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X-ray

X-ray

] ] 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.

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. 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.[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

phosphorescence phosphorescence 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. 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

Hermann von Helmholtz] 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[http://www.ratical.org/radiation/KillingOurOwn/KOO6.html].

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[http://www.birmingham.gov.uk/xray]. In the 1950s X-rays were first harnessed to produce an X-ray microscope. X-ray microscope of, and occultation of the X-ray background by, the Moon.]] 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

- [http://imagers.gsfc.nasa.gov/ems/xrays.html 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

Electromagnetic radiation

Electromagnetic radiation is a propagating wave in space with electric and magnetic components. These components oscillate at right angles to each other and to the direction of propagation. The term electromagnetic radiation is also used as a synonym for electromagnetic waves in general, even if they are not radiating or travelling in free space. This sense includes, for example, light travelling through an optical fiber, or electrical energy travelling within a coaxial cable. Electromagnetic (EM) radiation carries energy and momentum which may be imparted when it interacts with matter.

Physics

Theory

Electromagnetic waves of much lower frequency than visible light were predicted by Maxwell's equations and subsequently discovered by Heinrich Hertz. Maxwell derived a wave form of the electric and magnetic equations which made explicit the wave nature of the electric and magnetic fields. These equations displayed the symmetry of the fields. According to the theory, a time-varying electric field generates a magnetic field and vice versa. Thus, an oscillating electric field creates an oscillating magnetic field, which in turn creates an oscillating electric field, and so on. By this means an EM wave is produced which propagates through space.

Properties

Electric and magnetic fields exhibit the property of superposition. This means that the field due to a particular particle or time-varying electric or magnetic field adds to the fields due to other causes. (As magnetic and electric fields are vector fields, this is the vector addition of all the individual electric and magnetic field vectors.) As a result, EM radiation is influenced by various phenomena such as refraction and diffraction. For example, a travelling EM wave incident on a particular arrangement of atoms induces oscillation in the atoms and thus causes them to emit their own EM waves (called wavelets). These emissions interfere with the impinging wave and alter its form. In refraction, a wave moving from one medium to another of a different density changes its speed and direction when it enters the new medium. The ratio of the refractive indices of the media determines the extent of refraction. Refraction is the mechanism by which light disperses into a spectrum when it is shone through a prism. The physics of electromagnetic radiation is electrodynamics, a subfield of electromagnetism. EM radiation exhibits both wave properties and particle properties at the same time (see wave-particle duality). These characteristics are mutually exclusive and appear separately in different circumstances: the wave characteristics appear when EM radation is measured over relatively larger timescales and over larger distances, and the particle characteristics are evident when measuring smaller distances and timescales. EM radiation's behaviours as a wave and as a stream of particles have been confirmed by a large number of experiments.

Wave model

An important aspect of the wave nature of light is frequency. The frequency of a wave is its rate of oscillation and is measured in hertz, the SI unit of frequency, equal to one oscillation per second. Light usually comprises a spectrum of frequencies which sum to form the resultant wave. In addition, frequency affects properties like refraction, in which different frequencies undergo a different level of refraction. A wave has troughs and crests. The wavelength is the distance from crest to crest. Waves in the electromagnetic spectrum vary in size from very long radio waves the size of buildings, to very short gamma-rays smaller than the size of the nucleus of an atom. Frequency has an inverse relationship to the concept of wavelength. When waves travel from one medium to another, their frequency remains exactly the same - only their wavelength and/or speed changes. Waves can also be described by their radiant energy. Interference is the superposition of two or more waves resulting in a new wave pattern. The way that these coincide causes different types of interference.

Particle model

In the particle model of EM radiation, EM radiation is quantized as particles called photons. Quantisation of light represents the discrete packets of energy which constitute the radiation. The frequency of the radiation determines the magnitude of the energy of the particles. Moreover, these particles are emitted and absorbed by charged particles, so photons act as transporters of energy. A photon absorbed by an atom excites an electron and elevates it to a higher energy level. If the energy is great enough, the electron is liberated from the atom in a process called ionization. Conversely, an electron which descends to a lower energy level in an atom emits a photon of light equal to the energy difference. The energy levels of electrons in atoms are discrete. Therefore, each element has its own characteristic frequencies. Together these effects explain the absorption spectra of light. The dark bands in the spectrum are due to the atoms in the intervening medium which absorb different frequencies of the light. The composition of the medium through which the light travels determines the nature of the absorption spectrum. For instance, in a distant star, dark bands in the light it emits are due to the atoms in the atmosphere of the star. These bands correspond to the allowed energy levels in the atoms. A similar phenomenon occurs for emission. As the electrons descend to lower energy levels, a spectrum which represents the jumps between the energy levels of the electrons is exhibited. This is manifested in the emission spectrum of nebulae.

Speed of propagation

Any electric charge which accelerates, or any changing magnetic field, produces electromagnetic radiation. Electromagnetic information about the charge travels at the speed of light. Accurate treatment thus incorporates a concept known as retarded time (as opposed to advanced time, which is unphysical in light of causality), which adds to the expressions for the electrodynamic electric field and magnetic field. These extra terms are responsible for electromagnetic radiation. When any wire (or other conducting object such as an antenna) conducts alternating current, electromagnetic radiation is propagated at the same frequency as the electric current. Depending on the circumstances, it may behave as a wave or as particles. As a wave, it is characterized by a velocity (the speed of light), wavelength, and frequency. When considered as particles, they are known as photons, and each has an energy related to the frequency of the wave given by Planck's relation E = hν, where E is the energy of the photon, h = 6.626 × 10^-34 J·s is Planck's constant, and ν is the frequency of the wave. One rule is always obeyed regardless of the circumstances. EM radiation in a vacuum always travels at the speed of light, relative to the observer, regardless of the observer's velocity. (This observation led to Albert Einstein's development of the theory of special relativity.)

Electromagnetic spectrum

Generally, EM radiation is classified by wavelength into electrical energy, radio, microwave, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays. The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. Spectroscopy can detect a much wider region of the EM spectrum than the visible range of 400 nm to 700 nm. A common laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm. More in-depth information about the physical properties of objects, gases, or even stars can be obtained from this type of device. It is widely used in astrophysics. For example, many hydrogen atoms emit radio waves which have a wavelength of 21.12 cm.

Light

EM radiation with a wavelength between 400 nm and 700 nm is detected by the human eye and perceived as visible light. If radiation having a frequency in the visible region of the EM spectrum shines on an object, say, a bowl of fruit, this results in our visual perception of identifying information from the scene. Our brain's visual system processes the multitude of reflected frequencies into different shades and hues, and through this not-entirely-understood "psychophysical phenomenon," most humans perceive a bowl of fruit. In the vast majority of cases, however, the information carried by light is not directly apprehensible by human senses. Natural sources produce EM radiation across the spectrum; so, too, can human technology manipulate a broad range of wavelengths. Optical fiber transmits light which, although not suitable for direct viewing, can carry data. Those data can be translated into sound or an image. The coded form of such data is similar to that used with radio waves.

Radio waves

Radio waves carry information by varying amplitude and by varying frequency within a frequency band. When EM radiation impinges upon a conductor, it couples to the conductor, travels along it, and induces an electric current on the surface of that conductor by exciting the electrons of the conducting material. This effect (the skin effect) is used in antennas. EM radiation may also cause certain molecules to absorb energy and thus to heat up; this is exploited in microwave ovens.

Derivation

Electromagnetic waves as a general phenomenon were predicted by the classical laws of electricity and magnetism, known as Maxwell's equations. If you inspect Maxwell's equations without sources (charges or currents) then you will find that, along with the possibility of nothing happening, the theory will also admit nontrivial solutions of changing electric and magnetic fields. (For symbol definitions see magnetic field.) :

\nabla \cdot \mathbf = 0

\nabla \times \mathbf = -\frac \mathbf

\nabla \cdot \mathbf = 0

\nabla \times \mathbf = \mu_0 \epsilon_0 \frac \mathbf

\mathbf=\mathbf=\mathbf

is a solution, but there might be other solutions as well. Let us employ a useful identity from vector calculus. :

\nabla \times \left( \nabla \times \mathbf \right) = \nabla \left( \nabla \cdot \mathbf \right) - \nabla^2 \mathbf

Where

\mathbf

can be any vector function. Taking the curl of the curl equations and applying the identity, we get the following. :

\nabla^2 \mathbf = \mu_0 \epsilon_0 \frac \mathbf

\nabla^2 \mathbf = \mu_0 \epsilon_0 \frac \mathbf

These types of equations are identified as linear wave equations with wave speed

\frac

. Amazingly, this speed happens to be exactly the speed of light! Maxwell's equations have unified the permittivity of free space

\epsilon_0

, the permeability of free space

\mu_0

, and the speed of light itself:

c = \frac

. Before this derivation it was not known that there was such a strong relationship between light and electricity and magnetism. But these are only two equations and we started with four, so there is still more information pertaining to these waves hidden within Maxwell's equations. Let's consider a generic vector wave for the electric field. :

\mathbf = \mathbf_0 f\left( \hat \cdot \mathbf - c t \right)

Here

\mathbf_0

is the constant amplitude,

f

is any second differentiable function,

\hat

is a unit vector in the direction of propagation, and

\hat

is a position vector. We observe that

f\left( \hat \cdot \mathbf - c t \right)

is a generic solution to the wave equation. In other words :

\nabla^2 f\left( \hat \cdot \mathbf - c t \right) = \frac \frac f\left( \hat \cdot \mathbf - c t \right)

, for a generic wave traveling in the

\hat

direction. The proof of this is trivial. This form will satisfy the wave equation, but will it satisfy all of Maxwell's equations, and with what corresponding magnetic field? :

\nabla \cdot \mathbf = \hat \cdot \mathbf_0 f'\left( \hat \cdot \mathbf - c t \right) = 0

\mathbf \cdot \hat = 0

The first of Maxell's equations implies that electric field is orthogonal to the direction the wave propagates. :

\nabla \times \mathbf = \hat \times \mathbf_0 f'\left( \hat \cdot \mathbf - c t \right) = -\frac \mathbf

\mathbf = \frac \hat \times \mathbf

The second of Maxwell's equations yields the magnetic field. The remaining equations will be satisfied by this choice of

\mathbf,\mathbf

. Not only are the electric and magnetic field waves traveling at the speed of light, but they have a special restricted orientation and proportional magnitudes,

\mathbf_0 = c \mathbf_0

. The electric field, magnetic field, and direction of wave propagation are all orthogonal and the wave propagates in the same direction as

\mathbf \times \mathbf

. Visualizing yourself as an electromagnetic wave traveling forward, the electric field might be oscillating up and down, while the magnetic field oscillates right and left; but you can rotate this picture around with the electric field oscillating right and left and the magnetic field oscillating down and up. This is a different solution that is traveling in the same direction. This arbitrariness in the orientation, with respect to propagation direction, is known as polarization.

References

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External links

; General
- [http://www.sengpielaudio.com/calculator-wavelength.htm Conversion of frequency to wavelength and back - electromagnetic, radio and sound waves]
- [http://www.scienceofspectroscopy.info The Science of Spectroscopy - a learning tool for spectroscopy] ; Patents
- Greenleaf Whittier Pickard - - Intelligence intercommunication by magnetic wave component ko:전자기파 ja:電磁波

Wavelength

:For the album by Van Morrison, see Wavelength (album). The wavelength is the distance between repeating units of a wave pattern. It is commonly designated by the Greek letter lambda (λ). In a sine wave, the wavelength is the distance between the midpoints of the wave: Image:Wavelength.png The x axis represents distance, and I would be some varying quantity at a given point in time as a function of x, for instance air pressure for a sound wave or strength of the electric or magnetic field for light. Wavelength λ has an inverse relationship to frequency f, the number of peaks to pass a point in a given time. The wavelength is equal to the speed of the wave type divided by the frequency of the wave. When dealing with electromagnetic radiation in a vacuum, this speed is the speed of light c, for signals (waves) in air, this is the speed of sound in air. The relationship is given by: :

\lambda = \frac

where: :λ = wavelength of a sound wave or electromagnetic wave :c = speed of light in vacuum = 299,792.458 km/s ~ 300,000 km/s = 300,000,000 m/s or :c = speed of sound in air = 343 m/s at 20 °C (68 °F) :f = frequency of the wave in 1/s = Hz For radio waves this relationship is approximated with the formula: wavelength λ (in metres) = 300 / frequency (in megahertz).
For sound waves this relationship is approximated with the formula: wavelength λ (in metres) = 333 / frequency (in hertz). When light waves (and other electromagnetic waves) enter a medium, their wavelength is reduced by a factor equal to the refractive index n of the medium but the frequency of the wave is unchanged. The wavelength of the wave in the medium, λ' is given by: :

\lambda^\prime = \frac

where: :λ₀ is the vacuum wavelength of the wave Wavelengths of electromagnetic radiation, no matter what medium they are travelling through, are usually quoted in terms of the vacuum wavelength, although this is not always explicitly stated. Louis de Broglie discovered that all particles with momentum have a wavelength associated with their quantum mechanical wavefunction, called the de Broglie wavelength.

External link

- [http://www.sengpielaudio.com/calculator-wavelength.htm Conversion: Wavelength to frequency and vice versa - The calculator] Category:Length Category:Wave mechanics ko:파장 ja:波長 th:ความยาวคลื่น

Picometre

To help compare different orders of magnitude this page lists lengths between 10^-12 m (metre) and 10^-11 m (1 pm and 10 pm).
- Lengths shorter than 1 pm
- 1 pm = 1 picometre = 1000 femtometres
- 5 pm — shorter X-ray wavelengths (approx.)
- Lengths longer than 10 pm

SI prefix

An SI prefix is a prefix that can be applied to an SI unit to form a decimal multiple or submultiple. Many SI prefixes predate the introduction of the SI in 1960. They can be applied correctly to many non-SI units. As part of the SI system they are officially determined by the Bureau International des Poids et Mesures.

Overview

SI defines a number of SI prefixes to be used with the units: these combine with any unit name to give subdivisions and multiples. As an example, the prefix kilo denotes a multiple of a thousand, so the kilometre is 1000 metres, the kilogram is 1000 grams, a kilowatt is 1,000 watts, and so on. The prefix milli subdivides by a thousand, so a millimetre is one-thousandth of a metre (1000 millimetres in a metre), and a millilitre is one-thousandth of a litre. The prefixes are never combined; a millionth of a kilogram is a milligram, and not a 'microkilogram'. The ability to apply the same prefixes to any SI unit is one of the key strengths of the SI, since it considerably simplifies the system's learning and use. The following SI prefixes can be used to prefix any of the above units to produce a multiple or submultiple of the original unit. This includes the degree Celsius (e.g. "1.2 m°C"); however, to avoid confusion, prefixes are not used with the time-related unit symbols min (minute), h (hour), d (day). They are not recommended for use with the angle-related symbols ° (degree), ' (minute of arc), and " (second of arc)[http://physics.nist.gov/Pubs/SP811/sec06.html], but for astronomical usage, they are sometimes used with seconds of arc. See also: Non-SI unit prefixes Examples:
- 5 cm = 5 × 10⁻² m = 5 × 0.01 m = 0.05 m
- 3 MW = 3 × 10⁶ W = 3 × 1 000 000 W = 3 000 000 W Prefixes cannot be combined: for example 10⁻⁹ metre must be written as 1 nm, not as 1 mµm. The prefix always takes precedence over any exponentiation; thus "km²" means square kilometre and not kilo–square metre. For example, 3 km² is equal to 3 000 000 m² and not to 3000 m² (nor to 9 000 000 m²). Thus the SI prefixes provide steps of a factor one million instead of one thousand in the case of an exponent 2, of a billion in the case of an exponent 3, etc. As a result large numbers may be needed, even if the prefixes are fully used. Prefixes where the exponent is divisible by three are recommended. Hence "100 m" rather than "1 hm". The obsolete prefixes myria- and myrio- were dropped before SI was adopted in 1960, probably because they do not fit this pattern, no symbol was available (M, m and µ already being used), and were rarely used anyway. Double prefixes such as those formerly used in micromicrofarads (picofarads), hectokilometres (100 kilometres), and millimicrons or micromillimetres (both nanometres) were also dropped with the introduction of the SI. The kilogram stands out among all SI base units as the only one that has a prefix. It is derived from the mass of an actual object. The gram is defined as 1/1000 of this object's mass. Though in principle legal, most combinations of prefixes with quantities are very rarely used, even in a scientific or engineering context:
- Mass: hectogram, gram, milligram, microgram, and smaller are common. However, megagram or larger are rarely used; tonnes or scientific notation are used instead. Megagram is sometimes used to disambiguate the (metric) tonne from the various (non-metric) tons.
- Volume in litres: litre, decilitre, centilitre, millilitre, microlitre, and smaller are common. Larger volumes are sometimes denoted in hectolitres; otherwise in cubic metres or cubic kilometres. In Australia, large quantities of water are measured in kilolitres and megalitres.
- Length: kilometre, metre, decimetre, centimetre, millimetre, and smaller are common. The micrometre is still often referred to as a micron. In some fields such as chemistry, the angstrom (equal to 0.1 nm) competes with the nanometre. The femtometre, used mainly in particle physics, is usually called a fermi. At large scales, megametre, gigametre, and larger are rarely used. Often used are astronomical units, light years, and parsecs; the astronomical unit is mentioned in the SI standards as an accepted non-SI unit.
- Time: second, millisecond, microsecond, and shorter are common. The kilosecond and megasecond also have some use, though for these and longer times one usually uses either scientific notation or minutes, hours, and so on. ^† Britain, Ireland, Australia and New Zealand previously used the long scale number name conventions, but have now at least partly switched to the short scale usage. Note in particular that above a million and below a millionth, the same name has different values in the two naming systems, so billion and trillion (for example) become unfortunately potentially ambiguous terms internationally. Using the SI prefixes can circumvent this problem.

Pronunciation

The accepted English pronunciation of the initial G of giga was once soft, (like gigantic), but now the hard pronunciation, (like giggle), is significantly more common. However, both pronunciations are likely to be understood by most English speakers, though the second is likely to be preferred.

Use outside SI

The symbol "K" is often used to mean a multiple of a thousand, so one may talk of "a 40K salary" (40,000), or the Y2K problem. Note that in these cases an upper case K is often used, although it should be noted that using an uppercase K is never correct when writing under the rules of the SI. Also, it is often used as a prefix to designate the binary prefix kilo = 2¹⁰ = 1024.

Non-SI units

- Prefixes go back to the introduction of the metric system in the 1790s, long before the SI was introduced in 1960. The prefixes (including those introduced after the introduction of SI) are used with any metric units, SI or not (e.g. millidynes).
- SI prefixes rarely appear coupled with imperial units except in some specialised cases (e.g. microinches, kilofeet).
- They are also used with other specialized units used in particular fields (e.g. megaelectronvolts, gigaparsecs).
- They are also occasionally used with currency units (e.g., gigadollar), mainly by people who are familiar with the prefixes from scientific usage.

Computing

Main article: Binary prefix The prefixes K and greater are common in computing, where they are applied to information and storage units like the bit and the byte. Since 2¹⁰ = 1024, and 10³ = 1000, this led to the SI prefix letters being used to denote "binary" powers. Although these are incorrect usages according to the SI standards it seems common to apply base 10 prefixes, when relating to computers, as follows: ; K:= 2¹⁰ = 1,024 ; M:= 2²⁰ = 1,048,576 ; G:= 2³⁰ = 1,073,741,824 ; T:= 2⁴⁰ = 1,099,511,627,776 ; P:= 2⁵⁰ = 1,125,899,906,842,624. These prefixes, however, usually retain their powers-of-1000 meanings when used to describe rates of data transmission (bit rates): 10 Mbit/s Ethernet runs at 10,000,000 bit/s, not 10,485,760 bit/s. The problem is compounded by the fact that the units of information (the bit and the byte) are not part of SI, where the bit, byte, octet, baud or symbol rate would rather be given in hertz. Although some use "bit" for the bit and "b" for the byte, "b" is often used for bit and "B" for byte instead. (In SI, B stands for the bel, although its sub-unit, the decibel ("dB"), is almost universally used instead, preventing confusion between the symbols.) It is recommended by several standards bodies to use bit and B to keep the units very distinct, as in kbit or MiB. French-speaking countries often use "o" for "octet", nowadays a synonym for byte, but this is unacceptable in SI because of the risk of confusion with the zero. Consequently, the International Electrotechnical Commission (IEC) adopted new binary prefixes in 1998, formed from the first syllable of the decimal prefix plus 'bi' (pronounced 'bee'). The symbol is the decimal symbol plus 'i'. So now, one kilobyte (1 kB) equals 1000 bytes, whereas one kibibyte (1 KiB) equals 2¹⁰ = 1024 bytes. Likewise mebi (Mi; 2²⁰), gibi (Gi; 2³⁰), tebi (Ti; 2⁴⁰), pebi (Pi; 2⁵⁰), and exbi (Ei; 2⁶⁰). Although the IEC standard does not mention them, the sequence can be readily extended to zebi (Zi; 2⁷⁰) and yobi (Yi; 2⁸⁰). The adoption of these prefixes has been very limited.

Proposed extensions

Continuing backwards in the alphabet, after zetta and yotta, proposals for the next large number include xenta and xona (among others), the latter as an alteration of the Latin-derived numerical prefix nona-, and the next small number would also start with an x. Preserving the rule on abbreviating the prefixes (a Latin capital for the large number and a lower-case letter for the small number), even without consensus on the full name the following prefix symbols could be used without ambiguity: X, W, V, x, w, v. The logically next small prefix symbol, "u", is the accepted a substitution for "µ" (ISO 2955), the symbol for "micro". However, even some official prefixes may not be understood by all readers, let alone extrapolations of them, so giving an explanation is advisable when using them in communication (as opposed to using them in notes for oneself). Another proposal for xenta/xona is novetta, from the Italian nove. This does not have the convenience of backward alphabetic order. There are also proposals for further harmonization of the capitalisation. Therefore the symbols for deka, hecto and kilo would be changed from "k" to "K", from "h" to "H", and from "da" to "D". Likewise some lobby for the removal of prefixes that don't fit the 10^±3·n scheme, namely hecto, deka, deci and centi. The CGPM has tabled its decision on both matters for now. An unsolved (and maybe unsolvable) issue is the application of prefixes to units with exponents other than ±1. The prefix is always applied before the exponent currently. In volume measuring, for example, this inconvenience has lead to the continued use of the litre, which is one thousandth cubic metre (0.001 m³) or one cubic decimetre (1 dm³), where it could be handy to call it "one milli–cubic metre" ("1 m(m³)")—one cubic millimetre (mm³) is one thousand millionth of a cubic metre.

References

External links

- [http://www.bipm.fr/en/si/prefixes.html The International Bureau of Weights and Measures (BIPM): SI prefixes]
- [http://jimvb.home.mindspring.com/unitsystem.htm Proposal for an extension of the SI-prefix sytem to even larger and smaller units]
- ko:SI 접두어 ja:SI接頭辞

X-ray crystallography

X-ray crystallography is a technique in 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. This generally leads to an understanding of the material and molecular structure of a substance. The spacings in the crystal lattice can be determined using Bragg's law. The electrons that surround the atoms, rather than the atomic nuclei themselves, are the entities which physically interact with the incoming X-ray photons. This technique is widely used in chemistry and biochemistry to determine the structures of an immense variety of molecules, including inorganic compounds, DNA and proteins. X-ray diffraction is commonly carried out using single crystals of a material, but if these are not available, microcrystalline powdered samples may also be used, although this requires different equipment and is much less straightforward.

Inorganic Structures

In inorganic chemistry, x-ray crystallography is used to determine lattice structures as well as chemical formulas, bond lengths and angles. X-ray diffraction finds frequent use in materials science because sample preparation is relatively easy, and the test itself is often rapid and non-destructive. The vast majority of engineering materials are crystalline, and even those which are not yield some useful information in diffraction experiments. The pattern of diffraction peaks can be used to quickly identify materials (thanks to the JCPDS pattern database), and changes in peak width or position can be used to determine crystal size, purity, and texture. texture

Organic Structures

The first protein crystal structure was of sperm whale myoglobin, as determined by Max Perutz and Sir John Cowdery Kendrew in 1958, which led to a Nobel Prize in Chemistry. The X-ray diffraction analysis of myoglobin was originally motivated by the observation of myoglobin crystals in dried pools of blood on the decks of whaling ships. Today X-ray crystallography is often used to determine how drugs, such as anti-cancer medications, can be improved to better influence their protein targets. The molecule must be crystallized because one photon diffracted by one electron cannot be reliably detected. However, because of the regular crystalline structure, the photons are diffracted by corresponding electrons in many symmetrically arranged molecules. Because waves of the same frequency whose peaks match reinforce each other, the signal becomes detectable. To determine a structure, one must first grow crystals of the molecule of interest using some method of crystallization. This can be a painstaking procedure for macromolecules such as protein and DNA complexes. Many biomolecules of interest still have not been successfully crystallised. Discontinuities in the crystal structure impede the acquisition of high quality images. One cause of discontinuity is convection caused by temperature variations within the forming crystal, and one of the proposed scientific applications of the International Space Station is the growth of crystals, because convection does not occur in the absence of gravity. Once prepared the crystals are harvested and often frozen with liquid nitrogen. Freezing crystals both reduces radiation damage incurred during data collection and decreases thermal motion within the crystal. Crystals are placed on a diffractometer, a machine that emits a beam of X-rays. The X-rays diffract off the electrons in the crystal, and the pattern of diffraction is recorded on film and scanned into a computer. These diffraction images are combined and eventually used to construct a map of the electron density of the molecule that was crystallized, atoms are then fitted to the electron density map and various parameters such as position are refined to best fit the observed diffraction data. It is important to note that even after obtaining crystals suitable for diffraction analysis, current X-ray sources and detectors limit the measurement of only the diffracted photon intensities and not their respective phases, the latter encoding the majority of the information about the actual shape of electron density. A combination of experimental and computational methods are typically used to solve the phase problem, in order to estimate phases and obtain an initial map of the electron density. After phases are estimated, a model made up of atoms is built and refined against the observed data. Once a model of a molecule's structure has been determined, it is often deposited in a crystallographic database such as the [http://www.rcsb.org Protein Databank] or the [http://www.ccdc.cam.ac.uk/ Cambridge Structure Database]. Many structures obtained in private commercial ventures to crystallize medicinally relevant proteins, are not deposited in public crystallographic databases. Visit the X-factors webzine for the latest [http://www.sciencebase.com/speclines.html X-ray crystallography news]

References

- Drenth J. Principles of Protein X-Ray Crystallography. Springer-Verlag Inc. NY: 1999, ISBN 0387985875.
- Glusker JP, Lewis M, Rossi M. Crystal Structure Analysis for Chemists and Biologists. VCH Publishers. NY:1994, ISBN 0471185434.
- Rhodes G. Crystallography Made Crystal Clear. Academic Press. CA: 2000, ISBN 0125870728. Category:Diffraction Category:X-rays ja:X線回折

Ionizing radiation

Ionizing radiation is a type of particle radiation in which an individual particle (for example, a photon, electron, or helium nucleus) carries enough energy to ionize an atom or molecule (that is, to completely remove an electron from its orbit). If the individual particles do not carry this amount of energy, it is essentially impossible for even a large flood of particles to cause ionization. These ionizations, if enough occur, can be very destructive to living tissue. The composition of ionizing radiation can vary. Electromagnetic radiation can cause ionization if the energy per photon is high enough (that is, the wavelength is short enough). Far ultraviolet light, X-rays, and gamma rays are all ionizing radiation, while visible light, microwaves, and radio waves are not. Ionizing radiation may also consist of fast-moving particles such as electrons, positrons, or small atomic nuclei. Normally non-ionizing radiation such as near UV can also ionize materials when the intensity is high enough that nonlinear multiphoton absorption processes occur.

Types of radiation

UV, is halted by an aluminium plate. Gamma radiation is eventually absorbed as it penetrates a dense material.]] Ionizing radiation is produced by radioactive decay, nuclear fission and nuclear fusion, extremely hot objects (thermal or blackbody radiation), and accelerated charges (bremsstrahlung or synchrotron radiation). In order for radiation to be ionizing, the particles must both have a high enough energy and interact with electrons. Photons interact strongly with charged particles, so photons of sufficiently high energy are ionizing (the energy at which this begins to happen is in the ultraviolet region; sunburn is one of the effects of this ionization). Charged particles such as electrons, positrons, and alpha particles also interact strongly with electrons. Neutrons, on the other hand, do not interact strongly with electrons, and so they cannot directly ionize atoms. They can interact with atomic nuclei (depending on the nucleus and their velocity; see fast neutron and slow neutron), often producing radioactive nuclei, which produce ionizing radiation when they decay. The negatively charged electrons and positively charged nuclei created by ionizing radiation may cause damage in living tissue. If the dose is sufficient, the effect may be seen almost immediately, in the form of radiation poisoning. Lower doses may cause cancer or other long-term problems. The effect of the very low doses encountered in normal circumstances (from both natural and artificial sources, like cosmic rays, medical X-rays and nuclear power plants) is a subject of current debate. A 2005 report released by the National Research Council (the BEIR VII report, summarized in [http://www.nap.edu/execsumm_pdf/11340.pdf]) indicated that the overall cancer risk associated with background sources of radiation was relatively low. Radioactive materials usually release alpha rays (particles similar to the nuclei of helium), beta rays (quickly moving electrons or positrons) or gamma rays. Alpha and beta rays can often be shielded by a piece of paper or a sheet of aluminium, respectively. They cause most damage when they are emitted inside the human body. Gamma rays are less ionizing than either alpha or beta rays, but protection against them requires thicker shielding. They produce damage similar to that caused by X-rays such as burns, cancer, and genetic mutations. Human biology resists germ-line mutation by aborting most mutated conceptuses. Non-ionizing radiation is thought to be essentially harmless below the levels that cause heating. Ionizing radiation is dangerous in direct exposure, although the degree of danger is a subject of debate. Humans and animals can also be exposed to ionizing radiation internally: if radioactive isotopes are present in the environment, they may be taken into the body. For example, radioactive iodine is treated as normal iodine by the body and used by the thyroid; its accumulation there often leads to thyroid cancer. Some radioactive elements also bioaccumulate.

Example: Electromagnetic radiation

The energy of a photon (i.e., a quantum of electromagnetic radiation) is given by the Planck equation: :

E = h \nu

where :

E

is the energy of the photon :

h

is Planck's constant :

\nu

is the frequency of the photon The wavelength of a photon is related to its frequency by the equation of a wave's velocity: :

c = \lambda \nu

where :

c

is the speed of light :

\lambda

is the wavelength of light Plugging back in and solving for the wavelength, we get, :

\lambda = h c/E

The elements with the lowest and highest ionization potential are cesium (3.89 eV) and helium (24.6 eV), respectively. Photons with energies less than 3.89 eV (λ > 318.8 nm) are non-ionizing radiation, photons with energies greater than 24.6 eV (λ < 50.4 nm) are ionizing radiation, and photons with energies between 3.89 eV and 24.6 eV may be either ionizing or non-ionizing radiation depending on the nature of material (e.g., cesium or helium). Visible light corresponds to photons with energies from 1.77 eV (λ = 700.6 nm) to 3.10 eV (λ = 400 nm) and are thus non-ionizing electromagnetic radiation. Ultraviolet (UV) radiation spans the energy range from 3.10 eV (UV-A) to 12.4 eV (UV-C, λ = 100 nm). Because UV radiation, especially UV-C, exceeds the ionization energy of many of the elements, it is often considered ionizing radiation rather than non-ionizing radiation.

Sources of ionizing radiation

Natural background radiation

Natural background radiation comes from four primary sources: cosmic radiation, solar radiation, external terrestrial sources, and radon.

Cosmic radiation

The earth, and all living things on it, are constantly bombarded by radiation from outside our solar system of positively charged ions from protons to iron nuclei. This radiation interacts in the atmosphere to create secondary radiation that rains down, including x-rays, muons, protons, alpha particles, pions, electrons, and neutrons. The dose from cosmic radiation is largely from muons, neutrons, and electrons. The dose rate from cosmic radiation varies in different parts of the world based largely on the geomagnetic field and altitude.

Solar radiation

While most solar radiation is electro-magnetic radiation, the sun also produces particle radiation, Solar cosmic rays, which vary with the Solar Cycle. Solar cosmic rays are cosmic rays that originate from the Sun. Most are made of protons; these rays are relatively low in energy (10-100 keV). The average composition is similar to that of the Sun itself. High energy (Mev and above) cosmic rays come mainly from outside the solar system, while the particles in the solar case are energized near the Sun's surface by the action of magnetic fields. Solar cosmic rays vary widely in their intensity and spectrum, increasing in strength after some solar events such as solar flares. Further, an increase in the intensity of solar cosmic rays is followed by a decrease in the galactic cosmic rays, called a Forbush decrease after their discoverer, the physicist Scott Forbush. These decreases are due to the solar wind with its entrained magnetic field sweeping some of the galactic cosmic rays outwards, away from the Sun and Earth.

External terrestrial sources

Radioactive material is found throughout nature. It occurs naturally in the soil, rocks, water, air, and vegetation. The major radionuclides of concern for terrestrial radiation are potassium, uranium and thorium. Each of these sources has been decreasing in activity since the birth of the Earth so that our present dose from potassium-40 is about ½ what it would have been at the dawn of life on Earth.

Radon

Radon gas seeps out of uranium-containing soils found across most of the world and may concentrate in well-sealed homes. It is often the single largest contributor to an individual's background radiation dose and is certainly the most variable in the United States.

Man-made radiation sources

Natural and artificial radiation sources are identical in their nature and their effect. Above the background level of radiation exposure, the U.S. Nuclear Regulatory Commission (NRC) requires that its licensees limit man-made radiation exposure to individual members of the public to 100 mrem (1 mSv) per year, and limit occupational radiation exposure to adults working with radioactive material to 5,000 mrem (50 mSv) per year. The average exposure for Americans is about 360 mrem (3.6 mSv) per year, 81 percent of which comes from natural sources of radiation. The remaining 19 percent results from exposure to man-made radiation sources such as medical X-rays, most of which is deposited in people who have CAT scans. One important source of natural radiation is radon gas, which seeps continuously from bedrock but can, because of its high density, accumulate in poorly ventilated houses. The background rate varies considerably with location, being as low as 1.5 mSv/a in some areas and as over as 100 mSv/a in others. People in some areas of Ramsar, a city in northern Iran, receive an annual radiation absorbed dose from background radiation that is up to 260 mSv/a. Despite having lived for many generations in these high background areas, inhabitants of Ramsar show no significant cytogenetic differences compared to people in normal background areas; this has led to the suggestion that the body can sustain much higher steady levels of radiation than sudden bursts. Some man-made radiation sources affect man through direct radiation, while others take the form of radioactive contamination and irradiate man from the inside. By far, the most significant source of man-made radiation exposure to the general public is from medical procedures, such as diagnostic X-rays, nuclear medicine, and radiation therapy. Some of the major radionuclides used are I-131, Tc-99, Co-60, Ir-192, Cs-137. These are rarely released into the environment. In addition, members of the public are exposed to radiation from consumer products, such as tobacco (polonium-210), building materials, combustible fuels (gas, coal, etc.), ophthalmic glass, televisions, luminous watches and dials (tritium), airport X-ray systems, smoke detectors (americium), road construction materials, electron tubes, fluorescent lamp starters, lantern mantles (thorium), etc. Of lesser magnitude, members of the public are exposed to radiation from the nuclear fuel cycle, which includes the entire sequence from mining and milling of uranium to the disposal of the used (spent) fuel. The effects of such exposure have not been reliably measured. Estimates of exposure are low enough that proponents of nuclear power liken them to the mutagenic power of wearing trousers for two extra minutes per year (because heat causes mutation). Opponents use a cancer per dose model to prove that such activities cause several hundred cases of cancer per year. In a nuclear war, gamma rays from fallout of nuclear weapons would probably cause the largest number of casualties. Immediately downwind of targets, doses would exceed 300 Gy per hour. As a reference, 4.5 Gy (around 15,000 times the average annual background rate) is fatal to half of a normal population. Occupationally exposed individuals are exposed according to the sources with which they work. The radiation exposure of these individuals is carefully monitored with the use of pocket-pen-sized instruments called dosimeters. Some of the radionuclides of concern include cobalt-60, caesium-137, americium-241 and iodine-131. Examples of industries where occupational exposure is a concern include:
- airline crew (the most exposed population)
- Fuel cycle
- Industrial Radiography
- Radiology Departments (Medical)
- Radiation Oncology Departments
- Nuclear power plant
- Nuclear medicine Departments
- National (government) and university Research Laboratories

The effects of ionizing radiation on animals

We tend to think of biological effects of radiation in terms of their effect on living cells. For low levels of radiation exposure, the biological effects are so small they may not be detected. The body repairs many types of radiation and chemical damage. Biological effects of radiation on living cells may result in four outcomes: #Injured or damaged cells repair themselves, resulting in no residual damage. #Cells die, much like millions of body cells do every day, being replaced through normal biological processes. #Cells incorrectly repair themselves resulting in a biophysical change. #Low levels of ionizing radiation may be beneficial to many types of cells; this phenomenon is termed radiation hormesis and has not been shown in humans (see below).

Chronic radiation exposure

Exposure to ionizing radiation over an extended period of time is called chronic exposure. The natural background radiation is chronic exposure, but a normal level is difficult to determine due to variations. Location and occupation often affect chronic exposure.

Acute radiation exposure

Acute radiation exposure is an exposure to ionizing radiation which occurs during a short period of time. There are routine brief exposures, and the boundary at which it becomes significant is difficult to identify. Extreme examples include
- Instantaneous flashes from nuclear explosions.
- Exposures of minutes to hours during handling of radioactive material.
- Laboratory and manufacturing accidents.
- Intentional and accidental high medical doses. The effects of acute events are more easily studied than those of chronic exposure.

Radiation levels

The associations between ionizing radiation exposure and the development of cancer are mostly based on populations exposed to relatively high levels of ionizing radiation (e.g., Japanese atomic bomb survivors, and recipients of selected diagnostic or therapeutic medical procedures). Cancers associated with high dose exposure include leukemia, thyroid, breast, bladder, colon, liver, lung, esophagus, ovarian, multiple myeloma, and stomach cancers. Department of Health and Human Services literature also suggests a possible association between ionizing radiation exposure and prostate, nasal cavity/sinuses, pharyngeal and laryngeal, and pancreatic cancer. The period of time between radiation exposure and the detection of cancer is known as the latent period. Those cancers that may develop as a result of radiation exposure are indistinguishable from those that occur naturally or as a result of exposure to other chemical carcinogens. Furthermore, National Cancer Institute literature indicates that other chemical and physical hazards and lifestyle factors (e.g., smoking, alcohol consumption, and diet) significantly contribute to many of these same diseases. Although radiation may cause cancer at high doses and high dose rates, public health data do not certainly establish the occurrence of cancer following exposure to low doses and dose rates -- below about 10,000 mrem (100 mSv). To assess the health impacts of lower radiation doses, researchers rely on models of the process by which radiation causes cancer; several models have emerged which predict differing levels of risk. Most studies of occupational workers exposed to chronic low-levels of radiation above normal background have not shown conclusive adverse biological effects. Even so, the radiation protection community conservatively assumes that any amount of radiation may pose some risk for causing cancer and hereditary effect, and that the risk is higher for higher radiation exposures. The linear dose-response model suggests that any increase in dose, no matter how small, results in an incremental increase in risk. This LNT (Linear model, No Threshold) hypothesis is accepted by the NRC as a conservative model for estimating radiation risk. (See also the BEIR VII report, summarized in [http://www.nap.edu/execsumm_pdf/11340.pdf].) Under this model, about 1% of a population would develop cancer in their lifetime as a result of ionizing radiation from background levels of natural and manmade sources. All ionizing radiation attacks living tissue by causing ionization, which disrupts molecules directly and also produces highly reactive free radicals, which attack nearby cells. The net effect is that biological molecules suffer local disruption. Very high doses of radiation disrupt cells by wrecking large amounts of cellular machinery. Lower doses also wreck cellular machinery, but most cellular machinery can be effectively repaired, or doses sufficient to destroy cells outright affect cells in the process of replication more severely. This syndrome was observed in many atomic bomb survivors in 1945 and emergency workers responding to the 1986 Chernobyl nuclear power plant accident. Approximately 134 plant workers and firefighters battling the fire at the Chernobyl power plant received high radiation doses (70,000 to 1,340,000 mrem or 700 to 13,400 mSv) and suffered from acute radiation sickness. Of these, 28 died from their radiation injuries. Longer term effects of the Chernobyl accident have also been studied. There is a clear link (see [http://www.unscear.org/reports/2000_2.html UNSCEAR 2000 Report, Volume 2:Effects]) between the Chernobyl accident and the unusually large number (approximately 1800) of thyroid cancers (mostly in children) reported in contaminated areas. These were fatal in some cases. Other health effects of the Chernobyl accident are subject to current debate.

Ionizing radiation level examples

Recognized effects of acute radiation exposure are described in the article on radiation poisoning. The exact units of measurement vary, but light radiation sickness begins at about 50–100 rad (0.5–1 gray (Gy), 0.5–1 Sv, 50–100 rem, 50,000–100,000 mrem). Chronic radiation levels and standards are often measured in millirems, 1/1000th of a rem. The following table includes some short-term dosages for comparison purposes.

Level (mrem)	Ionizing radiation standards	Example
1 / yr		USA dose from nuclear fuel and nuclear power plants. [http://www.ornl.gov/sci/env_rpt/aser95/appa.htm]
1 / day		Daily natural background radiation, including radon. [http://www.ornl.gov/sci/env_rpt/aser95/tb-a-2.pdf]
2.5 / 6 h		Cosmic dose on flight from New York to Los Angeles. [http://www.ornl.gov/sci/env_rpt/aser95/tb-a-2.pdf]
2 / hour	USA NRC public area exposure limit.
10 / yr		USA average dose from consumer products. [http://www.ornl.gov/sci/env_rpt/aser95/appa.htm]
15 / yr	USA EPA cleanup standard.
25 / yr	USA NRC cleanup standard for individual sites/sources.
27 / yr		USA dose from natural cosmic radiation. 16 coastal plain - 63 eastern Rocky Mountains. [http://www.ornl.gov/sci/env_rpt/aser95/appa.htm]
28 / yr		USA dose from natural terrestrial sources. [http://www.ornl.gov/sci/env_rpt/aser95/appa.htm]
39 / yr		Global level of human internal radiation due to radioactive potassium.
46		Estimate of largest off-site dose possible from March 28 1979 Three Mile Island accident.
66 / yr		Average USA dose from human-made sources. [http://www.ornl.gov/sci/env_rpt/aser95/tb-a-2.pdf]
100 / yr	USA NRC total exposure limit to the public.
110 / yr		1980 average USA radiation worker occupational dose. [http://www.ornl.gov/sci/env_rpt/aser95/tb-a-2.pdf]
200 / yr		USA average medical and natural background. [http://www.ornl.gov/sci/env_rpt/aser95/tb-a-2.pdf] Human internal radiation due to radon, varies with radon levels. [http://www.ornl.gov/sci/env_rpt/aser95/appa.htm]
220		Average dose from upper gastrointestinal diagnostic X-ray series.
300 / yr		USA average dose from all natural sources. [http://www.ornl.gov/sci/env_rpt/aser95/tb-a-2.pdf]
366 / yr		USA average from all sources, including medical diagnostic radiation doses.
few hundred / yr		Estimate of cobalt-60 contamination within about 0.5 mile of dirty bomb.
500 / yr	USA NRC occupational limit for minors (10% of adult limit). USA NRC limit for visitors.	Orvieto town, Italy, natural. [http://www.unscear.org/pdffiles/annexb.pdf]
500 / 9 months	USA NRC occupational limit for pregnant women.
640 / yr		High Background Radiation Area (HBRA) of Yangjiang, China. [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11715418&dopt=Citation]
760 / yr		Fountainhead Rock Place, Santa Fe, NM natural.
1,000 - 5,000	USA EPA nuclear accident emergency action level. [http://www.ornl.gov/sci/env_rpt/aser95/tb-a-2.pdf]
1,000 - 19,000 acute		Nagasaki bomb survivors have lower incidence of cancer.
1,500 / yr		Taiwan cobalt-60 10-year exposure, 97% lower cancer than population.[http://www.jpands.org/vol9no1/chen.pdf]
5,000 / yr	USA NRC occupational limit ([http://www.nrc.gov/reading-rm/doc-collections/cfr/part020/ 10 CFR 20]).
10,000 acute	USA EPA acute dose level estimated to increase cancer risk 0.8%. [http://www.ornl.gov/sci/env_rpt/aser95/tb-a-2.pdf]
12,000 / yr		30 year exposure, Ural mountains, lower cancer mortality rate.[http://cnts.wpi.edu/RSH/Docs/Pollycove2000_Symp_on_Med_Ben.htm]
15,000 / yr	USA NRC occupational eye lens exposure limit.
17,500 / yr		Guarapari, Brazil natural radiation sources.[http://www.lewrockwell.com/miller/miller12.html]
25,000 acute	USA EPA voluntary maximum dose for emergency non-life-saving work. [http://www.ornl.gov/sci/env_rpt/aser95/tb-a-2.pdf]
50,000 / yr	USA NRC occupational whole skin, limb skin, or single organ exposure limit.	30 year exposure, Ural mountains, (exposed population lower cancer mortality rate).[http://cnts.wpi.edu/RSH/Docs/Pollycove2000_Symp_on_Med_Ben.htm]
75,000 acute	USA EPA voluntary maximum dose for emergency life-saving work. [http://www.ornl.gov/sci/env_rpt/aser95/tb-a-2.pdf]
70,000 / yr		Ramsar, Iran, natural background peak dose rate (in residences).[http://www.lewrockwell.com/miller/miller12.html] Guarapari, Brazil, natural, maximum on beach.
50,000 - 100,000 acute	Low-level radiation sickness due to short-term exposure.	World War II bomb victims.

Minimizing health effects of ionizing radiation

Although exposure to ionizing radiation carries a risk, it is impossible to completely avoid exposure. Radiation has always been present in the environment and in our bodies. We can, however, avoid undue exposure. Although people cannot sense ionizing radiation, there is a range of simple, sensitive instruments capable of detecting minute amounts of radiation from natural and man-made sources. Dosimeters measure an absolute dose received over a period of time. Ion-chamber dosimeters resemble pens, and can be clipped to one's clothing. Film-badge dosimeters enclose a piece of photographic film, which will become exposed as radiation passes through it. Ion-chamber dosimeters must be periodically recharged, and the result logged. Badge dosimeters must be developed as photographic emulsion so the exposures can be counted and logged; once developed, they are discarded. Geiger counters and scintillometers measure the dose rate of ionizing radiation directly. In addition, there are four ways in which we can protect ourselves: Time: For people who are exposed to radiation in addition to natural background radiation, limiting or minimizing the exposure time will reduce the dose from the radiation source. Distance: In the same way that the heat from a fire is less intense the further away you are, so the intensity of the radiation decreases the further you are form the source of the radiation. The dose decreases dramatically as you increase your distance from the source. Shielding: Barriers of lead, concrete, or water give good protection from penetrating radiation such as gamma rays and neutrons. This is why certain radioactive materials are stored or handled under water or by remote control in rooms constructed of thick concrete or lined with lead. There are special plastic shields which stop beta particles and air will stop alpha particles. Inserting the proper shield between you and the radiation source will greatly reduce or eliminate the extra radiation dose. Shielding can be designed using halving thicknesses, the thickness of material that reduces the radiation by half. Halving thicknesses for gamma rays are discussed in the article gamma rays. Containment: Radioactive materials are confined in the smallest possible space and kept out of the environment. Radioactive isotopes for medical use, for example, are dispensed in closed handling facilities, while nuclear reactors operate within closed systems with multiple barriers which keep the radioactive materials contained. Rooms have a reduced air pressure so that any leaks occur into the room and not out of it. In a nuclear war, an effective fallout shelter reduces human exposure at least 1000 times. Most people can accept doses as high as 100 R, distributed over several months, although with increased risk of cancer later in life. Other civil defense measures can help reduce exposure of populations by reducing ingestion of isotopes and occupational exposure during war time. One of these available measures could be the use of potassium iodide (KI) tablets which effectively block the uptake of dangerous radioactive iodine into the human thyroid gland.

External links

- [http://www.nrc.gov The Nuclear Regulatory Commission] regulates most commercial radiation sources and non-medical exposures in the US:
- [http://www.belleonline.com/ Biological Effects of Low Level Exposures: Radiation Hormesis]
- [http://www.unscear.org/reports/2000_1.html UNSCEAR 2000 Report, Volume 1:Sources]
- [http://www.unscear.org/reports/2000_2.html UNSCEAR 2000 Report, Volume 2:Effects] Category:Radioactivity Category:Radiobiology

Gamma rays

: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.

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. atomic numbers while those above and below the plane are thought to be quasars.]]
- 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 electron is the matter equivalent of an positron; it has the same weight as an positron, but it has a negative charge equal in strength to the positive charge of an positron. 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: :

^\hbox\;\to\;^\hbox\;+\;e^-\;+\;\overline_e.

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

^\hbox\;\to\;^\hbox\;+\;\gamma.

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. [http://www.inet.tsinghua.edu.cn/english2/academics4.htm]

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 Ruth