified gas (thus reducing problems of contamination or gas aging). These are called "flow counter."

Scintillators

Some materials such as sodium iodide (NaI) can "convert" an X-ray photon to a visible photon; an electronic detector can be built by adding a photomultiplier. These detectors are called "scintillators," filmscreens or "scintillation counters." The main advantage of using these is that an adequate image can be obtained while subjecting the patient to a much lower dose of X-rays.

Image Intensification

X-ray during Cholecystectomy.

X-rays are also used in "real-time" procedures such as angiography or contrast studies of the hollow organs (e.g. barium enema of the small or large intestine) using fluoroscopy acquired using an X-ray image intensifier. Angioplasty, medical interventions of the arterial system, rely heavily on X-ray-sensitive contrast to identify potentially treatable lesions.

Direct Semiconductor Detectors

Since the 1970s, new semiconductor detectors have been developed (silicon or germanium doped with lithium, Si(Li) or Ge(Li)). X-ray photons are converted to electron-hole pairs in the semiconductor and are collected to detect the X-rays. When the temperature is low enough (the detector is cooled by Peltier effect or best by liquid nitrogen), it is possible to directly determine the X-ray energy spectrum; this method is called energy dispersive X-ray spectroscopy (EDX or EDS); it is often used in small X-ray fluorescence spectrometers. These detectors are sometimes called "solid detectors." Cadmium telluride (CdTe) and its alloy with zinc, cadmium zinc telluride detectors have an increased sensitivity, which allows lower doses of X-rays to be used.

Practical application in Medical Imaging didn't start taking place until the 1990s. Currently amorphous selenium is used in commercial large area flat panel x-ray detectors for chest radiography and mammography.

Note: A standard semiconductor diode, such as a 1N4007, will produce a small amount of current when placed in an X-ray beam. A test device once used by Medical Imaging Service personnel was a small project box that contained several diodes of this type in series, which could be connected to an oscilloscope as a quick diagnostic.

Silicon drift detectors[2] (SDDs), produced by conventional semiconductor fabrication, now provide a cost-effective and high resolving radiation measurement. They replace conventional X-ray detectors, such as Si(Li)s, as they do not need to be cooled with liquid nitrogen.

Scintillator and semiconductor detectors (indirect detection)

With the advent of large semiconductor array detectors it has become possible to design detector systems using a scintillator screen to convert from X-rays to visible light which is then converted to electrical signals in an array detector. Indirect Flat Panel Detectors (FPDs) are in widespread use today in medical, dental, veterinary, and industrial applications. A common form of these detectors is based on amorphous silicon TFT/photodiode arrays.

The array technology is a variant on the amorphous silicon TFT arrays used in many flat panel displays, like the ones in computer laptops. The array consists of a sheet of glass covered with a thin layer of silicon that is in an amorphous or disordered state. At a microscopic scale, the silicon has been imprinted with millions of transistors arranged in a highly ordered array, like the grid on a sheet of graph paper. Each of these thin film transistors (TFT’s) are attached to a light-absorbing photodiode making up an individual pixel (picture element). Photons striking the photodiode are converted into two carriers of electrical charge, called electron-hole pairs. Since the number of charge carriers produced will vary with the intensity of incoming light photons, an electrical pattern is created that can be swiftly converted to a voltage and then a digital signal, which is interpreted by a computer to produce a digital image. Although silicon has outstanding electronic properties, it is not a particularly good absorber of X-ray photons. For this reason, X-rays first impinge upon scintillators made from either gadolinium oxysulfide or cesium iodide. The scintillator absorbs the X-rays and converts them into visible light photons that then pass onto the photodiode array.

Visibility to the human eye

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 that seemed to originate within the eye itself.[3] 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 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); or 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.

Medical uses

File:Dental x-ray.jpg
X-rays can reveal the details of bones and teeth.
X-Ray Image of a male skull.

Since Röntgen's discovery that X-rays can identify bony structures, X-rays have been developed for their use in medical imaging. Radiology is a specialized field of medicine. Radiographers employ radiography and other techniques for diagnostic imaging. Indeed, this is probably the most common use of X-ray technology.

X-rays are especially useful in the detection of pathology of the skeletal system, but are also useful 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 ileus (blockage of the intestine), free air (from visceral perforations) and free fluid (in ascites). In some cases, the use of X-rays is debatable, such as gallstones (which are rarely radiopaque) or kidney stones (which are often visible, but not always). Also, traditional plain X-rays pose very little use in the imaging of soft tissues such as the brain or muscle. Imaging alternatives for soft tissues are computed axial tomography (CAT or CT scanning), magnetic resonance imaging (MRI) or ultrasound. Since 2005, X-rays are listed as a carcinogen by the U.S. government.

Radiotherapy, a curative medical intervention, now used almost exclusively for cancer, employs higher energies of radiation.

The efficiency of X-ray tubes is less than 2 percent. Most of the energy is used to heat up the anode.

Other uses

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 analyzed to reveal the nature of that lattice.
  • 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.
  • Paintings are often X-rayed to reveal the underdrawing and pentimenti or alterations in the course of painting, or by later restorers. Many pigments such as lead white show well in X-ray photographs.

See also

Notes

  1. Major John Hall-Edwards, Birmingham City Council. Retrieved April 16, 2008.
  2. Silicon drift detectors, KETEK GmbH. Retrieved April 16, 2008.
  3. Tales from the Atomic Age, Thomas W. Frame. Retrieved April 16, 2008.

References
ISBN links support NWE through referral fees

  • Guinier, A. 1994. X-Ray Diffraction: In Crystals, Imperfect Crystals, and Amorphous Bodies. Mineola, NY: Dover Publications. ISBN 0486680118.
  • Fitzgerald, Frances. 2001. Way Out There in the Blue: Reagan, Star Wars and the End of the Cold War. New York, NY: Simon & Schuster. ISBN 0743200233.
  • Rothenberg, Mikel A. 1998. Understanding X-Rays: A Plain English Approach. Eau Claire, WI: PESI HealthCare. ISBN 1559579994.
  • Sample, Sharron. The Electromagnetic Spectrum: X-rays. NASA. Retrieved October 14, 2007.

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