Singlet oxygen
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Please help to improve this article by adding reliable references. Unverifiable material may be challenged and removed.
This article has been tagged since July 2007.
Molecular orbital diagram for singlet oxygen. Quantum mechanics predicts that this configuration with the paired electrons is higher in energy than the triplet ground state.Singlet oxygen is the common name used for the two metastable states of molecular oxygen (O2) with higher energy than the ground state triplet oxygen [1]. The energy difference between the lowest energy of O2 in the singlet state and the lowest energy in the triplet state is about 3625 kelvin (Te (a¹Δg <- X³Σg-) = 7918.1 cm-1.)
Molecular oxygen differs from most molecules in having an open-shell triplet ground state, O2(X³Σg-). Molecular orbital theory predicts two low-lying excited singlet states O2(a¹Δg) and O2(b¹Σg+) (for nomenclature see article on Molecular term symbol). These electronic states differ only in the spin and the occupancy of oxygen's two degenerate antibonding πg-orbitals (see degenerate energy level). The O2(b¹Σg+)-state is very short lived and relaxes quickly to the lowest lying excited state, O2(a¹Δg). Thus, the O2(a¹Δg)-state is commonly referred to as singlet oxygen.
Contents [hide]
1 Physics
2 Chemistry
3 Biochemistry
4 External links
5 References
[edit] Physics
The energy difference between ground state and singlet oxygen is 94.2 kJ/mol and corresponds to a transition in the near-infrared at ~1270 nm. In the isolated molecule, the transition is strictly forbidden by spin, symmetry and parity selection rules, making it one of nature's most forbidden transitions. In other words, direct excitation of ground state oxygen by light to form singlet oxygen is very improbable. As a consequence, singlet oxygen in the gas phase is extremely long lived (72 minutes). Interaction with solvents, however, reduces the lifetime to microsecond or even nanoseconds.
Direct detection of singlet oxygen is possible through its extremely weak phosphorescence at 1270 nm, which is not visible to the eye. However, at high singlet oxygen concentrations, the fluorescence of the so-called singlet oxygen dimol (simultaneous emission from two singlet oxygen molecules upon collision) can be observed as a red glow at 634 nm [2].
[edit] Chemistry
The chemistry of singlet oxygen is different from that of ground state oxygen. Singlet oxygen can participate in Diels-Alder reactions and ene reactions. It can be generated in a photosensitized process by energy transfer from dye molecules such as rose bengal, methylene blue or porphyrins, or by chemical processes such as spontaneous decomposition of hydrogen trioxide in water or the reaction of hydrogen peroxide with hypochlorite [3]. Singlet oxygen reacts with an alkene -C=C-CH- by abstraction of the allylic proton in an ene reaction type reaction to the allyl hydroperoxide HO-O-C-C=C. It can then be reduced to the allyl alcohol. With some substrates dioxetanes are formed and cyclic dienes such as 1,3-Cyclohexadiene form [4+2]cycloaddition adducts. [4].
[edit] Biochemistry
In photosynthesis, singlet oxygen can be produced from the light-harvesting chlorophyll molecules. One of the roles of carotenoids in photosynthetic systems is to prevent damage caused by produced singlet oxygen by either removing excess light energy from chlorophyll molecules or quenching the singlet oxygen molecules directly.
In mammalian biology, singlet oxygen is a form of reactive oxygen species, which is linked to oxidation of LDL cholesterol and resultant cardiovascular effects. Polyphenol antioxidants can scavenge and reduce concentrations of reactive oxygen species and may prevent such deleterious oxidative effects [5].
Singlet oxygen is the active species in photodynamic therapy.
[edit] External links
The NIST webbook on oxygen
Photochemistry & Photobiology tutorial on Singlet Oxygen
Demonstration of the Red Singlet Oxygen Dimol Emission (Purdue University)
[edit] References
^ David R. Kearns (1971). "Physical and chemical properties of singlet molecular oxygen". Chemical Reviews 71 (4): 395 - 427. DOI:10.1021/cr60272a004.
^ Interpretation of the atmospheric oxygen bands; electronic levels of the oxygen molecule R.S. Mulliken Nature (journal) Volume 122, Page 505 1928
^ Physical Mechanisms of Generation and Deactivation of Singlet Oxygen C. Schweitzer, R. Schmidt Chemical Reviews Volume 103, Pages 1685-1757 2003
^ Carey, Francis A.; Sundberg, Richard J.; (1984). Advanced Organic Chemistry Part A Structure and Mechanisms (2nd ed.). New York N.Y.: Plenum Press. ISBN 0-306-41198-9.
^ Cell and Molecular Cell Biology concepts and experiments Fourth Edition. Gerald Karp. Page 223 2005
Retrieved from "http://en.wikipedia.org/wiki/Singlet_oxygen"
Categories: Articles lacking reliable references from July 2007 | Reagents for organic chemistry | Spectroscopy | Physical chemistry | Oxygen
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neutron
Neutron
From Wikipedia, the free encyclopedia
Jump to: navigation, search
This article is about the subatomic particle. For other uses, see Neutron (disambiguation).
This article is a discussion of neutrons in general. For the specific case of a neutron found outside the nucleus, see free neutron.
Neutron
The quark structure of the neutron.
Composition: one up, two down
Family: Fermion
Group: Quark
Interaction: Gravity, Electromagnetic, Weak, Strong
Antiparticle: Antineutron
Discovered: James Chadwick[1]
Symbol: n
Mass: 1.674 927 29(28) × 10−27kg
939.565 560(81) MeV/c²
1.008665 u
Electric charge: 0 C
Spin: ½
In physics, the neutron is a subatomic particle with no net electric charge and a mass of 939.573 MeV/c² or 1.008 664 915 (78) u (1.6749 × 10−27 kg, slightly more than a proton). Its spin is ½. Its antiparticle is called the antineutron. The neutron, along with the proton, is a nucleon.
The nucleus of all atoms (except the lightest isotope of hydrogen, which has only a single proton) consists of protons and neutrons. The number of neutrons determines the isotope of an element. For example, the carbon-12 isotope has 6 protons and 6 neutrons, while the carbon-14 isotope has 6 protons and 8 neutrons. Isotopes are atoms of the same element that have the same atomic number but different masses due to a different number of neutrons.
A neutron consists of two down quarks and one up quark. Since it has three quarks, it is classified it as a baryon.
Contents [hide]
1 Neutron Stability and Beta Decay
2 Interactions
3 Detection
4 Uses
5 Sources
6 Discovery
7 Anti-Neutron
8 Current developments
8.1 Electric dipole moment
8.2 Tetraneutrons
9 Protection
10 See also
10.1 Fields concerning neutrons
10.2 Types of neutrons
10.3 Objects containing neutrons
10.4 Neutron sources
10.5 Processes involving neutrons
11 References
[edit] Neutron Stability and Beta Decay
The Feynman diagram of the neutron beta decay processOutside the nucleus, free neutrons are unstable and have a mean lifetime of 885.7±0.8 seconds (about 15 minutes), decaying by emission of a negative electron and antineutrino to become a proton:[2] . This decay mode, known as beta decay, can also transform the character of neutrons within unstable nuclei.
Inside of a bound nucleus, protons can also transform via beta decay into neutrons. In this case, the transformation may occur by emission of a positive electron (also called a positron or an antielectron) and neutrino (instead of an antineutrino): . The transformation of a proton to a neutron inside of a nucleus is also possible through electron capture: . Positron capture by neutrons in nuclei that contain an excess of neutrons would also be possible, but is hindered due to the fact positrons are repelled by the nucleus, and furthermore, quickly annihilate when they encounter negative electrons.
When bound inside of a nucleus, the instability of a single neutron to beta decay is balanced against the instability that would be acquired by the nucleus as a whole if an additional proton were to participate in repulsive interactions with the other protons that are already present in the nucleus. As such, although free neutrons are unstable, bound neutrons are not necessarily so. The same reasoning explains why protons, which are stable in empty space, may transform into neutrons when bound inside of a nucleus.
Beta decay and electron capture are types of radioactive decay and are both governed by the weak interaction.
[edit] Interactions
The neutron interacts through all four fundamental interactions: the electromagnetic, weak nuclear, strong nuclear and gravitational interactions.
Although the neutron has zero net charge, it may interact electromagnetically in two ways: first, the neutron has a magnetic moment of the same order as the proton;[3] second, it is composed of electrically charged quarks. Thus, the electromagnetic interaction is primarily important to the neutron in deep inelastic scattering and in magnetic interactions.
The neutron experiences the weak interaction through beta decay into a proton, electron and electron antineutrino. It experiences the gravitational force as does any energetic body; however, gravity is so weak that it may be neglected in particle physics experiments.
The most important force to neutrons is the strong interaction. This interaction is responsible for the binding of the neutron's three quarks into a single particle. The residual strong force is responsible for the binding of neutrons and protons together into nuclei. This nuclear force plays the leading role when neutrons pass through matter. Unlike charged particles or photons, the neutron cannot lose energy by ionizing atoms. Rather, the neutron goes on its way unchecked until it makes a head-on collision with an atomic nucleus. For this reason, neutron radiation is extremely penetrating.
[edit] Detection
Main article: neutron detection
The common means of detecting a charged particle by looking for a track of ionization (such as in a cloud chamber) does not work for neutrons directly. Neutrons that elastically scatter off atoms can create an ionization track that is detectable, but the experiments are not as simple to carry out; other means for detecting neutrons, consisting of allowing them to interact with atomic nuclei, are more commonly used.
A common method for detecting neutrons involves converting the energy released from such reactions into electrical signals. The nuclides 3He, 6Li, 10B, 233U, 235U, 237Np and 239Pu are useful for this purpose. A good discussion on neutron detection is found in chapter 14 of the book Radiation Detection and Measurement by Glenn F. Knoll (John Wiley & Sons, 1979).
[edit] Uses
The neutron plays an important role in many nuclear reactions. For example, neutron capture often results in neutron activation, inducing radioactivity. In particular, knowledge of neutrons and their behavior has been important in the development of nuclear reactors and nuclear weapons.
Cold, thermal and hot neutron radiation is commonly employed in neutron scattering facilities, where the radiation is used in a similar way one uses X-rays for the analysis of condensed matter. Neutrons are complementary to the latter in terms of atomic contrasts by different scattering cross sections; sensitivity to magnetism; energy range for inelastic neutron spectroscopy; and deep penetration into matter.
The development of "neutron lenses" based on total internal reflection within hollow glass capillary tubes or by reflection from dimpled aluminum plates has driven ongoing research into neutron microscopy and neutron/gamma ray tomography.[4][5][6]
One use of neutron emitters is the detection of light nuclei, particularly the hydrogen found in water molecules. When a fast neutron collides with a light nucleus, it loses a large fraction of its energy. By measuring the rate at which slow neutrons return to the probe after reflecting off of hydrogen nuclei, a neutron probe may determine the water content in soil.
[edit] Sources
Due to the fact that free neutrons are unstable, they can be obtained only from nuclear disintegrations, nuclear reactions, and high-energy reactions (such as in cosmic radiation showers or accelerator collisions). Free neutron beams are obtained from neutron sources by neutron transport. For access to intense neutron sources, researchers must go to specialist facilities, such as the ISIS facility in the UK, which is currently the world's most intense pulsed neutron and muon source.
Neutrons' lack of total electric charge prevents engineers or experimentalists from being able to steer or accelerate them. Charged particles can be accelerated, decelerated, or deflected by electric or magnetic fields. However, these methods have no effect on neutrons except for a small effect of a magnetic field because of the neutron's magnetic moment.
[edit] Discovery
In 1930 Walther Bothe and H. Becker in Germany found that if the very energetic alpha particles emitted from polonium fell on certain light elements, specifically beryllium, boron, or lithium, an unusually penetrating radiation was produced. At first this radiation was thought to be gamma radiation although it was more penetrating than any gamma rays known, and the details of experimental results were very difficult to interpret on this basis. The next important contribution was reported in 1932 by Irène Joliot-Curie and Frédéric Joliot in Paris. They showed that if this unknown radiation fell on paraffin or any other hydrogen-containing compound it ejected protons of very high energy. This was not in itself inconsistent with the assumed gamma ray nature of the new radiation, but detailed quantitative analysis of the data became increasingly difficult to reconcile with such a hypothesis. Finally (later in 1932) the physicist James Chadwick in England performed a series of experiments showing that the gamma ray hypothesis was untenable. He suggested that in fact the new radiation consisted of uncharged particles of approximately the mass of the proton, and he performed a series of experiments verifying his suggestion. Such uncharged particles were eventually called neutrons, apparently from the Latin root for neutral and the Greek ending -on (by imitation of electron and proton).
[edit] Anti-Neutron
Main article: antineutron
The antineutron is the antiparticle of the neutron. It was discovered by Bruce Cork in the year 1956, a year after the antiproton was discovered.
CPT-symmetry puts strong constraints on the relative properties of particles and antiparticles and, therefore, is open to stringent tests. The fractional difference in the masses of the neutron and antineutron is (9±5)×10−5. Since the difference is only about 2 standard deviations away from zero, this does not give any convincing evidence of CPT-violation.[3]
[edit] Current developments
[edit] Electric dipole moment
An experiment at the Institut Laue-Langevin (ILL) has attempted to measure an electric dipole, or separation of charges, within the neutron, and is consistent with an electric dipole moment of zero. These results are important in developing theories that go beyond the Standard Model. See FRONTIERS article, and the experiment's web page.
[edit] Tetraneutrons
The existence of stable clusters of four neutrons, or tetraneutrons, has been hypothesised by a team led by Francisco-Miguel Marqués at the CNRS Laboratory for Nuclear Physics based on observations of the disintegration of beryllium-14 nuclei. This is particularly interesting, because current theory suggests that these clusters should not be stable.
[edit] Protection
Exposure to neutrons can be hazardous, since the interaction of neutrons with molecules in the body can cause disruption to molecules and atoms, and can also cause reactions which give rise to other forms of radiation. The normal expectations of radiation protection apply: avoid exposure, stay as far from the source as possible, and keep exposure time to the minimum. Some thought must however be given to how to protect oneselves from such exposure. For other types of radiation, e.g. alpha particles, beta particles, or gamma rays, material of a high atomic number and with high density makes for good shielding; frequently lead is used. However, this approach will not work with neutrons, since the absorption of neutrons does not increase straightforwardly with atomic number as it does with alpha, beta, and gamma radiation. Instead one needs to look at the particular interactions neutrons have with matter (see the section on detection above). For example, hydrogen rich materials are often used since ordinary hydrogen scatters neutrons, so this often means simple concrete blocks, or paraffin loaded plastic blocks may be the best protection.
[edit] See also
[edit] Fields concerning neutrons
particle physics
quark model
chemistry
Neutron Detection
Neutron Scattering
[edit] Types of neutrons
nucleon
fast neutron
free neutron
thermal neutron
neutron radiation and the Sievert radiation scale
neutron temperature, used to classify neutron types
[edit] Objects containing neutrons
nucleus
dineutron
tetraneutron
neutronium
neutron star
[edit] Neutron sources
Neutron sources
Neutron generator
[edit] Processes involving neutrons
neutron transport
neutron diffraction
neutron bomb
[hide]v • d • eParticles in physics
elementary particles Elementary fermions: Quarks: u · d · s · c · b · t • Leptons: e · μ · τ · νe · νμ · ντ
Elementary bosons: Gauge bosons: γ · g · W± · Z0 • Ghosts
Composite particles Hadrons: Baryons(list)/Hyperons/Nucleons: p · n · Δ · Λ · Σ · Ξ · Ω · Ξb • Mesons(list)/Quarkonia: π · K · ρ · J/ψ · Υ
Other: Atomic nucleus • Atoms • Molecules • Positronium
Hypothetical elementary particles Superpartners: Axino · Dilatino · Chargino · Gluino · Gravitino · Higgsino · Neutralino · Sfermion · Slepton · Squark
Other: Axion · Dilaton · Goldstone boson · Graviton · Higgs boson · Tachyon · X · Y · W' · Z'
Hypothetical composite particles Exotic hadrons: Exotic baryons: Pentaquark • Exotic mesons: Glueball · Tetraquark
Other: Mesonic molecule
Quasiparticles Davydov soliton · Exciton · Magnon · Phonon · Plasmon · Polariton · Polaron
[edit] References
^ 1935 Nobel Prize in Physics
^ Particle Data Group Summary Data Table on Baryons
^ a b Particle Data Group's Review of Particle Physics 2006
^ Nature 357, 390-391 (04 June 1992); doi:10.1038/357390a0
^ Physorg.com, "New Way of 'Seeing': A 'Neutron Microscope'"
^ NASA.gov: "NASA Develops a Nugget to Search for Life in Space"
Retrieved from "http://en.wikipedia.org/wiki/Neutron"
Categories: Neutron | Fundamental physics concepts
From Wikipedia, the free encyclopedia
Jump to: navigation, search
This article is about the subatomic particle. For other uses, see Neutron (disambiguation).
This article is a discussion of neutrons in general. For the specific case of a neutron found outside the nucleus, see free neutron.
Neutron
The quark structure of the neutron.
Composition: one up, two down
Family: Fermion
Group: Quark
Interaction: Gravity, Electromagnetic, Weak, Strong
Antiparticle: Antineutron
Discovered: James Chadwick[1]
Symbol: n
Mass: 1.674 927 29(28) × 10−27kg
939.565 560(81) MeV/c²
1.008665 u
Electric charge: 0 C
Spin: ½
In physics, the neutron is a subatomic particle with no net electric charge and a mass of 939.573 MeV/c² or 1.008 664 915 (78) u (1.6749 × 10−27 kg, slightly more than a proton). Its spin is ½. Its antiparticle is called the antineutron. The neutron, along with the proton, is a nucleon.
The nucleus of all atoms (except the lightest isotope of hydrogen, which has only a single proton) consists of protons and neutrons. The number of neutrons determines the isotope of an element. For example, the carbon-12 isotope has 6 protons and 6 neutrons, while the carbon-14 isotope has 6 protons and 8 neutrons. Isotopes are atoms of the same element that have the same atomic number but different masses due to a different number of neutrons.
A neutron consists of two down quarks and one up quark. Since it has three quarks, it is classified it as a baryon.
Contents [hide]
1 Neutron Stability and Beta Decay
2 Interactions
3 Detection
4 Uses
5 Sources
6 Discovery
7 Anti-Neutron
8 Current developments
8.1 Electric dipole moment
8.2 Tetraneutrons
9 Protection
10 See also
10.1 Fields concerning neutrons
10.2 Types of neutrons
10.3 Objects containing neutrons
10.4 Neutron sources
10.5 Processes involving neutrons
11 References
[edit] Neutron Stability and Beta Decay
The Feynman diagram of the neutron beta decay processOutside the nucleus, free neutrons are unstable and have a mean lifetime of 885.7±0.8 seconds (about 15 minutes), decaying by emission of a negative electron and antineutrino to become a proton:[2] . This decay mode, known as beta decay, can also transform the character of neutrons within unstable nuclei.
Inside of a bound nucleus, protons can also transform via beta decay into neutrons. In this case, the transformation may occur by emission of a positive electron (also called a positron or an antielectron) and neutrino (instead of an antineutrino): . The transformation of a proton to a neutron inside of a nucleus is also possible through electron capture: . Positron capture by neutrons in nuclei that contain an excess of neutrons would also be possible, but is hindered due to the fact positrons are repelled by the nucleus, and furthermore, quickly annihilate when they encounter negative electrons.
When bound inside of a nucleus, the instability of a single neutron to beta decay is balanced against the instability that would be acquired by the nucleus as a whole if an additional proton were to participate in repulsive interactions with the other protons that are already present in the nucleus. As such, although free neutrons are unstable, bound neutrons are not necessarily so. The same reasoning explains why protons, which are stable in empty space, may transform into neutrons when bound inside of a nucleus.
Beta decay and electron capture are types of radioactive decay and are both governed by the weak interaction.
[edit] Interactions
The neutron interacts through all four fundamental interactions: the electromagnetic, weak nuclear, strong nuclear and gravitational interactions.
Although the neutron has zero net charge, it may interact electromagnetically in two ways: first, the neutron has a magnetic moment of the same order as the proton;[3] second, it is composed of electrically charged quarks. Thus, the electromagnetic interaction is primarily important to the neutron in deep inelastic scattering and in magnetic interactions.
The neutron experiences the weak interaction through beta decay into a proton, electron and electron antineutrino. It experiences the gravitational force as does any energetic body; however, gravity is so weak that it may be neglected in particle physics experiments.
The most important force to neutrons is the strong interaction. This interaction is responsible for the binding of the neutron's three quarks into a single particle. The residual strong force is responsible for the binding of neutrons and protons together into nuclei. This nuclear force plays the leading role when neutrons pass through matter. Unlike charged particles or photons, the neutron cannot lose energy by ionizing atoms. Rather, the neutron goes on its way unchecked until it makes a head-on collision with an atomic nucleus. For this reason, neutron radiation is extremely penetrating.
[edit] Detection
Main article: neutron detection
The common means of detecting a charged particle by looking for a track of ionization (such as in a cloud chamber) does not work for neutrons directly. Neutrons that elastically scatter off atoms can create an ionization track that is detectable, but the experiments are not as simple to carry out; other means for detecting neutrons, consisting of allowing them to interact with atomic nuclei, are more commonly used.
A common method for detecting neutrons involves converting the energy released from such reactions into electrical signals. The nuclides 3He, 6Li, 10B, 233U, 235U, 237Np and 239Pu are useful for this purpose. A good discussion on neutron detection is found in chapter 14 of the book Radiation Detection and Measurement by Glenn F. Knoll (John Wiley & Sons, 1979).
[edit] Uses
The neutron plays an important role in many nuclear reactions. For example, neutron capture often results in neutron activation, inducing radioactivity. In particular, knowledge of neutrons and their behavior has been important in the development of nuclear reactors and nuclear weapons.
Cold, thermal and hot neutron radiation is commonly employed in neutron scattering facilities, where the radiation is used in a similar way one uses X-rays for the analysis of condensed matter. Neutrons are complementary to the latter in terms of atomic contrasts by different scattering cross sections; sensitivity to magnetism; energy range for inelastic neutron spectroscopy; and deep penetration into matter.
The development of "neutron lenses" based on total internal reflection within hollow glass capillary tubes or by reflection from dimpled aluminum plates has driven ongoing research into neutron microscopy and neutron/gamma ray tomography.[4][5][6]
One use of neutron emitters is the detection of light nuclei, particularly the hydrogen found in water molecules. When a fast neutron collides with a light nucleus, it loses a large fraction of its energy. By measuring the rate at which slow neutrons return to the probe after reflecting off of hydrogen nuclei, a neutron probe may determine the water content in soil.
[edit] Sources
Due to the fact that free neutrons are unstable, they can be obtained only from nuclear disintegrations, nuclear reactions, and high-energy reactions (such as in cosmic radiation showers or accelerator collisions). Free neutron beams are obtained from neutron sources by neutron transport. For access to intense neutron sources, researchers must go to specialist facilities, such as the ISIS facility in the UK, which is currently the world's most intense pulsed neutron and muon source.
Neutrons' lack of total electric charge prevents engineers or experimentalists from being able to steer or accelerate them. Charged particles can be accelerated, decelerated, or deflected by electric or magnetic fields. However, these methods have no effect on neutrons except for a small effect of a magnetic field because of the neutron's magnetic moment.
[edit] Discovery
In 1930 Walther Bothe and H. Becker in Germany found that if the very energetic alpha particles emitted from polonium fell on certain light elements, specifically beryllium, boron, or lithium, an unusually penetrating radiation was produced. At first this radiation was thought to be gamma radiation although it was more penetrating than any gamma rays known, and the details of experimental results were very difficult to interpret on this basis. The next important contribution was reported in 1932 by Irène Joliot-Curie and Frédéric Joliot in Paris. They showed that if this unknown radiation fell on paraffin or any other hydrogen-containing compound it ejected protons of very high energy. This was not in itself inconsistent with the assumed gamma ray nature of the new radiation, but detailed quantitative analysis of the data became increasingly difficult to reconcile with such a hypothesis. Finally (later in 1932) the physicist James Chadwick in England performed a series of experiments showing that the gamma ray hypothesis was untenable. He suggested that in fact the new radiation consisted of uncharged particles of approximately the mass of the proton, and he performed a series of experiments verifying his suggestion. Such uncharged particles were eventually called neutrons, apparently from the Latin root for neutral and the Greek ending -on (by imitation of electron and proton).
[edit] Anti-Neutron
Main article: antineutron
The antineutron is the antiparticle of the neutron. It was discovered by Bruce Cork in the year 1956, a year after the antiproton was discovered.
CPT-symmetry puts strong constraints on the relative properties of particles and antiparticles and, therefore, is open to stringent tests. The fractional difference in the masses of the neutron and antineutron is (9±5)×10−5. Since the difference is only about 2 standard deviations away from zero, this does not give any convincing evidence of CPT-violation.[3]
[edit] Current developments
[edit] Electric dipole moment
An experiment at the Institut Laue-Langevin (ILL) has attempted to measure an electric dipole, or separation of charges, within the neutron, and is consistent with an electric dipole moment of zero. These results are important in developing theories that go beyond the Standard Model. See FRONTIERS article, and the experiment's web page.
[edit] Tetraneutrons
The existence of stable clusters of four neutrons, or tetraneutrons, has been hypothesised by a team led by Francisco-Miguel Marqués at the CNRS Laboratory for Nuclear Physics based on observations of the disintegration of beryllium-14 nuclei. This is particularly interesting, because current theory suggests that these clusters should not be stable.
[edit] Protection
Exposure to neutrons can be hazardous, since the interaction of neutrons with molecules in the body can cause disruption to molecules and atoms, and can also cause reactions which give rise to other forms of radiation. The normal expectations of radiation protection apply: avoid exposure, stay as far from the source as possible, and keep exposure time to the minimum. Some thought must however be given to how to protect oneselves from such exposure. For other types of radiation, e.g. alpha particles, beta particles, or gamma rays, material of a high atomic number and with high density makes for good shielding; frequently lead is used. However, this approach will not work with neutrons, since the absorption of neutrons does not increase straightforwardly with atomic number as it does with alpha, beta, and gamma radiation. Instead one needs to look at the particular interactions neutrons have with matter (see the section on detection above). For example, hydrogen rich materials are often used since ordinary hydrogen scatters neutrons, so this often means simple concrete blocks, or paraffin loaded plastic blocks may be the best protection.
[edit] See also
[edit] Fields concerning neutrons
particle physics
quark model
chemistry
Neutron Detection
Neutron Scattering
[edit] Types of neutrons
nucleon
fast neutron
free neutron
thermal neutron
neutron radiation and the Sievert radiation scale
neutron temperature, used to classify neutron types
[edit] Objects containing neutrons
nucleus
dineutron
tetraneutron
neutronium
neutron star
[edit] Neutron sources
Neutron sources
Neutron generator
[edit] Processes involving neutrons
neutron transport
neutron diffraction
neutron bomb
[hide]v • d • eParticles in physics
elementary particles Elementary fermions: Quarks: u · d · s · c · b · t • Leptons: e · μ · τ · νe · νμ · ντ
Elementary bosons: Gauge bosons: γ · g · W± · Z0 • Ghosts
Composite particles Hadrons: Baryons(list)/Hyperons/Nucleons: p · n · Δ · Λ · Σ · Ξ · Ω · Ξb • Mesons(list)/Quarkonia: π · K · ρ · J/ψ · Υ
Other: Atomic nucleus • Atoms • Molecules • Positronium
Hypothetical elementary particles Superpartners: Axino · Dilatino · Chargino · Gluino · Gravitino · Higgsino · Neutralino · Sfermion · Slepton · Squark
Other: Axion · Dilaton · Goldstone boson · Graviton · Higgs boson · Tachyon · X · Y · W' · Z'
Hypothetical composite particles Exotic hadrons: Exotic baryons: Pentaquark • Exotic mesons: Glueball · Tetraquark
Other: Mesonic molecule
Quasiparticles Davydov soliton · Exciton · Magnon · Phonon · Plasmon · Polariton · Polaron
[edit] References
^ 1935 Nobel Prize in Physics
^ Particle Data Group Summary Data Table on Baryons
^ a b Particle Data Group's Review of Particle Physics 2006
^ Nature 357, 390-391 (04 June 1992); doi:10.1038/357390a0
^ Physorg.com, "New Way of 'Seeing': A 'Neutron Microscope'"
^ NASA.gov: "NASA Develops a Nugget to Search for Life in Space"
Retrieved from "http://en.wikipedia.org/wiki/Neutron"
Categories: Neutron | Fundamental physics concepts
Decay Product
Decay product
From Wikipedia, the free encyclopedia
Jump to: navigation, search
In nuclear physics, a decay product, also known as a daughter product, daughter isotope or daughter nuclide, is a nuclide resulting from the radioactive decay of a parent isotope or precursor nuclide. The daughter product may be stable or it may decay to form a daughter product of its own. The daughter of a daughter product is sometimes called a granddaughter product.
Decay products are extremely important in understanding radioactive decay and the management of radioactive waste.
In practice nearly all decay products are themselves radioactive. The result of this is that most radionuclides do not have simply a decay product, but rather a decay chain, leading eventually to a stable nuclide. For elements above lead in atomic number, this is nearly always an isotope of lead. Lead is generally the stable point at which decay chains stop.
In many cases members of the decay chain are far more radioactive than the original nuclide. Thus, although uranium is not dangerously radioactive when pure, some pieces of naturally-occurring pitchblende are quite dangerous owing to their radium content. Similarly, thorium gas mantles are very slightly radioactive when new, but become far more radioactive after only a few months of storage.
Although it cannot be predicted whether any given atom of a radioactive substance will decay at any given time, the decay products of a radioactive substance are extremely predictable. Because of this, decay products are important to scientists in many fields who need to know the quantity or type of the parent product. Such studies are done to measure pollution levels (in and around nuclear facilities) and for other matters.
Retrieved from "http://en.wikipedia.org/wiki/Decay_product"
Categories: Nuclear physics | Nuclear chemistry
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In nuclear physics, a decay product, also known as a daughter product, daughter isotope or daughter nuclide, is a nuclide resulting from the radioactive decay of a parent isotope or precursor nuclide. The daughter product may be stable or it may decay to form a daughter product of its own. The daughter of a daughter product is sometimes called a granddaughter product.
Decay products are extremely important in understanding radioactive decay and the management of radioactive waste.
In practice nearly all decay products are themselves radioactive. The result of this is that most radionuclides do not have simply a decay product, but rather a decay chain, leading eventually to a stable nuclide. For elements above lead in atomic number, this is nearly always an isotope of lead. Lead is generally the stable point at which decay chains stop.
In many cases members of the decay chain are far more radioactive than the original nuclide. Thus, although uranium is not dangerously radioactive when pure, some pieces of naturally-occurring pitchblende are quite dangerous owing to their radium content. Similarly, thorium gas mantles are very slightly radioactive when new, but become far more radioactive after only a few months of storage.
Although it cannot be predicted whether any given atom of a radioactive substance will decay at any given time, the decay products of a radioactive substance are extremely predictable. Because of this, decay products are important to scientists in many fields who need to know the quantity or type of the parent product. Such studies are done to measure pollution levels (in and around nuclear facilities) and for other matters.
Retrieved from "http://en.wikipedia.org/wiki/Decay_product"
Categories: Nuclear physics | Nuclear chemistry
Decay Energy
Decay energy
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The decay energy is the energy released by a nuclear decay.
The difference between the mass of the reactants and the mass of products is often written as Q:
Q = (mass of reactants) - (mass of products)
This can be expressed as energy by Albert Einstein's famous formula E=mc².
Types of radioactive decay include
gamma radiation
beta decay
alpha decay
[edit] External links
University of Waterloo science
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From Wikipedia, the free encyclopedia
Jump to: navigation, search
The decay energy is the energy released by a nuclear decay.
The difference between the mass of the reactants and the mass of products is often written as Q:
Q = (mass of reactants) - (mass of products)
This can be expressed as energy by Albert Einstein's famous formula E=mc².
Types of radioactive decay include
gamma radiation
beta decay
alpha decay
[edit] External links
University of Waterloo science
This chemistry article is a stub. You can help Wikipedia by expanding it.
Retrieved from "http://en.wikipedia.org/wiki/Decay_energy"
Categories: Chemistry stubs | Nuclear chemistry
ViewsArticle Discussion Edit this page History Personal toolsSign in / create account Navigation
Main page
Contents
Featured content
Current events
Random article
interaction
About Wikipedia
Community portal
Recent changes
File upload wizard
Contact Wikipedia
Make a donation
Help
Search
Toolbox
What links here
Related changes
Upload file
Special pages
Printable version
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Cite this article
In other languages
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Asturianu
Català
Deutsch
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Français
Magyar
Nederlands
日本語
Polski
Português
Svenska
This page was last modified 20:19, 21 May 2007. All text is available under the terms of the GNU Free Documentation License. (See Copyrights for details.)
Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a US-registered 501(c)(3) tax-deductible nonprofit charity.
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Radioactive Decay
Radioactive decay
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"Radioactive" and "Radioactivity" redirect here. For other uses see Radioactive (disambiguation).
For decay rate in a more general context see Particle decay.
Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, called the daughter nuclide. For example: a carbon-14 atom (the "parent") emits radiation and transforms to a nitrogen-14 atom (the "daughter.") This is a random process on the atomic level, in that it is impossible to predict when a particular atom will decay, but given a large number of similar atoms, the decay rate, on average, is predictable.
The trefoil symbol is used to indicate radioactive material.The SI unit of radioactive decay is the becquerel (Bq). One Bq is defined as one transformation (or decay) per second. Since any reasonably-sized sample of radioactive material contains many atoms, a Bq is a tiny measure of activity; amounts on the order of TBq (terabecquerels) or GBq (gigabecquerels) are commonly used. Another unit of decay is the curie, which was originally defined as the radioactivity of one gram of pure radium, and is equal to 3.7 × 1010 Bq.
Contents [hide]
1 Explanation
2 Discovery
3 Modes of decay
4 Decay chains and multiple modes
5 Occurrence and applications
6 Radioactive decay rates
6.1 Activity measurements
7 Decay timing
8 References
9 See also
10 External links
[edit] Explanation
The neutrons and protons that constitute nuclei, as well as other particles that may approach them, are governed by several interactions. The strong nuclear force, not observed at the familiar macroscopic scale, is the most powerful force over subatomic distances. The electrostatic force is also significant. Of lesser importance is the weak nuclear force.
The interplay of these forces is simple. Some configurations of the particles in a nucleus have the property that, should they shift ever so slightly, the particles could fall into a lower-energy arrangement (with the extra energy moving elsewhere). One might draw an analogy with a snowfield on a mountain: while friction between the snow crystals can support the snow's weight, the system is inherently unstable with regards to a lower-potential-energy state, and a disturbance may facilitate the path to a greater entropy state (i.e., towards the ground state where heat will be produced, and thus total energy is distributed over a larger number of quantum states). Thus, an avalanche results. The total energy does not change in this process, but because of entropy effects, avalanches only happen in one direction, and the end of this direction, which is dictated by the largest number of chance-mediated ways to distribute available energy, is what we commonly refer to as the "ground state."
Such a collapse (a decay event) requires a specific activation energy. In the case of a snow avalanche, this energy classically comes as a disturbance from outside the system, although such disturbances can be arbitrarily small. In the case of an excited atomic nucleus, the arbitrarily small disturbance comes from quantum vacuum fluctuations. A nucleus (or any excited system in quantum mechanics) is unstable, and can thus spontaneously stabilize to a less-excited system. This process is driven by entropy considerations: the energy does not change, but at the end of the process, the total energy is more diffused in spacial volume. The resulting transformation alters the structure of the nucleus. Such a reaction is thus a nuclear reaction, in contrast to chemical reactions, which also are driven by entropy, but which involve changes in the arrangement of the outer electrons of atoms, rather than their nuclei.
Some nuclear reactions do involve external sources of energy, in the form of collisions with outside particles. However, these are not considered decay. Rather, they are examples of induced nuclear reactions. Nuclear fission and fusion are common types of induced nuclear reactions.
[edit] Discovery
Radioactivity was first discovered in 1896 by the French scientist Henri Becquerel while working on phosphorescent materials. These materials glow in the dark after exposure to light, and he thought that the glow produced in cathode ray tubes by X-rays might somehow be connected with phosphorescence. So he tried wrapping a photographic plate in black paper and placing various phosphorescent minerals on it. All results were negative until he tried using uranium salts. The result with these compounds was a deep blackening of the plate.
However, it soon became clear that the blackening of the plate had nothing to do with phosphorescence because the plate blackened when the mineral was kept in the dark. Also non-phosphorescent salts of uranium and even metallic uranium blackened the plate. Clearly there was some new form of radiation that could pass through paper that was causing the plate to blacken.
Alpha particles may be completely stopped by a sheet of paper, beta particles by aluminum shielding. Gamma rays, however, can only be reduced by much more substantial obstacles, such as a very thick piece of lead.At first it seemed that the new radiation was similar to the then recently discovered X-rays. However further research by Becquerel, Marie Curie, Pierre Curie, Ernest Rutherford and others discovered that radioactivity was significantly more complicated. Different types of decay can occur, but Rutherford was the first to realize that they all occur with the same mathematical approximately exponential formula (see below).
As for types of radioactive radiation, it was found that an electric or magnetic field could split such emissions into three types of beams. For lack of better terms, the rays were given the alphabetic names alpha, beta, and gamma, names they still hold today. It was immediately obvious from the direction of electromagnetic forces that alpha rays carried a positive charge, beta rays carried a negative charge, and gamma rays were neutral. From the magnitude of deflection, it was also clear that alpha particles were much more massive than beta particles. Passing alpha rays through a thin glass membrane and trapping them in a discharge tube allowed researchers to study the emission spectrum of the resulting gas, and ultimately prove that alpha particles are in fact helium nuclei. Other experiments showed the similarity between beta radiation and cathode rays; they are both streams of electrons, and between gamma radiation and X-rays, which are both high energy electromagnetic radiation.
Although alpha, beta, and gamma are most common, other types of decay were eventually discovered. Shortly after discovery of the neutron in 1932, it was discovered by Enrico Fermi that certain rare decay reactions give rise to neutrons as a decay particle. Isolated proton emission was also eventually observed in some elements. Shortly after the discovery of the positron in cosmic ray products, it was realized that the same process that operates in classical beta decay can also produce positrons (positron emission), analogously to negative electrons. Each of the two types of beta decay acts to move a nucleus toward a ratio of neutrons and protons which has the least energy for the combination. Finally, in a phenomenon called cluster decay, specific combinations of neutrons and protons other than alpha particles were found to occasionally spontaneously be emitted from atoms.
Still other types of radioactive decay were found which emit previously seen particles, but by different mechanisms. An example is internal conversion, which results in electron and sometimes high energy photon emission, even though it involves neither beta nor gamma decay.
The early researchers also discovered that many other chemical elements besides uranium have radioactive isotopes. A systematic search for the total radioactivity in uranium ores also guided Marie Curie to isolate a new element polonium and to separate a new element radium from barium; the two elements' chemical similarity would otherwise have made them difficult to distinguish.
The dangers of radioactivity and of radiation were not immediately recognized. Acute effects of radiation were first observed in the use of X-rays when the Serbo-Croatian-American electric engineer Nikola Tesla intentionally subjected his fingers to X-rays in 1896. He published his observations concerning the burns that developed, though he attributed them to ozone rather than to the X-rays. Fortunately his injuries healed later.
The genetic effects of radiation, including the effects on cancer risk, were recognized much later. It was only in 1927 that Hermann Joseph Muller published his research that showed the genetic effects. In 1946 he was awarded the Nobel prize for his findings.
Before the biological effects of radiation were known, many physicians and corporations had begun marketing radioactive substances as patent medicine and Radioactive quackery; particularly alarming examples were radium enema treatments, and radium-containing waters to be drunk as tonics. Marie Curie spoke out against this sort of treatment, warning that the effects of radiation on the human body were not well understood (Curie later died from aplastic anemia assumed due to her own work with radium, but later examination of her bones showed that she had been a careful laboratory worker and had a low burden of radium; a better candidate for her disease was her long exposure to unshielded X-ray tubes while a volunteer medical worker in WW I). By the 1930s, after a number of cases of bone-necrosis and death in enthusiasts, radium-containing medical products had nearly vanished from the market.
[edit] Modes of decay
Radionuclides can undergo a number of different reactions. These are summarized in the following table. A nucleus with positive charge (atomic number) Z and atomic weight A is represented as (A, Z).
Mode of decay Participating particles Daughter nucleus
Decays with emission of nucleons:
Alpha decay An alpha particle (A=4, Z=2) emitted from nucleus (A-4, Z-2)
Proton emission A proton ejected from nucleus (A-1, Z-1)
Neutron emission A neutron ejected from nucleus (A-1, Z)
Double proton emission Two protons ejected from nucleus simultaneously (A-2, Z-2)
Spontaneous fission Nucleus disintegrates into two or more smaller nuclei and other particles -
Cluster decay Nucleus emits a specific type of smaller nucleus (A1, Z1) larger than an alpha particle (A-A1, Z-Z1) + (A1,Z1)
Different modes of beta decay:
Beta-Negative decay A nucleus emits an electron and an antineutrino (A, Z+1)
Positron emission, also Beta-Positive decay A nucleus emits a positron and a neutrino (A, Z-1)
Electron capture A nucleus captures an orbiting electron and emits a neutrino - The daughter nucleus is left in an excited and unstable state (A, Z-1)
Double beta decay A nucleus emits two electrons and two antineutrinos (A, Z+2)
Double electron capture A nucleus absorbs two orbital electrons and emits two neutrinos - The daughter nucleus is left in an excited and unstable state (A, Z-2)
Electron capture with positron emission A nucleus absorbs one orbital electron, emits one positron and two neutrinos (A, Z-2)
Double positron emission A nucleus emits two positrons and two neutrinos (A, Z-2)
Transitions between states of the same nucleus:
Gamma decay Excited nucleus releases a high-energy photon (gamma ray) (A, Z)
Internal conversion Excited nucleus transfers energy to an orbital electron and it is ejected from the atom (A, Z)
Radioactive decay results in a reduction of summed rest mass, which is converted to energy (the disintegration energy) according to the formula E = mc2. This energy is released as kinetic energy of the emitted particles. The energy remains associated with a measure of mass of the decay system invariant mass, inasmuch the kinetic energy of emitted particles contributes also to the total invariant mass of systems. Thus, the sum of rest masses of particles is not conserved in decay, but the system mass or system invariant mass (as also system total energy) is conserved.
[edit] Decay chains and multiple modes
The daughter nuclide of a decay event is usually also unstable, sometimes even more unstable than the parent. If this is the case, it will proceed to decay again. A sequence of several decay events, producing in the end a stable nuclide, is a decay chain.
Many radionuclides have several different observed modes of decay. Bismuth-212, for example, has three. Thus a given nuclide may lead to several different decay chains.
Of the commonly occurring forms of radioactive decay, the only one that changes the number of aggregate protons and neutrons (nucleons) contained in the nucleus is alpha emission, which reduces it by four. Thus, the number of nucleons modulo 4 is preserved across any decay chain.
[edit] Occurrence and applications
According to the Big Bang theory, radioactive isotopes of the lightest elements (H, He, and traces of Li) were produced very shortly after the emergence of the universe. However, these nuclides are so highly unstable that virtually none of them have survived to today. Most radioactive nuclei are therefore relatively young, having formed in stars (particularly supernovae) and during ongoing interactions between stable isotopes and energetic particles. For example, carbon-14, a radioactive nuclide with a half-life of only 5730 years, is constantly produced in Earth's upper atmosphere due to interactions between cosmic rays and nitrogen.
Radioactive decay has been put to use in the technique of radioisotopic labeling, used to track the passage of a chemical substance through a complex system (such as a living organism). A sample of the substance is synthesized with a high concentration of unstable atoms. The presence of the substance in one or another part of the system is determined by detecting the locations of decay events.
On the premise that radioactive decay is truly random (rather than merely chaotic), it has been used in hardware random-number generators. Because the process is not thought to vary significantly in mechanism over time, it is also a valuable tool in estimating the absolute ages of certain materials. For geological materials, the radioisotopes and certain of their decay products become trapped when a rock solidifies, and can then later be used (subject to many well-known qualifications) to estimate the date of the solidification. These include checking the results of several simultaneous processes and their products against each other, within the same sample.
[edit] Radioactive decay rates
The decay rate, or activity, of a radioactive substance are characterized by:
Constant quantities:
half life — symbol t1 / 2 — the time for half of a substance to decay.
mean lifetime — symbol τ — the average lifetime of any given particle.
decay constant — symbol λ — the inverse of the mean lifetime.
(Note that although these are constants, they are associated with statistically random behavior of substances, and predictions using these constants are less accurate for small number of atoms.)
Time-variable quantities:
Total activity — symbol A — number of decays an object undergoes per second.
Number of particles — symbol N — the total number of particles in the sample.
Specific activity — symbol SA — number of decays per second per amount of substance. The "amount of substance" can be the unit of either mass or volume.)
These are related as follows:
where
is the initial amount of active substance — substance that has the same percentage of unstable particles as when the substance was formed.
[edit] Activity measurements
The units in which activities are measured are: becquerel (symbol Bq) = number of disintegrations per second; curie (Ci) = 3.7 × 1010 disintegrations per second. Low activities are also measured in disintegrations per minute (dpm).
[edit] Decay timing
See also: exponential decay
As discussed above, the decay of an unstable nucleus is entirely random and it is impossible to predict when a particular atom will decay. However, it is equally likely to decay at any time. Therefore, given a sample of a particular radioisotope, the number of decay events –dN expected to occur in a small interval of time dt is proportional to the number of atoms present. If N is the number of atoms, then the probability of decay (– dN/N) is proportional to dt:
Particular radionuclides decay at different rates, each having its own decay constant (λ). The negative sign indicates that N decreases with each decay event. The solution to this first-order differential equation is the following function:
This function represents exponential decay. It is only an approximate solution, for two reasons. Firstly, the exponential function is continuous, but the physical quantity N can only take non-negative integer values. Secondly, because it describes a random process, it is only statistically true. However, in most common cases, N is a very large number and the function is a good approximation.
In addition to the decay constant, radioactive decay is sometimes characterized by the mean lifetime. Each atom "lives" for a finite amount of time before it decays, and the mean lifetime is the arithmetic mean of all the atoms' lifetimes. It is represented by the symbol τ, and is related to the decay constant as follows:
A more commonly used parameter is the half-life. Given a sample of a particular radionuclide, the half-life is the time taken for half the radionuclide's atoms to decay. The half life is related to the decay constant as follows:
This relationship between the half-life and the decay constant shows that highly radioactive substances are quickly spent, while those that radiate weakly endure longer. Half-lives of known radionuclides vary widely, from more than 1019 years (such as for very nearly stable nuclides, e.g. 209Bi), to 10-23 seconds for highly unstable ones.
[edit] References
"Radioactivity", Encyclopædia Britannica. 2006. Encyclopædia Britannica Online. 18 Dec. 2006
[edit] See also
Nuclear pharmacy
Nuclear physics
Radioactivity in biology
Poisson process
Radiation
Radiation therapy
Radioactive contamination
Radiometric dating
Actinides in the environment
Half-life
Fallout shelter
Particle decay
[edit] External links
Look up radioactivity in
Wiktionary, the free dictionary.General information
General information, with emphasis on different modes
Some numerical calculations based on the Uranium-232 decay chain
Nomenclature of nuclear chemistry
Some theoretical questions of nuclear stability
Decay heat rate|quantity calculation
Specific activity and related topics.
The Lund/LBNL Nuclear Data Search - Contains tabulated information on radioactive decay types and energies.
Retrieved from "http://en.wikipedia.org/wiki/Radioactive_decay"
Categories: Exponentials | Radioactivity
From Wikipedia, the free encyclopedia
Jump to: navigation, search
"Radioactive" and "Radioactivity" redirect here. For other uses see Radioactive (disambiguation).
For decay rate in a more general context see Particle decay.
Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, called the daughter nuclide. For example: a carbon-14 atom (the "parent") emits radiation and transforms to a nitrogen-14 atom (the "daughter.") This is a random process on the atomic level, in that it is impossible to predict when a particular atom will decay, but given a large number of similar atoms, the decay rate, on average, is predictable.
The trefoil symbol is used to indicate radioactive material.The SI unit of radioactive decay is the becquerel (Bq). One Bq is defined as one transformation (or decay) per second. Since any reasonably-sized sample of radioactive material contains many atoms, a Bq is a tiny measure of activity; amounts on the order of TBq (terabecquerels) or GBq (gigabecquerels) are commonly used. Another unit of decay is the curie, which was originally defined as the radioactivity of one gram of pure radium, and is equal to 3.7 × 1010 Bq.
Contents [hide]
1 Explanation
2 Discovery
3 Modes of decay
4 Decay chains and multiple modes
5 Occurrence and applications
6 Radioactive decay rates
6.1 Activity measurements
7 Decay timing
8 References
9 See also
10 External links
[edit] Explanation
The neutrons and protons that constitute nuclei, as well as other particles that may approach them, are governed by several interactions. The strong nuclear force, not observed at the familiar macroscopic scale, is the most powerful force over subatomic distances. The electrostatic force is also significant. Of lesser importance is the weak nuclear force.
The interplay of these forces is simple. Some configurations of the particles in a nucleus have the property that, should they shift ever so slightly, the particles could fall into a lower-energy arrangement (with the extra energy moving elsewhere). One might draw an analogy with a snowfield on a mountain: while friction between the snow crystals can support the snow's weight, the system is inherently unstable with regards to a lower-potential-energy state, and a disturbance may facilitate the path to a greater entropy state (i.e., towards the ground state where heat will be produced, and thus total energy is distributed over a larger number of quantum states). Thus, an avalanche results. The total energy does not change in this process, but because of entropy effects, avalanches only happen in one direction, and the end of this direction, which is dictated by the largest number of chance-mediated ways to distribute available energy, is what we commonly refer to as the "ground state."
Such a collapse (a decay event) requires a specific activation energy. In the case of a snow avalanche, this energy classically comes as a disturbance from outside the system, although such disturbances can be arbitrarily small. In the case of an excited atomic nucleus, the arbitrarily small disturbance comes from quantum vacuum fluctuations. A nucleus (or any excited system in quantum mechanics) is unstable, and can thus spontaneously stabilize to a less-excited system. This process is driven by entropy considerations: the energy does not change, but at the end of the process, the total energy is more diffused in spacial volume. The resulting transformation alters the structure of the nucleus. Such a reaction is thus a nuclear reaction, in contrast to chemical reactions, which also are driven by entropy, but which involve changes in the arrangement of the outer electrons of atoms, rather than their nuclei.
Some nuclear reactions do involve external sources of energy, in the form of collisions with outside particles. However, these are not considered decay. Rather, they are examples of induced nuclear reactions. Nuclear fission and fusion are common types of induced nuclear reactions.
[edit] Discovery
Radioactivity was first discovered in 1896 by the French scientist Henri Becquerel while working on phosphorescent materials. These materials glow in the dark after exposure to light, and he thought that the glow produced in cathode ray tubes by X-rays might somehow be connected with phosphorescence. So he tried wrapping a photographic plate in black paper and placing various phosphorescent minerals on it. All results were negative until he tried using uranium salts. The result with these compounds was a deep blackening of the plate.
However, it soon became clear that the blackening of the plate had nothing to do with phosphorescence because the plate blackened when the mineral was kept in the dark. Also non-phosphorescent salts of uranium and even metallic uranium blackened the plate. Clearly there was some new form of radiation that could pass through paper that was causing the plate to blacken.
Alpha particles may be completely stopped by a sheet of paper, beta particles by aluminum shielding. Gamma rays, however, can only be reduced by much more substantial obstacles, such as a very thick piece of lead.At first it seemed that the new radiation was similar to the then recently discovered X-rays. However further research by Becquerel, Marie Curie, Pierre Curie, Ernest Rutherford and others discovered that radioactivity was significantly more complicated. Different types of decay can occur, but Rutherford was the first to realize that they all occur with the same mathematical approximately exponential formula (see below).
As for types of radioactive radiation, it was found that an electric or magnetic field could split such emissions into three types of beams. For lack of better terms, the rays were given the alphabetic names alpha, beta, and gamma, names they still hold today. It was immediately obvious from the direction of electromagnetic forces that alpha rays carried a positive charge, beta rays carried a negative charge, and gamma rays were neutral. From the magnitude of deflection, it was also clear that alpha particles were much more massive than beta particles. Passing alpha rays through a thin glass membrane and trapping them in a discharge tube allowed researchers to study the emission spectrum of the resulting gas, and ultimately prove that alpha particles are in fact helium nuclei. Other experiments showed the similarity between beta radiation and cathode rays; they are both streams of electrons, and between gamma radiation and X-rays, which are both high energy electromagnetic radiation.
Although alpha, beta, and gamma are most common, other types of decay were eventually discovered. Shortly after discovery of the neutron in 1932, it was discovered by Enrico Fermi that certain rare decay reactions give rise to neutrons as a decay particle. Isolated proton emission was also eventually observed in some elements. Shortly after the discovery of the positron in cosmic ray products, it was realized that the same process that operates in classical beta decay can also produce positrons (positron emission), analogously to negative electrons. Each of the two types of beta decay acts to move a nucleus toward a ratio of neutrons and protons which has the least energy for the combination. Finally, in a phenomenon called cluster decay, specific combinations of neutrons and protons other than alpha particles were found to occasionally spontaneously be emitted from atoms.
Still other types of radioactive decay were found which emit previously seen particles, but by different mechanisms. An example is internal conversion, which results in electron and sometimes high energy photon emission, even though it involves neither beta nor gamma decay.
The early researchers also discovered that many other chemical elements besides uranium have radioactive isotopes. A systematic search for the total radioactivity in uranium ores also guided Marie Curie to isolate a new element polonium and to separate a new element radium from barium; the two elements' chemical similarity would otherwise have made them difficult to distinguish.
The dangers of radioactivity and of radiation were not immediately recognized. Acute effects of radiation were first observed in the use of X-rays when the Serbo-Croatian-American electric engineer Nikola Tesla intentionally subjected his fingers to X-rays in 1896. He published his observations concerning the burns that developed, though he attributed them to ozone rather than to the X-rays. Fortunately his injuries healed later.
The genetic effects of radiation, including the effects on cancer risk, were recognized much later. It was only in 1927 that Hermann Joseph Muller published his research that showed the genetic effects. In 1946 he was awarded the Nobel prize for his findings.
Before the biological effects of radiation were known, many physicians and corporations had begun marketing radioactive substances as patent medicine and Radioactive quackery; particularly alarming examples were radium enema treatments, and radium-containing waters to be drunk as tonics. Marie Curie spoke out against this sort of treatment, warning that the effects of radiation on the human body were not well understood (Curie later died from aplastic anemia assumed due to her own work with radium, but later examination of her bones showed that she had been a careful laboratory worker and had a low burden of radium; a better candidate for her disease was her long exposure to unshielded X-ray tubes while a volunteer medical worker in WW I). By the 1930s, after a number of cases of bone-necrosis and death in enthusiasts, radium-containing medical products had nearly vanished from the market.
[edit] Modes of decay
Radionuclides can undergo a number of different reactions. These are summarized in the following table. A nucleus with positive charge (atomic number) Z and atomic weight A is represented as (A, Z).
Mode of decay Participating particles Daughter nucleus
Decays with emission of nucleons:
Alpha decay An alpha particle (A=4, Z=2) emitted from nucleus (A-4, Z-2)
Proton emission A proton ejected from nucleus (A-1, Z-1)
Neutron emission A neutron ejected from nucleus (A-1, Z)
Double proton emission Two protons ejected from nucleus simultaneously (A-2, Z-2)
Spontaneous fission Nucleus disintegrates into two or more smaller nuclei and other particles -
Cluster decay Nucleus emits a specific type of smaller nucleus (A1, Z1) larger than an alpha particle (A-A1, Z-Z1) + (A1,Z1)
Different modes of beta decay:
Beta-Negative decay A nucleus emits an electron and an antineutrino (A, Z+1)
Positron emission, also Beta-Positive decay A nucleus emits a positron and a neutrino (A, Z-1)
Electron capture A nucleus captures an orbiting electron and emits a neutrino - The daughter nucleus is left in an excited and unstable state (A, Z-1)
Double beta decay A nucleus emits two electrons and two antineutrinos (A, Z+2)
Double electron capture A nucleus absorbs two orbital electrons and emits two neutrinos - The daughter nucleus is left in an excited and unstable state (A, Z-2)
Electron capture with positron emission A nucleus absorbs one orbital electron, emits one positron and two neutrinos (A, Z-2)
Double positron emission A nucleus emits two positrons and two neutrinos (A, Z-2)
Transitions between states of the same nucleus:
Gamma decay Excited nucleus releases a high-energy photon (gamma ray) (A, Z)
Internal conversion Excited nucleus transfers energy to an orbital electron and it is ejected from the atom (A, Z)
Radioactive decay results in a reduction of summed rest mass, which is converted to energy (the disintegration energy) according to the formula E = mc2. This energy is released as kinetic energy of the emitted particles. The energy remains associated with a measure of mass of the decay system invariant mass, inasmuch the kinetic energy of emitted particles contributes also to the total invariant mass of systems. Thus, the sum of rest masses of particles is not conserved in decay, but the system mass or system invariant mass (as also system total energy) is conserved.
[edit] Decay chains and multiple modes
The daughter nuclide of a decay event is usually also unstable, sometimes even more unstable than the parent. If this is the case, it will proceed to decay again. A sequence of several decay events, producing in the end a stable nuclide, is a decay chain.
Many radionuclides have several different observed modes of decay. Bismuth-212, for example, has three. Thus a given nuclide may lead to several different decay chains.
Of the commonly occurring forms of radioactive decay, the only one that changes the number of aggregate protons and neutrons (nucleons) contained in the nucleus is alpha emission, which reduces it by four. Thus, the number of nucleons modulo 4 is preserved across any decay chain.
[edit] Occurrence and applications
According to the Big Bang theory, radioactive isotopes of the lightest elements (H, He, and traces of Li) were produced very shortly after the emergence of the universe. However, these nuclides are so highly unstable that virtually none of them have survived to today. Most radioactive nuclei are therefore relatively young, having formed in stars (particularly supernovae) and during ongoing interactions between stable isotopes and energetic particles. For example, carbon-14, a radioactive nuclide with a half-life of only 5730 years, is constantly produced in Earth's upper atmosphere due to interactions between cosmic rays and nitrogen.
Radioactive decay has been put to use in the technique of radioisotopic labeling, used to track the passage of a chemical substance through a complex system (such as a living organism). A sample of the substance is synthesized with a high concentration of unstable atoms. The presence of the substance in one or another part of the system is determined by detecting the locations of decay events.
On the premise that radioactive decay is truly random (rather than merely chaotic), it has been used in hardware random-number generators. Because the process is not thought to vary significantly in mechanism over time, it is also a valuable tool in estimating the absolute ages of certain materials. For geological materials, the radioisotopes and certain of their decay products become trapped when a rock solidifies, and can then later be used (subject to many well-known qualifications) to estimate the date of the solidification. These include checking the results of several simultaneous processes and their products against each other, within the same sample.
[edit] Radioactive decay rates
The decay rate, or activity, of a radioactive substance are characterized by:
Constant quantities:
half life — symbol t1 / 2 — the time for half of a substance to decay.
mean lifetime — symbol τ — the average lifetime of any given particle.
decay constant — symbol λ — the inverse of the mean lifetime.
(Note that although these are constants, they are associated with statistically random behavior of substances, and predictions using these constants are less accurate for small number of atoms.)
Time-variable quantities:
Total activity — symbol A — number of decays an object undergoes per second.
Number of particles — symbol N — the total number of particles in the sample.
Specific activity — symbol SA — number of decays per second per amount of substance. The "amount of substance" can be the unit of either mass or volume.)
These are related as follows:
where
is the initial amount of active substance — substance that has the same percentage of unstable particles as when the substance was formed.
[edit] Activity measurements
The units in which activities are measured are: becquerel (symbol Bq) = number of disintegrations per second; curie (Ci) = 3.7 × 1010 disintegrations per second. Low activities are also measured in disintegrations per minute (dpm).
[edit] Decay timing
See also: exponential decay
As discussed above, the decay of an unstable nucleus is entirely random and it is impossible to predict when a particular atom will decay. However, it is equally likely to decay at any time. Therefore, given a sample of a particular radioisotope, the number of decay events –dN expected to occur in a small interval of time dt is proportional to the number of atoms present. If N is the number of atoms, then the probability of decay (– dN/N) is proportional to dt:
Particular radionuclides decay at different rates, each having its own decay constant (λ). The negative sign indicates that N decreases with each decay event. The solution to this first-order differential equation is the following function:
This function represents exponential decay. It is only an approximate solution, for two reasons. Firstly, the exponential function is continuous, but the physical quantity N can only take non-negative integer values. Secondly, because it describes a random process, it is only statistically true. However, in most common cases, N is a very large number and the function is a good approximation.
In addition to the decay constant, radioactive decay is sometimes characterized by the mean lifetime. Each atom "lives" for a finite amount of time before it decays, and the mean lifetime is the arithmetic mean of all the atoms' lifetimes. It is represented by the symbol τ, and is related to the decay constant as follows:
A more commonly used parameter is the half-life. Given a sample of a particular radionuclide, the half-life is the time taken for half the radionuclide's atoms to decay. The half life is related to the decay constant as follows:
This relationship between the half-life and the decay constant shows that highly radioactive substances are quickly spent, while those that radiate weakly endure longer. Half-lives of known radionuclides vary widely, from more than 1019 years (such as for very nearly stable nuclides, e.g. 209Bi), to 10-23 seconds for highly unstable ones.
[edit] References
"Radioactivity", Encyclopædia Britannica. 2006. Encyclopædia Britannica Online. 18 Dec. 2006
[edit] See also
Nuclear pharmacy
Nuclear physics
Radioactivity in biology
Poisson process
Radiation
Radiation therapy
Radioactive contamination
Radiometric dating
Actinides in the environment
Half-life
Fallout shelter
Particle decay
[edit] External links
Look up radioactivity in
Wiktionary, the free dictionary.General information
General information, with emphasis on different modes
Some numerical calculations based on the Uranium-232 decay chain
Nomenclature of nuclear chemistry
Some theoretical questions of nuclear stability
Decay heat rate|quantity calculation
Specific activity and related topics.
The Lund/LBNL Nuclear Data Search - Contains tabulated information on radioactive decay types and energies.
Retrieved from "http://en.wikipedia.org/wiki/Radioactive_decay"
Categories: Exponentials | Radioactivity
Half Life Period
Half-life
From Wikipedia, the free encyclopedia
Jump to: navigation, search
This article is about the scientific and mathematical term. For other uses, see Half-life (disambiguation).
The half-life of a quantity, subject to exponential decay, is the time required for the quantity to decay to half of its initial value. The concept originated in the study of radioactive decay, but applies to many other fields as well, including phenomena which are described by non-exponential decays.
The term half-life was coined in 1907, but it was always referred to as half-life period. It was not until the early 1950s that the word period was dropped from the name. [1]
Number of
half-lives
elapsed Fraction
remaining As
power
of 2
0 1/1 1 / 20
1 1/2 1 / 21
2 1/4 1 / 22
3 1/8 1 / 23
4 1/16 1 / 24
5 1/32 1 / 25
6 1/64 1 / 26
7 1/128 1 / 27
... ... ...
N 1 / 2N 1 / 2N
The table at right shows the reduction of the quantity in terms of the number of half-lives elapsed.
It can be shown that, for exponential decay, the half-life t1 / 2 obeys this relation:
where
ln(2) is the natural logarithm of 2 (approximately 0.693), and
λ is the decay constant, a positive constant used to describe the rate of exponential decay.
The half-life is related to the mean lifetime τ by the following relation:
Contents [hide]
1 Examples
2 Decay by two or more processes
3 Derivation
4 Experimental determination
5 See also
6 References
7 External links
[edit] Examples
Main article: Exponential decay--Applications and examples
The constant λ can represent many different specific physical quantities, depending on what process is being described.
In an RC circuit or RL circuit, λ is the reciprocal of the circuit's time constant. For simple RC and RL circuits, λ equals 1 / RC or R / L, respectively.
In first-order chemical reactions, λ is the reaction rate constant.
In radioactive decay, it describes the probability of decay per unit time: dN = λNdt, where dN is the number of nuclei decayed during the time dt, and N is the quantity of radioactive nuclei.
In biology (specifically pharmacokinetics), from MeSH: Half-Life: The time it takes for a substance (drug, radioactive nuclide, or other) to lose half of its pharmacologic, physiologic, or radiologic activity. Year introduced: 1974 (1971).
[edit] Decay by two or more processes
Some quantities decay by two processes simultaneously (see Decay by two or more processes). In a fashion similar to the previous section, we can calculate the new total half-life T1 / 2 and we'll find it to be:
or, in terms of the two half-lives t1 and t2
i.e., half their harmonic mean.
[edit] Derivation
Quantities that are subject to exponential decay are commonly denoted by the symbol N. (This convention suggests a decaying number of discrete items. This interpretation is valid in many, but not all, cases of exponential decay.) If the quantity is denoted by the symbol N, the value of N at a time t is given by the formula:
where N0 is the initial value of N (at t = 0)
When t = 0, the exponential is equal to 1, and N(t) is equal to N0. As t approaches infinity, the exponential approaches zero. In particular, there is a time such that
Substituting into the formula above, we have
[edit] Experimental determination
The half-life of a process can be determined easily by experiment. In fact, some methods do not require advance knowledge of the law governing the decay rate, be it exponential decay or another pattern.
Most appropriate to validate the concept of half-life for radioactive decay, in particular when dealing with a small number of atoms, is to perform experiments and correct computer simulations. See in [1] how to test the behavior of the last atoms. Validation of physics-math models consists in comparing the model's behavior with experimental observations of real physical systems or valid simulations (physical and/or computer). The references given here describe how to test the validity of the exponential formula for small number of atoms with simple simulations, experiments, and computer code.
In radioactive decay, the exponential model does not apply for a small number of atoms (or a small number of atoms is not within the domain of validity of the formula or equation or table). The DIY experiments use pennies or M&M's candies. [2], [3]. A similar experiment is performed with isotopes of a very short half-life, for example, see Fig 5 in [4]. See how to write a computer program that simulates radioactive decay including the required randomness in [5] and experience the behavior of the last atoms. Of particular note, atoms undergo radioactive decay in whole units, and so after enough half-lives the remaining original quantity becomes an actual zero rather than asymptotically approaching zero as with continuous systems.
[edit] See also
Look up half-life in
Wiktionary, the free dictionary.Exponential decay
Mean lifetime
Elimination half-life
For non-exponential decays, see half-life in the article Rate equation
[edit] References
^ John Ayto "20th Century Words" (1999) Cambridge University Press.
[edit] External links
Time constant [6]
Retrieved from "http://en.wikipedia.org/wiki/Half-life"
Categories: Radioactivity | Exponentials | Chemical kinetics
From Wikipedia, the free encyclopedia
Jump to: navigation, search
This article is about the scientific and mathematical term. For other uses, see Half-life (disambiguation).
The half-life of a quantity, subject to exponential decay, is the time required for the quantity to decay to half of its initial value. The concept originated in the study of radioactive decay, but applies to many other fields as well, including phenomena which are described by non-exponential decays.
The term half-life was coined in 1907, but it was always referred to as half-life period. It was not until the early 1950s that the word period was dropped from the name. [1]
Number of
half-lives
elapsed Fraction
remaining As
power
of 2
0 1/1 1 / 20
1 1/2 1 / 21
2 1/4 1 / 22
3 1/8 1 / 23
4 1/16 1 / 24
5 1/32 1 / 25
6 1/64 1 / 26
7 1/128 1 / 27
... ... ...
N 1 / 2N 1 / 2N
The table at right shows the reduction of the quantity in terms of the number of half-lives elapsed.
It can be shown that, for exponential decay, the half-life t1 / 2 obeys this relation:
where
ln(2) is the natural logarithm of 2 (approximately 0.693), and
λ is the decay constant, a positive constant used to describe the rate of exponential decay.
The half-life is related to the mean lifetime τ by the following relation:
Contents [hide]
1 Examples
2 Decay by two or more processes
3 Derivation
4 Experimental determination
5 See also
6 References
7 External links
[edit] Examples
Main article: Exponential decay--Applications and examples
The constant λ can represent many different specific physical quantities, depending on what process is being described.
In an RC circuit or RL circuit, λ is the reciprocal of the circuit's time constant. For simple RC and RL circuits, λ equals 1 / RC or R / L, respectively.
In first-order chemical reactions, λ is the reaction rate constant.
In radioactive decay, it describes the probability of decay per unit time: dN = λNdt, where dN is the number of nuclei decayed during the time dt, and N is the quantity of radioactive nuclei.
In biology (specifically pharmacokinetics), from MeSH: Half-Life: The time it takes for a substance (drug, radioactive nuclide, or other) to lose half of its pharmacologic, physiologic, or radiologic activity. Year introduced: 1974 (1971).
[edit] Decay by two or more processes
Some quantities decay by two processes simultaneously (see Decay by two or more processes). In a fashion similar to the previous section, we can calculate the new total half-life T1 / 2 and we'll find it to be:
or, in terms of the two half-lives t1 and t2
i.e., half their harmonic mean.
[edit] Derivation
Quantities that are subject to exponential decay are commonly denoted by the symbol N. (This convention suggests a decaying number of discrete items. This interpretation is valid in many, but not all, cases of exponential decay.) If the quantity is denoted by the symbol N, the value of N at a time t is given by the formula:
where N0 is the initial value of N (at t = 0)
When t = 0, the exponential is equal to 1, and N(t) is equal to N0. As t approaches infinity, the exponential approaches zero. In particular, there is a time such that
Substituting into the formula above, we have
[edit] Experimental determination
The half-life of a process can be determined easily by experiment. In fact, some methods do not require advance knowledge of the law governing the decay rate, be it exponential decay or another pattern.
Most appropriate to validate the concept of half-life for radioactive decay, in particular when dealing with a small number of atoms, is to perform experiments and correct computer simulations. See in [1] how to test the behavior of the last atoms. Validation of physics-math models consists in comparing the model's behavior with experimental observations of real physical systems or valid simulations (physical and/or computer). The references given here describe how to test the validity of the exponential formula for small number of atoms with simple simulations, experiments, and computer code.
In radioactive decay, the exponential model does not apply for a small number of atoms (or a small number of atoms is not within the domain of validity of the formula or equation or table). The DIY experiments use pennies or M&M's candies. [2], [3]. A similar experiment is performed with isotopes of a very short half-life, for example, see Fig 5 in [4]. See how to write a computer program that simulates radioactive decay including the required randomness in [5] and experience the behavior of the last atoms. Of particular note, atoms undergo radioactive decay in whole units, and so after enough half-lives the remaining original quantity becomes an actual zero rather than asymptotically approaching zero as with continuous systems.
[edit] See also
Look up half-life in
Wiktionary, the free dictionary.Exponential decay
Mean lifetime
Elimination half-life
For non-exponential decays, see half-life in the article Rate equation
[edit] References
^ John Ayto "20th Century Words" (1999) Cambridge University Press.
[edit] External links
Time constant [6]
Retrieved from "http://en.wikipedia.org/wiki/Half-life"
Categories: Radioactivity | Exponentials | Chemical kinetics
Natural Abundance
Natural abundance
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In chemistry, natural abundance (NA) refers to the prevalence of isotopes of a chemical element as naturally found on a planet. The relative atomic mass (a weighted average) of these isotopes is the atomic weight listed for the element in the periodic table. The abundance of an isotope varies from planet to planet but remains relatively constant in time.
As an example, uranium has three naturally occurring isotopes: U-238, U-235 and U-234. Their respective NA is 99.2745%, 0.72% and 0.0055%. For example, if 100,000 uranium atoms were analyzed, one would expect to find approximately 99,275 U-238 atoms, 720 U-235 atoms, and no more than 5 or 6 U-234 atoms. This is because U-238 is much more stable than U-235 or U-234, as the half-life of each isotope reveals: 4.468×109 years for U-238 compared to 7.038×108 years for U-235 and 245,500 years for U-234.
[edit] See also
Abundance of the chemical elements
This chemistry article is a stub. You can help Wikipedia by expanding it.
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From Wikipedia, the free encyclopedia
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This article or section is in need of attention from an expert on the subject.
WikiProject Chemistry or the Chemistry Portal may be able to help recruit one.
If a more appropriate WikiProject or portal exists, please adjust this template accordingly.
In chemistry, natural abundance (NA) refers to the prevalence of isotopes of a chemical element as naturally found on a planet. The relative atomic mass (a weighted average) of these isotopes is the atomic weight listed for the element in the periodic table. The abundance of an isotope varies from planet to planet but remains relatively constant in time.
As an example, uranium has three naturally occurring isotopes: U-238, U-235 and U-234. Their respective NA is 99.2745%, 0.72% and 0.0055%. For example, if 100,000 uranium atoms were analyzed, one would expect to find approximately 99,275 U-238 atoms, 720 U-235 atoms, and no more than 5 or 6 U-234 atoms. This is because U-238 is much more stable than U-235 or U-234, as the half-life of each isotope reveals: 4.468×109 years for U-238 compared to 7.038×108 years for U-235 and 245,500 years for U-234.
[edit] See also
Abundance of the chemical elements
This chemistry article is a stub. You can help Wikipedia by expanding it.
This physics-related article is a stub. You can help Wikipedia by expanding it.
Retrieved from "http://en.wikipedia.org/wiki/Natural_abundance"
Categories: Chemistry articles needing expert attention | Articles needing expert attention | Chemical properties | Chemistry stubs | Physics stubs
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This page was last modified 16:16, 5 July 2007. All text is available under the terms of the GNU Free Documentation License. (See Copyrights for details.)
Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a US-registered 501(c)(3) tax-deductible nonprofit charity.
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