Carbon-14

Carbon-14, (14C), or radiocarbon, is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and colleagues (1949) to date archaeological, geological and hydrogeological samples. Carbon-14 was discovered on February 27, 1940, by Martin Kamen and Sam Ruben at the University of California Radiation Laboratory in Berkeley, California. Its existence had been suggested by Franz Kurie in 1934.[2]

There are three naturally occurring isotopes of carbon on Earth: carbon-12, which makes up 99% of all carbon on Earth; carbon-13, which makes up 1%; and carbon-14, which occurs in trace amounts, making up about 1 or 1.5 atoms per 1012 atoms of carbon in the atmosphere. Carbon-12 and carbon-13 are both stable, while carbon-14 is unstable and has a half-life of 5,730±40 years.[3] Carbon-14 decays into nitrogen-14 through beta decay.[4] A gram of carbon containing 1 atom of carbon-14 per 1012 atoms will emit ~0.2[5] beta particles per second. The primary natural source of carbon-14 on Earth is cosmic ray action on nitrogen in the atmosphere, and it is therefore a cosmogenic nuclide. However, open-air nuclear testing between 1955–1980 contributed to this pool.

The different isotopes of carbon do not differ appreciably in their chemical properties. This resemblance is used in chemical and biological research, in a technique called carbon labeling: carbon-14 atoms can be used to replace nonradioactive carbon, in order to trace chemical and biochemical reactions involving carbon atoms from any given organic compound.

Carbon-14,  14C
General
Name, symbolradiocarbon,14C
Neutrons8
Protons6
Nuclide data
Natural abundance1 part per trillion
Half-life5,730 ± 40 years
Isotope mass14.003241 u
Spin0+
Decay modes
Decay modeDecay energy (MeV)
Beta0.156476[1]
Complete table of nuclides

Radioactive decay and detection

Carbon-14 goes through radioactive beta decay:

14
6
C
14
7
N
+
e
+
ν
e

By emitting an electron and an electron antineutrino, one of the neutrons in the carbon-14 atom decays to a proton and the carbon-14 (half-life of 5700 ± 40 years[6]) decays into the stable (non-radioactive) isotope nitrogen-14.

The emitted beta particles have a maximum energy of 156 keV, while their weighted mean energy is 49 keV.[6] These are relatively low energies; the maximum distance traveled is estimated to be 22 cm in air and 0.27 mm in body tissue. The fraction of the radiation transmitted through the dead skin layer is estimated to be 0.11. Small amounts of carbon-14 are not easily detected by typical Geiger–Müller (G-M) detectors; it is estimated that G-M detectors will not normally detect contamination of less than about 100,000 disintegrations per minute (0.05 µCi). Liquid scintillation counting is the preferred method.[7] The G-M counting efficiency is estimated to be 3%. The half-distance layer in water is 0.05 mm.[8]

Radiocarbon dating

Radiocarbon dating is a radiometric dating method that uses (14C) to determine the age of carbonaceous materials up to about 60,000 years old. The technique was developed by Willard Libby and his colleagues in 1949[9] during his tenure as a professor at the University of Chicago. Libby estimated that the radioactivity of exchangeable carbon-14 would be about 14 disintegrations per minute (dpm) per gram of pure carbon, and this is still used as the activity of the modern radiocarbon standard.[10][11] In 1960, Libby was awarded the Nobel Prize in chemistry for this work.

One of the frequent uses of the technique is to date organic remains from archaeological sites. Plants fix atmospheric carbon during photosynthesis, so the level of 14C in plants and animals when they die approximately equals the level of 14C in the atmosphere at that time. However, it decreases thereafter from radioactive decay, allowing the date of death or fixation to be estimated. The initial 14C level for the calculation can either be estimated, or else directly compared with known year-by-year data from tree-ring data (dendrochronology) up to 10,000 years ago (using overlapping data from live and dead trees in a given area), or else from cave deposits (speleothems), back to about 45,000 years before the present. A calculation or (more accurately) a direct comparison of carbon-14 levels in a sample, with tree ring or cave-deposit carbon-14 levels of a known age, then gives the wood or animal sample age-since-formation.

Origin

Natural production in the atmosphere

Carbon 14 formation and decay
1: Formation of carbon-14
2: Decay of carbon-14
3: The "equal" equation is for living organisms, and the unequal one is for dead organisms, in which the C-14 then decays (See 2).

Carbon-14 is produced in the upper layers of the troposphere and the stratosphere by thermal neutrons absorbed by nitrogen atoms. When cosmic rays enter the atmosphere, they undergo various transformations, including the production of neutrons. The resulting neutrons (1n) participate in the following reaction:

n + 14
7
N
14
6
C
+ p

The highest rate of carbon-14 production takes place at altitudes of 9 to 15 km (30,000 to 49,000 ft) and at high geomagnetic latitudes.

The rate of 14C production can be modelled, yielding values of 16,400[12] or 18,800[13] atoms of 14C per second per square meter of the Earth's surface, which agrees with the global carbon budget that can be used to backtrack,[14] but attempts to directly measure the production rate in situ were not very successful. Production rates vary because of changes to the cosmic ray flux caused by the heliospheric modulation (solar wind and solar magnetic field), and due to variations in the Earth's magnetic field. The latter can create significant variations in 14C production rates, although the changes of the carbon cycle can make these effects difficult to tease out.[14][15] Occasional spikes may occur; for example, there is evidence for an unusually strong increase of the production rate in AD 774–775,[16] caused by an extreme solar energetic particle event, strongest for the last ten millennia.[17][18] Another "extraordinarily large" 14C increase (20‰) has been recently (2017) associated with the 5480 BC event, which is however unlikely to be a solar energetic particle event.[19]

Carbon-14 may also be produced by lightning bolts [20][21] but in the amounts negligible compared to cosmic rays.

Other carbon-14 sources

Carbon-14 can also be produced by other neutron reactions, including in particular 13C(n,γ)14C and 17O(n,α)14C with thermal neutrons, and 15N(n,d)14C and 16O(n,3He)14C with fast neutrons.[22] The most notable routes for 14C production by thermal neutron irradiation of targets (e.g., in a nuclear reactor) are summarized in the table.

Carbon-14 may also be radiogenic (cluster decay of 223Ra, 224Ra, 226Ra). However, this origin is extremely rare.

14C production routes[23]
Parent isotope Natural abundance, % Cross section for thermal neutron capture, b Reaction
14N 99.634 1.81 14N(n,p)14C
13C 1.103 0.0009 13C(n,γ)14C
17O 0.0383 0.235 17O(n,α)14C

Formation during nuclear tests

Radiocarbon bomb spike
Atmospheric 14C, New Zealand[24] and Austria.[25] The New Zealand curve is representative for the Southern Hemisphere, the Austrian curve is representative for the Northern Hemisphere. Atmospheric nuclear weapon tests almost doubled the concentration of 14C in the Northern Hemisphere.[26]

The above-ground nuclear tests that occurred in several countries between 1955 and 1980 (see nuclear test list) dramatically increased the amount of carbon-14 in the atmosphere and subsequently in the biosphere; after the tests ended, the atmospheric concentration of the isotope began to decrease.

One side-effect of the change in atmospheric carbon-14 is that this has enabled some options (e.g., bomb-pulse dating[27]) for determining the birth year of an individual, in particular, the amount of carbon-14 in tooth enamel,[28][29] or the carbon-14 concentration in the lens of the eye.[30]

Emissions from nuclear power plants

Carbon-14 is produced in coolant at boiling water reactors (BWRs) and pressurized water reactors (PWRs). It is typically released to the atmosphere in the form of carbon dioxide at BWRs, and methane at PWRs.[31] Best practice for nuclear power plant operator management of carbon-14 includes releasing it at night, when plants are not photosynthesizing.[32]

Occurrence

Dispersion in the environment

After production in the upper atmosphere, the carbon-14 atoms react rapidly to form mostly (about 93%) 14CO (carbon monoxide), which subsequently oxidizes at a slower rate to form 14CO2, radioactive carbon dioxide. The gas mixes rapidly and becomes evenly distributed throughout the atmosphere (the mixing timescale in the order of weeks). Carbon dioxide also dissolves in water and thus permeates the oceans, but at a slower rate.[15] The atmospheric half-life for removal of 14CO2 has been estimated to be roughly 12 to 16 years in the northern hemisphere. The transfer between the ocean shallow layer and the large reservoir of bicarbonates in the ocean depths occurs at a limited rate.[23] In 2009 the activity of 14C was 238 Bq per kg carbon of fresh terrestrial biomatter, close to the values before atmospheric nuclear testing (226 Bq/kg C; 1950).[33]

Total inventory

The inventory of carbon-14 in Earth's biosphere is about 300 megacuries (11 EBq), of which most is in the oceans.[34] The following inventory of carbon-14 has been given:[35]

  • Global inventory: ~8500 PBq (about 50 t)
    • Atmosphere: 140 PBq (840 kg)
    • Terrestrial materials: the balance
  • From nuclear testing (till 1990): 220 PBq (1.3 t)

In fossil fuels

Many man-made chemicals are derived from fossil fuels (such as petroleum or coal) in which 14C is greatly depleted. 14CO2--or rather, its relative absence—is therefore used to determine the relative contribution (or mixing ratio) of fossil fuel oxidation to the total carbon dioxide in a given region of the Earth's atmosphere.[36]

Dating a specific sample of fossilized carbonaceous material is more complicated. Such deposits often contain trace amounts of carbon-14. These amounts can vary significantly between samples, ranging up to 1% of the ratio found in living organisms, a concentration comparable to an apparent age of 40,000.[37] This may indicate possible contamination by small amounts of bacteria, underground sources of radiation causing the 14N(n,p) 14C reaction, direct uranium decay (although reported measured ratios of 14C/U in uranium-bearing ores[38] would imply roughly 1 uranium atom for every two carbon atoms in order to cause the 14C/12C ratio, measured to be on the order of 10−15), or other unknown secondary sources of carbon-14 production. The presence of carbon-14 in the isotopic signature of a sample of carbonaceous material possibly indicates its contamination by biogenic sources or the decay of radioactive material in surrounding geologic strata. In connection with building the Borexino solar neutrino observatory, petroleum feedstock (for synthesizing the primary scintillant) was obtained with low 14C content. In the Borexino Counting Test Facility, a 14C/12C ratio of 1.94×10−18 was determined;[39] probable reactions responsible for varied levels of 14C in different petroleum reservoirs, and the lower 14C levels in methane, have been discussed by Bonvicini et al.[40]

In the human body

Since many sources of human food are ultimately derived from terrestrial plants, the carbon that comprises our bodies contains carbon-14 at almost the same concentration as the atmosphere. The rates of disintegration of potassium-40 and carbon-14 in the normal adult body are comparable (a few thousand disintegrated nuclei per second).[41] The beta-decays from external (environmental) radiocarbon contribute approximately 0.01 mSv/year (1 mrem/year) to each person's dose of ionizing radiation.[42] This is small compared to the doses from potassium-40 (0.39 mSv/year) and radon (variable).

Carbon-14 can be used as a radioactive tracer in medicine. In the initial variant of the urea breath test, a diagnostic test for Helicobacter pylori, urea labeled with approximately 37 kBq (1.0 μCi) carbon-14 is fed to a patient (i.e., 37,000 decays per second). In the event of a H. pylori infection, the bacterial urease enzyme breaks down the urea into ammonia and radioactively-labeled carbon dioxide, which can be detected by low-level counting of the patient's breath.[43] The 14C urea breath test has been largely replaced by the 13C urea breath test, which has no radiation issues.

See also

References

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Further reading

  • Kamen, Martin D. (1985). Radiant Science, Dark Politics: A Memoir of the Nuclear Age. Berkeley: University of California Press. ISBN 978-0-520-04929-1.

External links

Lighter:
carbon-13
Carbon-14 is an
isotope of carbon
Heavier:
carbon-15
Decay product of:
boron-14, nitrogen-18
Decay chain
of carbon-14
Decays to:
nitrogen-14
(n-p) reaction

The (n-p) reaction is an example of a nuclear reaction. It is the reaction which occurs when a neutron enters a nucleus and a proton leaves the nucleus simultaneously.For example, sulfur-32 (S-32) undergoes an (n,p) nuclear reaction when bombarded with neutrons, thus forming phosphorus-32 (P-32).

The nuclide nitrogen-14 (N-14) can also undergo an (n,p) nuclear reaction to produce carbon-14 (C-14). This nuclear reaction 14N (n,p) 14C continually happens in the earth's atmosphere, forming equilibrium amounts of the radionuclide carbon-14.

Most (n,p) reactions have threshold neutron energies below which the reaction cannot take place as a result of the charged particle in the exit channel requiring energy (usually more than a MeV) to overcome the Coulomb barrier experienced by the emitted proton. The (n,p) nuclear reaction 14N (n,p) 14C is an exception to this rule, and is exothermic - it can take place at all incident neutron energies. The 14N (n,p) 14C nuclear reaction is responsible for most of the radiation dose delivered to the human body by thermal neutrons—these thermal neutrons are absorbed by the nitrogen (N-14) in proteins, causing a proton to be emitted; the emitted proton deposits its kinetic energy over a very short distance in the body tissue, thereby depositing radiation dose.

774–775 carbon-14 spike

The 774–775 carbon-14 spike is an observed increase of 1.2% in the concentration of carbon-14 isotope in tree rings dated to the years 774 or 775 AD, which is about 20 times as high as the normal background rate of variation. It was discovered during a study of Japanese cedar trees, with the year of occurrence determined through dendrochronology. A surge in beryllium isotope 10Be, detected in Antarctic ice cores, has also been associated with the 774–775 event.The event appears to have been global, with the same carbon-14 signal found in tree rings from Germany, Russia, the United States, and New Zealand.

The signal exhibits a sharp increase of ≈1.2% followed by a slow decline (see Figure 1), which is typical for an instant production of carbon-14 in the atmosphere, indicating that the event was short in duration. The globally averaged production of carbon-14 for this event is calculated as Q = (1.1 – 1.5) × 108 atoms/cm2.

993–994 carbon-14 spike

The 993–994 carbon-14 spike was a rapid increase in carbon-14 content from tree rings, and followed the 774–775 carbon-14 spike. This event is also confirmed by a sharp increase of beryllium-10 and hence considered as solar-origin. It may have come from a massive solar storm as a series of auroral observations are known to be observed in late 992.

Absolute dating

Absolute dating is the process of determining an age on a specified chronology in archaeology and geology. Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty of accuracy. Absolute dating provides a numerical age or range in contrast with relative dating which places events in order without any measure of the age between events.

In archaeology, absolute dating is usually based on the physical, chemical, and life properties of the materials of artifacts, buildings, or other items that have been modified by humans and by historical associations with materials with known dates (coins and written history). Techniques include tree rings in timbers, radiocarbon dating of wood or bones, and trapped-charge dating methods such as thermoluminescence dating of glazed ceramics. Coins found in excavations may have their production date written on them, or there may be written records describing the coin and when it was used, allowing the site to be associated with a particular calendar year.

In historical geology, the primary methods of absolute dating involve using the radioactive decay of elements trapped in rocks or minerals, including isotope systems from very young (radiocarbon dating with 14C) to systems such as uranium–lead dating that allow acquisition of absolute ages for some of the oldest rocks on earth.

Ard Tlaili

Ard Tlaili or Tell Ard Tlaili is a small tell mound archaeological site in a plain at the foot of the Lebanon Mountains 11 km (7 mi) northwest of Baalbeck, in the Beqaa Valley in Lebanon.IIt was first surveyed and studied in 1965–66 by Lorraine Copeland and Peter Wescombe with excavations later in the 1960s by Diana Kirkbride. The perimeter of the mound was buried under a metre of soil but the remains of rectangular buildings were found in 2 phases. Building walls were of wall made of stiff earth or clay with pebble bases and large stones in the upper layers. The floors were layered with white plaster with plastered and even burnished walls. Hearths and other areas were constructed of plaster or clay.

The wide variety of materials recovered included a stone assemblage of tools, obsidian blades, basalt bowls and hammers, clay sling ammunition, finely denticulated flint blades, scrapers, borers and a few axes. Pottery included Halafian painted shards both pattern and plain burnished with incised decoration including horizontal or vertical lines with dots, waves, zig-zags and cross-hatched designs. some with an application of red wash. These finds were significant as they represented the most southerly Halaf type painted pottery yet found. Red, orange, brown and black burnished bowls and jars were found in upper levels, with lower levels showing more coarse shards smoothed by hand or with straw. This little farming village shares the material culture of Byblos and southern Syrian and Halaf sites to the north.The carbon 14 dating of charcoal from the different levels at Ard Tlaili gave a short date range between c. 5710 until c.5780 BC.

Before Present

Before Present (BP) years is a time scale used mainly in geology and other scientific disciplines to specify when events occurred in the past. Because the "present" time changes, standard practice is to use 1 January 1950 as the commencement date of the age scale, reflecting the origin of practical radiocarbon dating in the 1950s. The abbreviation "BP" has alternatively been interpreted as "Before Physics"; that is, before nuclear weapons testing artificially altered the proportion of the carbon isotopes in the atmosphere, making dating after that time likely to be unreliable.

Geochemical Ocean Sections Study

The Geochemical Ocean Sections Study (GEOSECS) was a global survey of the three-dimensional distributions of chemical, isotopic, and radiochemical tracers in the ocean. A key objective was to investigate the deep thermohaline circulation of the ocean, using chemical tracers, including radiotracers, to establish the pathways taken by this.Expeditions undertaken during GEOSECS took place in the Atlantic Ocean from July 1972 to May 1973, in the Pacific Ocean from August 1973 to June 1974, and in the Indian Ocean from December 1977 to March 1978.Measurements included those of physical oceanographic quantities such as temperature, salinity, pressure and density, chemical / biological quantities such as total inorganic carbon, alkalinity, nitrate, phosphate, silicic acid, oxygen and apparent oxygen utilisation (AOU), and radiochemical / isotopic quantities such as carbon-13, carbon-14 and tritium.

Grotte de Cussac

The Grotte de Cussac is a cave containing over 150 Paleolithic artworks as well as several human remains. It is located in the Dordogne River valley in Le Buisson-de-Cadouin, Dordogne, Aquitaine, France.

The cave was discovered on September 30, 2000, by amateur speleologist Marc Delluc and formally announced by the French Ministry of Culture on December 8, 2000. It is currently under protection for scientific study, and closed to the public.

The cave's artworks are estimated to be 25,000 years old, and are almost exclusively engravings, often very large, made with stone tools on the walls, or with fingers on clay soil. Pigments are limited to very few red dots. They include both classic instances of Upper Paleolithic animal art (bison, horses, mammoths, rhinoceroses, ibex) and rarer images including birds, enigmatic figures, and perhaps four female profiles. All appear close in theme and style of those known to Gravettien in the Quercy caves, in particular Pech Merle.

The cave's human remains appear to represent one of very few associations of parietal works and human burials in Paleolithic Europe. At least five people, four adults and a teenager, were deposited in the cavities, with bones dated by Carbon 14 measurements to approximately 25,000 years in age.

Grotto of the Gentio

Cave of Gentio is a parietal art-bearing archaeological site situated about 30 km from Unaí, Minas Gerais, 180 km away from the Federal District.

Its importance was verified by UFMG archaeologists who began their exploration in the 70's. In one of the stages of the excavations, the body of a naturally mummified child was found, classified as the oldest ever found in Brazil.

The cave is located in a limestone massif. During the surveys conducted in this area (1973) some technical procedures were taken in order to reveal the evidence of different occupations, which has been confirmed by carbon-14 dating obtained for the older occupational layers, and also because the high degree of preservation of the archaeological material found there, which has the occurrence of pictographs and petroglyphs.

The oldest layers from the site, which yielded lithic, osteological and wooden artefacts associated with hunter-gatherers, is dated to around 8,125 BP. A later occupational layer ranged from approximately 3,490 to 340 BP. This later layer yielded wooden art artefacts associated with a ceramic-agriculturalist culture. 36 human coprolites were recovered from this layer. The coprolites yielded evidence of helminth eggs from hookworms and Trichuris trichiura. This layer also yielded evidence for the presence of peanuts, maize, squash and gourd. These domesticated plants were present at the site by around 1,900 BC.

Hypolith

In Arctic and Antarctic ecology, a hypolith is a photosynthetic organism, and an extremophile, that lives

underneath rocks in climatically extreme deserts such as Cornwallis Island and Devon Island in the Canadian high Arctic. The community itself is the hypolithon.

Hypolithons are protected by their rock from harsh ultraviolet radiation and wind scouring. The rocks can also trap moisture and are generally translucent allowing light to penetrate. Writing in Nature, ecologist Charles S. Cockell of the British Antarctic Survey and Dale Stokes (Scripps Institution of Oceanography) describe how hypoliths reported to date (until 2004) had been found under quartz, which is one of the most common translucent minerals.However, Cockell reported that on Cornwallis Island and Devon Island, 94-95% of a random sample of 850 opaque dolomitic rocks were colonized by hypoliths, and found that the communities were dominated by cyanobacteria. The rocks chosen were visually indistinguishable from those nearby, and were about 10 cm across; the hypolithon was visible as a greenish coloured band. Cockell proposed that rock sorting by periglacial action, including that during freeze–thaw cycles, improves light penetration around the edges of rocks (see granular material and Brazil nut effect).

Cockell and Stokes went on to estimate the productivity of the Arctic communities by monitoring the uptake of sodium bicarbonate labelled with Carbon-14 and found that (for Devon Island) productivity of the hypolithon was comparable to that of plants, lichens, and bryophytes combined (0.8 ± 0.3 g m−2 y−1 and 1 ± 0.4 g m−2 y−1 respectively) and concluded that the polar hypolithon may double previous estimates of the productivity of that region of the rocky polar desert.

Isotope

Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom.The term isotope is formed from the Greek roots isos (ἴσος "equal") and topos (τόπος "place"), meaning "the same place"; thus, the meaning behind the name is that different isotopes of a single element occupy the same position on the periodic table. It was coined by a Scottish doctor and writer Margaret Todd in 1913 in a suggestion to chemist Frederick Soddy.

The number of protons within the atom's nucleus is called atomic number and is equal to the number of electrons in the neutral (non-ionized) atom. Each atomic number identifies a specific element, but not the isotope; an atom of a given element may have a wide range in its number of neutrons. The number of nucleons (both protons and neutrons) in the nucleus is the atom's mass number, and each isotope of a given element has a different mass number.

For example, carbon-12, carbon-13 and carbon-14 are three isotopes of the element carbon with mass numbers 12, 13 and 14 respectively. The atomic number of carbon is 6, which means that every carbon atom has 6 protons, so that the neutron numbers of these isotopes are 6, 7 and 8 respectively.

Karbon (software)

Karbon (formerly Karbon14, Kontour, and KIllustrator) is a vector graphics editor. It is a component of Calligra Suite, an integrated graphic art and office suite by KDE. The name is a play on KDE and the radioactive isotope Carbon-14.

On 26 February 2012 the lead developer clarified on the Calligra development mailing list that the application’s name is simply "Karbon" without "14".

Martin Kamen

Martin David Kamen (August 27, 1913, Toronto – August 31, 2002) was a chemist briefly involved with the Manhattan project. Together with Sam Ruben, he co-discovered the synthesis of the isotope carbon-14 on February 27, 1940, at the University of California Radiation Laboratory, Berkeley.

Pengtoushan

The Pengtoushan culture, dating 7500–6100 BC, was a Neolithic culture centered primarily around the central Yangtze River region in northwestern Hunan, China. It was roughly contemporaneous with its northern neighbor, the Peiligang culture. The two primary examples of Pengtoushan culture are the type site at Pengtoushan and the later site at Bashidang.

The type site at Pengtoushan was discovered in Li County, Hunan. This site is the earliest permanently settled village yet discovered in China. Excavated in 1988, Pengtoushan has been difficult to date accurately, with a large variability in dates ranging from 9000 BC to 5500 BC. Cord-marked pottery was discovered among the burial goods.

Analysis of Chinese rice residues which were Carbon-14 dated to 8200–7800 BC show that rice had been domesticated by this time. The size of the Pengtoushan rice was larger than the size of naturally occurring wild rice; however, Pengtoushan lacked evidence of tools used in cultivating rice. Although not found at Pengtoushan, rice-cultivating tools were found in later sites associated with the Pengtoushan culture.

Radioactive decay

Radioactive decay (also known as nuclear decay, radioactivity or nuclear radiation) is the process by which an unstable atomic nucleus loses energy (in terms of mass in its rest frame) by emitting radiation, such as an alpha particle, beta particle with neutrino or only a neutrino in the case of electron capture, or a gamma ray or electron in the case of internal conversion. A material containing such unstable nuclei is considered radioactive. Certain highly excited short-lived nuclear states can decay through neutron emission, or more rarely, proton emission.

Radioactive decay is a stochastic (i.e. random) process at the level of single atoms. According to quantum theory, it is impossible to predict when a particular atom will decay, regardless of how long the atom has existed. However, for a collection of atoms, the collection's expected decay rate is characterized in terms of their measured decay constants or half-lives. This is the basis of radiometric dating. The half-lives of radioactive atoms have no known upper limit, spanning a time range of over 55 orders of magnitude, from nearly instantaneous to far longer than the age of the universe.

A radioactive nucleus with zero spin can have no defined orientation, and hence emits the total momentum of its decay products isotropically (all directions and without bias). If there are multiple particles produced during a single decay, as in beta decay, their relative angular distribution, or spin directions may not be isotropic. Decay products from a nucleus with spin may be distributed non-isotropically with respect to that spin direction, either because of an external influence such as an electromagnetic field, or because the nucleus was produced in a dynamic process that constrained the direction of its spin. Such a parent process could be a previous decay, or a nuclear reaction.The decaying nucleus is called the parent radionuclide (or parent radioisotope), and the process produces at least one daughter nuclide. Except for gamma decay or internal conversion from a nuclear excited state, the decay is a nuclear transmutation resulting in a daughter containing a different number of protons or neutrons (or both). When the number of protons changes, an atom of a different chemical element is created.

The first decay processes to be discovered were alpha decay, beta decay, and gamma decay. Alpha decay occurs when the nucleus ejects an alpha particle (helium nucleus). This is the most common process of emitting nucleons, but highly excited nuclei can eject single nucleons, or in the case of cluster decay, specific light nuclei of other elements. Beta decay occurs in two ways:

(i) beta-minus decay, when the nucleus emits an electron and an antineutrino in a process that changes a neutron to a proton, or

(ii) beta-plus decay, when the nucleus emits a positron and a neutrino in a process that changes a proton to a neutron.

Highly excited neutron-rich nuclei, formed as the product of other types of decay, occasionally lose energy by way of neutron emission, resulting in a change from one isotope to another of the same element. The nucleus may capture an orbiting electron, causing a proton to convert into a neutron in a process called electron capture. All of these processes result in a well-defined nuclear transmutation.

By contrast, there are radioactive decay processes that do not result in a nuclear transmutation. The energy of an excited nucleus may be emitted as a gamma ray in a process called gamma decay, or that energy may be lost when the nucleus interacts with an orbital electron causing its ejection from the atom, in a process called internal conversion.

Another type of radioactive decay results in products that vary, appearing as two or more "fragments" of the original nucleus with a range of possible masses. This decay, called spontaneous fission, happens when a large unstable nucleus spontaneously splits into two (or occasionally three) smaller daughter nuclei, and generally leads to the emission of gamma rays, neutrons, or other particles from those products.

For a summary table showing the number of stable and radioactive nuclides in each category, see radionuclide. There are 28 naturally occurring chemical elements on Earth that are radioactive, consisting of 33 radionuclides (5 elements have 2 different radionuclides) that date before the time of formation of the solar system. These 33 are known as primordial nuclides. Well-known examples are uranium and thorium, but also included are naturally occurring long-lived radioisotopes, such as potassium-40. Another 50 or so shorter-lived radionuclides, such as radium and radon, found on Earth, are the products of decay chains that began with the primordial nuclides, or are the product of ongoing cosmogenic processes, such as the production of carbon-14 from nitrogen-14 in the atmosphere by cosmic rays. Radionuclides may also be produced artificially in particle accelerators or nuclear reactors, resulting in 650 of these with half-lives of over an hour, and several thousand more with even shorter half-lives. (See List of nuclides for a list of these sorted by half-life.)

Radiocarbon dating

Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon.

The method was developed in the late 1940s by Willard Libby, who received the Nobel Prize in Chemistry for his work in 1960. It is based on the fact that radiocarbon (14C) is constantly being created in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen. The resulting 14C combines with atmospheric oxygen to form radioactive carbon dioxide, which is incorporated into plants by photosynthesis; animals then acquire 14C by eating the plants. When the animal or plant dies, it stops exchanging carbon with its environment, and from that point onwards the amount of 14C it contains begins to decrease as the 14C undergoes radioactive decay. Measuring the amount of 14C in a sample from a dead plant or animal such as a piece of wood or a fragment of bone provides information that can be used to calculate when the animal or plant died. The older a sample is, the less 14C there is to be detected, and because the half-life of 14C (the period of time after which half of a given sample will have decayed) is about 5,730 years, the oldest dates that can be reliably measured by this process date to around 50,000 years ago, although special preparation methods occasionally permit accurate analysis of older samples.

Research has been ongoing since the 1960s to determine what the proportion of 14C in the atmosphere has been over the past fifty thousand years. The resulting data, in the form of a calibration curve, is now used to convert a given measurement of radiocarbon in a sample into an estimate of the sample's calendar age. Other corrections must be made to account for the proportion of 14C in different types of organisms (fractionation), and the varying levels of 14C throughout the biosphere (reservoir effects). Additional complications come from the burning of fossil fuels such as coal and oil, and from the above-ground nuclear tests done in the 1950s and 1960s. Because the time it takes to convert biological materials to fossil fuels is substantially longer than the time it takes for its 14C to decay below detectable levels, fossil fuels contain almost no 14C, and as a result there was a noticeable drop in the proportion of 14C in the atmosphere beginning in the late 19th century. Conversely, nuclear testing increased the amount of 14C in the atmosphere, which attained a maximum in about 1965 of almost twice what it had been before the testing began.

Measurement of radiocarbon was originally done by beta-counting devices, which counted the amount of beta radiation emitted by decaying 14C atoms in a sample. More recently, accelerator mass spectrometry has become the method of choice; it counts all the 14C atoms in the sample and not just the few that happen to decay during the measurements; it can therefore be used with much smaller samples (as small as individual plant seeds), and gives results much more quickly. The development of radiocarbon dating has had a profound impact on archaeology. In addition to permitting more accurate dating within archaeological sites than previous methods, it allows comparison of dates of events across great distances. Histories of archaeology often refer to its impact as the "radiocarbon revolution". Radiocarbon dating has allowed key transitions in prehistory to be dated, such as the end of the last ice age, and the beginning of the Neolithic and Bronze Age in different regions.

Radiometric dating

Radiometric dating, radioactive dating or radioisotope dating is a technique used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. The use of radiometric dating was first published in 1907 by Bertram Boltwood and is now the principal source of information about the absolute age of rocks and other geological features, including the age of fossilized life forms or the age of the Earth itself, and can also be used to date a wide range of natural and man-made materials.

Together with stratigraphic principles, radiometric dating methods are used in geochronology to establish the geologic time scale. Among the best-known techniques are radiocarbon dating, potassium–argon dating and uranium–lead dating. By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change. Radiometric dating is also used to date archaeological materials, including ancient artifacts.

Different methods of radiometric dating vary in the timescale over which they are accurate and the materials to which they can be applied.

Waste-to-energy

Waste-to-energy (WtE) or energy-from-waste (EfW) is the process of generating energy in the form of electricity and/or heat from the primary treatment of waste, or the processing of waste into a fuel source. WtE is a form of energy recovery. Most WtE processes generate electricity and/or heat directly through combustion, or produce a combustible fuel commodity, such as methane, methanol, ethanol or synthetic fuels.

Willard Libby

Willard Frank Libby (December 17, 1908 – September 8, 1980) was an American physical chemist noted for his role in the 1949 development of radiocarbon dating, a process which revolutionized archaeology and palaeontology. For his contributions to the team that developed this process, Libby was awarded the Nobel Prize in Chemistry in 1960.

A 1927 chemistry graduate of the University of California at Berkeley, from which he received his doctorate in 1933, he studied radioactive elements and developed sensitive Geiger counters to measure weak natural and artificial radioactivity. During World War II he worked in the Manhattan Project's Substitute Alloy Materials (SAM) Laboratories at Columbia University, developing the gaseous diffusion process for uranium enrichment.

After the war, Libby accepted professorship at the University of Chicago's Institute for Nuclear Studies, where he developed the technique for dating organic compounds using carbon-14. He also discovered that tritium similarly could be used for dating water, and therefore wine. In 1950, he became a member of the General Advisory Committee (GAC) of the Atomic Energy Commission (AEC). He was appointed a commissioner in 1954, becoming its sole scientist. He sided with Edward Teller on pursuing a crash program to develop the hydrogen bomb, participated in the Atoms for Peace program, and defended the administration's atmospheric nuclear testing.

Libby resigned from the AEC in 1959 to become Professor of Chemistry at University of California, Los Angeles (UCLA), a position he held until his retirement in 1976. In 1962, he became the Director of the University of California statewide Institute of Geophysics and Planetary Physics (IGPP). He started the first Environmental Engineering program at UCLA in 1972, and as a member of the California Air Resources Board, he worked to develop and improve California's air pollution standards.

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