Radiometric Dating Types Of Radiometric Dating Fundamentals Of

Radiometric dating is a technique used to date materials based on a knowledge of the decay rates of naturally occuring isotopes, and the current abundances.

1 Types of radiometric dating

2 Fundamentals of radiometric dating

Most chemical elements decay radioactively to some extent. Decaying so slowly that, for all practical purposes, they can be considered inert. An element whose decay can be easily detected (for example, with a Geiger counter), is said to be radioactive. Radiometric dating is based on forms of radioactive decay in nucleus different element or isotope.

The decay rate of an element depends solely on its nuclear properties; it does not depend on temperature, chemical environment, or any other external factors. The abunance of a particular element may be used as a type of clock. For dating purposes, the important parameter is the half-life of the nuclear reaction - the time it takes for half the material to have decayed. Isotopes useful for radiometric dating have half lives ranging from a few thousand to a few billion years.

Some types of radiometric dating assume that the initial proportions of a radioactive substance and its decay product are known. The decay product should not be a small-molecule gas that can leak out, and must itself have a long enough half life that it will be present in significant amounts. In addition, the initial element and the decay product should not be produced or depleted in significant amounts by other reactions. The procedures used to isolate and analyze the reaction products must be straightforward and reliable.

In contrast to most radiometric dating techniques, isochron dating using rubidium-strontium does not require knowledge of the initial proportions.

2.1 Limitation of techiniques

The accuracy of a method of dating depends in part on the half-life of the radioactive isotope involved. For instance, carbon-14 has a half-life of less than 6000 years. After an organism has been dead for 60,000 years, so little carbon-14 is left in it that accurate dating becomes impossible. On the other hand, the concentration of carbon-14 falls off so steeply that the age of relatively young remains can be determined accurately to within a few decades. The isotope used in uranium-thorium dating has a longer half-life, but other factors make it more accurate than radiocarbon dating.

3 Modern dating techniques

The decay of radioactive materials can take place through alpha decay, in which an isotope loses two neutrons and two protons, or through beta decay, in which an isotope loses an electron, converting a neutron into a proton.

Radioactive or "radiometric" dating works by determining the relative concentrations of the "parent" and "daughter" isotopes in a decay process. Radiometric dating schemes are regarded as accurate because radioactive decay is not influenced by any normal physical process. It is neither slowed nor accelerated by heat, pressure, or magnetic and electric fieldIn physics, an electric field is an effect produced by an electric charge that exerts a force on charged objects in its vicinity. Definition and derivation The mathematical definition of the electric field is developed as follows. Coulomb's law gives thes. It can be accelerated by radioactive bombardment, but such bombardment tends to leave evidence of its occurrence.

Furthermore, the processes that form specific materials are often conveniently selective as to what elements they incorporate during their formation. In the ideal case, the material will incorporate a parent isotope and reject the daughter isotope. In this case, the only daughter isotopes found through examination of a sample were created since the sample was formed.

However, if a material that selectively rejects the daughter isotope is heated, any daughter isotopes that have been accumulated over time will be lost through diffusion, setting the isotopic "clock" to zero. The temperature at which this happens is known as the "blocking temperature" and is specific to a particular material.

Although radiometric dating is accurate in principle, the accuracy is very dependent on the care with which the procedure is performed. The possible confounding effects of initial contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created. Accuracy is enhanced if measurements are taken on different samples taken from the same rock body but at different locations. This permits some compensation for variations.

Of course, the half-life of the parent isotope has to be known, and the measurements of the parent and daughter isotopes have to be accurate as well. Radiometric dating can be performed on samples as small as a billionth of a gram using a mass spectrometer. The mass spectrometer was invented in the 1940sCenturies: 19th century 20th century 21st century Decades: 1890s 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s Years: 1940 1941 1942 1943 1944 1945 1946 1947 1948 1949 Events and trends Technology First nuclear bomb First cruise missile, the and began to be used in radiometric dating in the 1950sCenturies: 19th century 20th century 21st century Decades: 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s Years: 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 Events and trends Technology United States tests the first fusion bomb..

The mass spectrometer operates by generating a beam of ionized atoms from the sample under test. The ions then travel through a magnetic field, which diverts them into different sampling sensors, known as "Faraday cups", depending on their mass and level of ionization. On impact in the cups, the ions set up a very weak current that can be measured to determine the rate of impacts and the relative concentrations of different atoms in the beams.

The uranium-lead radiometric dating scheme is one of the oldest available, as well as one of the most highly respected. It has been refined to the point that the error in dates of rocks about three billion years old is no more than two million years.

Uranium-lead dating is best performed on the mineralMinerals are natural compounds formed through geological processes. The term "mineral" encompasses not only the material's chemical composition but also the mineral structures. Minerals range in composition from pure elements and simple salts to very comp " zirconFor the spy satellite of this codename see Zircon (satellite). Zircon (from Persian: sarkun golden) is a mineral belonging to the group of nesosilicates. Its chemical formula is Zr Si O. The crystal structure of zircon is tetragonal (crystal class: 4/m 2/" (ZrSiO₄), though it can be used on other materials. Zircon incorporates uranium atoms into its crystalline structure as substitutes for zirconium, but strongly rejects lead. It has a very high blocking temperature, and is very chemically inert.

One of its great advantages is that any sample provides two clocks, one based on uranium-235's decay to lead-207 with a half-life of about 4.5 billion years, and one based on uranium-238's decay to lead-206 with a half-life of about 700 million years, providing a built-in crosscheck that allows accurate determination of the age of the sample even if some of the lead has been lost.

Two other radiometric techniques are used for long-term dating. Potassium-argon dating involves the beta decay of potassium-40 to argon-40. Potassium-40 has a half-life of 1.3 billion years, and so this method is applicable to the oldest rocks. Radioactive potassium-40 is common in micas, feldspars, and hornblendes, though the blocking temperature is fairly low in these materials, about 125C.

Rubidium-strontium dating is based on the beta decay of rubidium-87 to strontium-87, with a half-life of 50 billion years. This scheme is used to date old igneous and metamorphic rocks, and has also been used to date lunar samples. Blocking temperatures are so high that they are not a concern. Rubidium-strontium dating is not as precise as the uranium-lead method, with errors of 30 to 50 million years for a 3-billion-year-old sample.

4 Short-range dating techniques

There are a number of other dating techniques that have short ranges and are so used for historical or archaelogical studies. One of the best-known is the carbon-14 (C14) radiometric technique.

Carbon-14 is a radioactive isotope of carbon-12, with a very short half-life of 5,730 years. In other radiometric dating methods, the heavy parent isotopes were synthesized in the explosions of massive stars that scattered materials through the Galaxy, to be formed into planets and other stars. The parent isotopes have been decaying since that time, and so any parent isotope with a short half-life should be extinct by now.

Carbon-14 is an exception. It is continuously created through collisions of neutrons generated by cosmic rays with nitrogen in the upper atmosphere. The carbon-14 ends up as a trace component in atmospheric carbon dioxide (CO2).

An organism acquires carbon from carbon dioxide during its lifetime. Plants acquire it through respiration and photosynthesis, and animals acquire it from consumption of plants and other animals. When the organism dies, the carbon-14 begins to decay, and the proportion of carbon-14 left when the remains of the organism are examined provides an indication of the date of its death. Carbon-14 radiometric dating has a range of about 50,000 years.

The rate of creation of carbon-14 appears to be roughly constant, as cross-checks of carbon-14 dating with other dating methods show it gives consistent results. However, local eruptions of volcanoes or other events that give off large amounts of carbon dioxide can reduce local concentrations of carbon-14 and give inaccurate dates. The releases of carbon dioxide into the biosphere as a consequence of industrialization have also depressed the proportion of carbon-14 by a few percent; conversely, the amount of carbon-14 was increased by above-ground nuclear bomb tests that were conducted into the early 1960s.

Another relatively short-range dating technique is based on the decay of uranium-238 into thorium-230, a process with a half-life of 80,000 years It is accompanied by a sister process, in which uranium-235 decays into protactinium-231, which has a half-life of 34,300 years.

While uranium is water-soluble, thorium and protactinium are not, and so they are selectively precipitated into ocean-floor sediments, from which their ratios are measured. The scheme has a range of several hundred thousand years.

Archaeologists use tree-ring dating ( dendrochronology) to determine the age of old pieces of wood. Trees grow rings on a yearly basis, with the spacing of rings being wider in good growth years than in bad growth years. These spacings can be used to help pin down the age of old wood samples, and also give some hints to climate change. The technique is only useful to about 4,000 years in the past, however, because it requires overlapping tree ring series.

Although determining geologic time by measuring the rate of deposition of sediments is not reliable over the large scale, it is still useful for certain scenarios, such as the deposition of layers of sediment on the bottom of a stable lake. The approach is now known as "varve analysis" (the term "varve" means a layer or layers of sediment).

Another technique used by archaelogists is to inspect the depth of penetration of water vapor into chipped obsidian (volcanic glass) artifacts. The water vapor creates a "hydration rind" in the obsidian, and so this approach is known as "hydration dating" or "obsidian dating", and is useful for determining dates as far back as 200,000 years.

Natural sources of radiation in the environment knock loose electrons in, say, a piece of pottery, and these electron accumulate in defects in the material's crystal lattice structure. When the sample is heated, at a certain temperature it will glow from the emission of electrons released from the defects, and this glow can be used to estimate the age of the sample to a threshold of a few hundred thousand years.

Finally, "fission track dating" involves inspection of a polished slice of a material to determine the density of "track" markings left in it by radioactive decay of uranium-238 impurities.

The uranium content has to be understood, but that can be determined by placing a plastic film over the polished slice of the material, and then bombarding it with slow neutrons. This causes induced fission of U-235, as opposed to spontaneous fission of U-238. The fission tracks produced by this process are recorded by a thin plastic film placed against the surface of the sample. The uranium content of the material can then be calculated so long as the neutron dose is known.

This scheme has a maximum range of about a million years and works best with micas, tektites (glass fragments from volcanic eruptions), and meteorites. However, the dates may be inaccurate if the sample was heated to high temperatures in the past as blocking temperatures are generally low, or if the sample was exposed on the surface of the Earth where it was bombarded with cosmic rays.

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