Help building the largest human-edited directory of the web Suggest URL - Open Directory Project - Become an editor

directopedia.org uses links and structure from dmoz Open Directory Project.
The contents has been generating using technology developed by scientec.

An X-ray or Röntgen ray is a form of electromagnetic radiation with a wavelength in the range of 10 nanometers to 100 picometers (corresponding to frequencies in the range 30 PHz to 3 EHz). X-rays are primarily used for diagnostic medical imaging and crystallography. X-rays are a form of ionizing radiation and as such can be dangerous.

Contents 1 Physics 2 Detectors 2.1 Photographic plates 2.2 Geiger counters 2.3 Scintillators 2.4 Direct semiconductor detectors 2.5 Scintillator + Semiconductor detectors 2.6 Visibility to the Human Eye 3 Medical uses 4 History 4.1 Tesla 4.2 Hertz 4.3 Röntgen 4.4 Edison 4.5 The 20th century and beyond 5 References 6 See also

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 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

Unverified: if a reliable reference exists, please check the values and cite the reference.

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 (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. 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 synchrotrons (see synchrotron light).

Detectors

Photographic plates

The detection of X-rays is based on various methods. The most commonly known method are a photographic plate and a fluorescent screen.

The X-ray photographic plate is frequently used in hospitals 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.

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. The plastic sheet can be used over and over again.

Geiger counters

Initially, most common detection methods were based on the ionisation of gases, as in the 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 diffracting crystal to first separate the different photons, the method is called wavelength dispersive X-ray spectroscopy (WDX or WDS).

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 "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.

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 have an increased sensitivity, which allows lower doses of X-rays to be used.

Silicon drift detectors (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 + Semiconductor detectors

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.

Visibility to the Human Eye

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 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.[1] 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

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 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 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.

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.

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

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.

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. 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.

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 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.

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, 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.

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.

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[2].

The 20th 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.

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[3]. In the 1950s X-rays were first harnessed 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.

References

• Nasa Goddard Space Flight centre introduction to x-rays.
• Way Out There in the Blue: Reagan, Star Wars and the End of the Cold War, Frances Fitzgerald, Simon & Schuster (2001). ISBN 0743200233

See also

• X-ray crystallography
• X-ray astronomy
• X-ray machine
• X-ray microscopy
• Geiger counter
• N-ray
• X-ray vision
• X-Ray Specs

Electromagnetic Spectrum Gamma ray | X-ray | Ultraviolet | Optical spectrum | Infrared | Terahertz radiation | Microwave | Radio waves Optical (visible) spectrum: Violet | Indigo | Blue | Green | Yellow | Orange | Red

This article is based on the article "X-ray" from Wikipedia - the free encyclopedia created and edited by online user community. This article is distributed under the terms of GNU Free Documentation License. Here you find the list of authors of this article. The article can only edited within Wikipedia. Edit this article in Wikipedia.

This article is about electromagnetic radiation. For the power metal band, see Gamma Ray (band)

Gamma rays (often denoted by the Greek letter gamma, γ) are an energetic form of electromagnetic radiation produced by radioactivity or other nuclear or subatomic processes such as electron-positron annihilation.

Gamma rays form the highest-energy end of the electromagnetic spectrum. They are often defined to begin at an energy of 10 keV, a frequency of/ 2.42 EHz, or a wavelength of/ 124 pm, although electromagnetic radiation from around 10 keV to several hundred keV is also referred to as hard X rays. It is important to note that there is no physical difference between gamma rays and X rays of the same energy — they are two names for the same electromagnetic radiation, just as sunlight and moonlight are two names for visible light. Rather, gamma rays are distinguished from X rays by their origin. Gamma ray is a term for high-energy electromagnetic radiation produced by nuclear transitions, while X ray is a term for high-energy electromagnetic radiation produced by energy transitions due to accelerating electrons. Because it is possible for some electron transitions to be of higher energy than some nuclear transitions, there is an overlap between what we call low energy gamma rays and high energy X-rays.

Gamma rays are a form of ionizing radiation; they are more penetrating than either alpha or beta radiation (neither of which is electromagnetic radiation), but less ionizing. They produce damage similar to that caused by X-rays, such as burns, cancer, and genetic mutations. Gamma rays from nuclear fallout would probably cause the largest number of casualties in the event of the use of nuclear weapons in a nuclear war. An effective fallout shelter reduces human exposure at least 1000 times.

Gamma sources are used for a range of applications in both medicine and industry for further details see commonly used gamma emitting isotopes.

Nuclear processes

Radioactive decay processes Alpha decay Beta decay Cluster decay Double beta decay Double electron capture Electron capture Gamma radiation Internal conversion Isomeric transition Neutron emission Positron emission Proton emission Spontaneous fission Nucleosynthesis Neutron Capture The R-process The S-process Proton capture: The P-process

Contents 1 Shielding 2 Interaction with matter 3 Uses 4 History 5 Culture 6 References 7 See also

Shielding

Shielding for γ rays requires large amounts of mass. The material used for shielding takes into account that gamma rays are better absorbed by materials with high atomic number and high density. Also, the higher the energy of the gamma rays, the thicker the shielding required. Materials for shielding gamma rays are typically illustrated by the thickness required to reduce the intensity of the gamma rays by one half (the half value layer or HVL). For example, gamma rays that require 1 cm (0.4 inches) of lead to reduce their intensity by 50% will also have their intensity reduced in half by 6 cm (2½ inches) of concrete or 9 cm (3½ inches) of packed dirt.

Interaction with matter

In terms of ionization, gamma radiation interacts with matter via three main processes: the photoelectric effect, Compton scattering, and pair production.

• Photoelectric Effect: This describes the case in which a gamma photon interacts with and transfers all of its energy to an orbital electron, ejecting that electron from the atom. The kinetic energy of the resulting photoelectron is equal to the energy of the incident gamma photon minus the binding energy of the electron. The photoelectric effect is thought to be the dominant energy transfer mechanism for x-ray and gamma ray photons with energies below 50 keV (thousand electron volts), but it is much less important at higher energies.

• Compton Scattering: This is an interaction in which an incident gamma photon loses enough energy to an orbital electron to cause its ejection, with the remainder of the original photon's energy being emitted as a new, lower energy gamma photon with an emission direction different from that of the incident gamma photon. The probability of Compton scatter decreases with increasing photon energy. Compton scattering is thought to be the principal absorption mechanism for gamma rays in the intermediate energy range 100 keV to 10 MeV (megaelectronvolts), an energy spectrum which includes most gamma radiation present in a nuclear explosion. Compton scattering is relatively independent of the atomic number of the absorbing material.

• Pair Production: By interaction in the vicinity of the coulomb force of the nucleus, the energy of the incident photon is spontaneously converted into the mass of an electron-positron pair. A positron is the matter equivalent of an electron; it has the same weight as an electron, but it has a positive charge equal in strength to the negative charge of an electron. Energy in excess of the equivalent rest mass of the two particles (1.02 MeV) appears as the kinetic energy of the pair and the recoil nucleus. The electron of the pair, frequently referred to as the secondary electron, is densely ionizing. The positron has a very short lifetime. It combines within 10^-8 seconds with a free electron. The entire mass of these two particles is then converted into two gamma photons of 0.51 MeV energy each.

Gamma rays are often produced alongside other forms of radiation such as alpha or beta. When a nucleus emits an α or β particle, the daughter nucleus is sometimes left in an excited state. It can then jump down to a lower level by emitting a gamma ray in much the same way that an atomic electron can jump to a lower level by emitting ultraviolet radiation.

Gamma rays, x-rays, visible light, and UV rays are all forms of electromagnetic radiation. The only difference is the frequency and hence the energy of the photons. Gamma rays are the most energetic. An example of gamma ray production follows.

First cobalt-60 decays to excited nickel-60 by beta decay:

Then the nickel-60 drops down to the ground state (see nuclear shell model) by emitting a gamma ray:

Gamma rays of 1.17 MeV and 1.33 MeV are produced.

Another example is that Am-241 decays by alpha decay to form Np-237, this alpha decay is accompanied by gamma emission. In some cases the gamma emission spectrum for a nucleus is quite simple (eg Co-60/Ni-60) while in other cases such as (Am-241/Np-237 and Ir-192/Pt-192) the gamma emission spectrum is complex revealing that a series of nuclear energy levels can exist. The fact that an alpha spectrum can have a series of different peaks with different energies reinforces the idea that several nuclear energy levels are possible.

Because a beta decay is accompanied by the emission of a neutrino which also carries away energy, the beta spectrum does not have sharp lines, but instead it is a broad peak. Hence from beta decay alone it is not possible to probe the different energy levels found in the nucleus.

In optical spectroscopy it is well known that an entity which is an emitter of light can also absorb light at the same wavelength (photon energy), for instance a sodium flame can emit yellow light, but also it can absorb the yellow light from a sodium vapour lamp. In the case of gamma rays this can be seen in Mössbauer spectroscopy, here a correction for the energy lost by the recoil of the nucleus is made and the exact conditions for gamma ray absorption through resonance can be attained.

This can be thought of as being similar to the Frank Condon effects seen in optical spectroscopy.

Uses

The powerful nature of gamma rays have made them useful in the sterilizing of medical equipment by killing bacteria. They are also used to kill bacteria and insects in foodstuffs, particularly meat and vegetables, to maintain freshness.

In spite of their cancer-causing properties, gamma rays are also used to treat some types of cancer. In the procedure called gamma-knife surgery, multiple concentrated beams of gamma rays are directed on the growth in order to kill the cancerous cells. The beams are aimed from different angles to focus the radiation on the growth while minimising damage to the surrounding tissues.

Gamma rays are also used for diagnostic purposes in nuclear medicine. Several gamma-emitting radioisotopes are used, one of which is technetium-99m. When administered to a patient, a gamma camera can be used to form an image of the radioisotope's distribution by detecting the gamma radiation emitted. Such a technique can be employed to diagnose a wide range of conditions (e.g. spread of cancer to the bones).

Gamma ray detectors are also starting to be used in Singapore and Pakistan as part of the Container Security Initiative (CSI). These US$5 million machines are advertised to scan 30 containers per hour. The objective of this technique is to pre-screen merchant ship containers before they enter U.S. ports. [1]

History

Gamma rays were discovered by the French chemist and physicist, Paul Ulrich Villard in 1900 while he was studying uranium. Working in the chemistry department of the École Normale in rue d'Ulm, Paris with self-constructed equipment, he found that the rays were not bent by a magnetic field.

For a time, it was assumed that gamma rays were particles. The fact that they were rays was demonstrated by the British Physicist, William Henry Bragg in 1910 when he showed that the rays ionized gas in a similar way to X-rays.

In 1914, Ernest Rutherford and Edward Andrade showed that gamma rays were a form of electromagnetic radiation by measuring their wavelengths using crystal diffraction. The wavelengths are similar to those of X-rays and are very short, in the range 10^-11m to 10^-14m. It was Rutherford who coined the name 'gamma rays', after naming 'alpha' and 'beta' rays; the natures of the different rays were unknown at that time.

Gamma-ray astronomy did not develop until it was possible to get our detectors above all or most of the atmosphere, using balloons or spacecraft. The first gamma-ray telescope, carried into orbit on the Explorer XI satellite in 1961, picked up fewer than 100 cosmic gamma-ray photons! Perhaps the most spectacular discovery in gamma-ray astronomy came in the late 1960s and early 1970s. Detectors on board the Vela satellite series, originally military satellites, began to record bursts of gamma-rays, not from Earth, but from deep space.

Culture

Gamma radiation, for the common man, is basically a ray that changes significantly the living organism. This is a result of the many cartoons and movies, for example Marvel's, in which science and technology play a huge role in the action. The Marvel Comics character, The Incredible Hulk, whose experiences formed the subject of a TV show and a recent movie, was created when scientist Bruce Banner was bombarded by a heavy dose of gamma radiation, which activated his genes that he had inherited from his father, who had injected himself with an experimental chemical. Spider Man, on the other hand, is treated with radiation in order for his mutation to be kept under control(althought this is impossible - radiation may only increase the chaos in which Peter Parker's organism is).

There is also a European metal band called Gamma Ray. They sing Power metal.

References

See also

• Alpha rays
• Beta rays
• Delta rays
• Epsilon rays
• Gamma-ray astronomy
• Gamma ray bursts
• Radiation therapy
• Food irradiation

Electromagnetic Spectrum Gamma ray | X-ray | Ultraviolet | Optical spectrum | Infrared | Terahertz radiation | Microwave | Radio waves Optical (visible) spectrum: Violet | Indigo | Blue | Green | Yellow | Orange | Red

This article is based on the article "Gamma-ray" from Wikipedia - the free encyclopedia created and edited by online user community. This article is distributed under the terms of GNU Free Documentation License. Here you find the list of authors of this article. The article can only edited within Wikipedia. Edit this article in Wikipedia.