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Potassium 40 is useful for dating very old fossils

Potassium-argon datingmethod of determining the time of origin of rocks by measuring the ratio of radioactive argon to radioactive potassium in the rock.

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This has been archived and is no longer updated. Despite seeming like a relatively stable place, the Earth's surface has changed dramatically over the past 4. Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free.

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O'Connell, A. Potassium decays to 40 Ar with a half-life of My. In the traditional approach, the potassium content of a sample was measured using techniques such as atomic absorption spectrometry, and the 40 Ar in a high-resolution gas source mass spectrometer. Single grains of potassium-rich minerals can be heated with a laser to release the argon allowing detection of older contaminating grains. Archaeological applications of K—Ar dating require volcanic material to be associated with cultural layers. New archaeological applications are being developed with the increasing sensitivity of the technique, which enables younger samples to be dated.

According to current morphological classification, these fossils represent the earliest known examples of modern humans. The weakly radioactive 40 K, naturally present in most of the earth's crust, decays to two daughters, 40 Ar and 40 Ca, with a half-life of million years. After resolidification of the rock, argon gas generated can be trapped within the glassy matrix. Given a closed system, collection and measurement by mass spectrometry of trapped 40 Ar can therefore be used to date the formation of the lava flow.

Potassium concentration can then be determined by measurement of the 39 Ar. This has the advantage of requiring only one measurement procedure, of the isotopic ratios of argon, but more importantly allows a controlled release of argon by gradual temperature increase, which can be used to detect sample contamination.

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Lustig, B. The naturally occurring radioactive isotope of potassium, 40 K, is present in a known, constant, very low percentage of total potassium. Since body potassium is essentially intracellular, and not present in stored fat, measurement of 40 K not only provides an estimate of total-body potassium, but also allows estimation of body cell mass.

If total body water is known from deuterium dilutionextracellular water can be calculated. The technique requires a highly shielded environment in which to detect the 40 K in the body, as 40 K also occurs in most environmental structures.

Aitken, in Encyclopedia of Materials: Science and Technology In naturally occurring potassium there is a weakly radioactive isotope, 40 K, which on decay produces an isotope, 40 Ar, of the gas argon; dating is based on the amount of 40 Ar which has accumulated since time-zero—the event dated. In volcanic products this is the eruption. Traditionally, the potassium content of separated minerals from a sample is measured by atomic absorption spectrometry and the argon by mass spectrometry after release by fusion.

Argon—argon dating has the same basis but is particularly advantageous for samples of heterogeneous potassium content because 40 K and 40 Ar are measured on exactly the same subsample. By irradiation with high-energy neutrons in a nuclear reactor, 40 K is converted into 39 Ar and after release by fusion both isotopes, 39 Ar and 40 Ar, are measured in the mass spectrometer at the same time; the age is then determined from the ratio between the two. By using a high-power laser for release of argon, determinations can be made on single grains.

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Such single crystal laser fusion SCLF has the advantage that intrusive grains from older or younger strata can be detected and ignored. The dating of archaeological materials is through association, such as when sandwiched between two sequential lava flows. The most important application is to early hominids and the long half-life of 40 K more than encompasses the required range of several million years.

The young end of the age range is limited by the amount of accumulated argon; for minerals rich in potassium, such as sanidine, the order of years can be reached. Natural radioactivity occurs in all foods, arising from the radioactive isotopes carbon and potassium There is no legislation governing this radioactivity since it cannot be eliminated, and consumption of such foods does not ificantly alter the potassium content, and hence the potassium radioactivity of the body.

Human exposure to radiation is laid down in law and national measures are usually based on recommendations of the International Committee for Radiological Protection ICRP. Drinking water contributes a very small radiation dose to total population exposure and is largely due to naturally occurring radionuclides in the uranium and thorium decay series.

WHO has established guideline levels for radioactivity in drinking water, based on ICRP recommendations and available data on risk exposure. Some countries lay down limits for radionuclides in bottled waters, for example in the USA, Sweden, Israel, and Switzerland.

Before the accident at Chernobyl nuclear power station invery few countries had radioactivity limits for food since the policy was, and still is, to control emission and waste at source, with local incidents being handled by monitoring of food and population.

Contamination of food on a continental scale arising from Chernobyl required immediate action and resulted in the EU setting limits for radioactive contamination of imports from outside its member states.

Since then, contamination levels have decreased to a negligible risk of health, and later rules exclude a of products from the scope of the original regulation. Inthe EU established procedures for carrying out checks on the radiocesium content of such foods. A of countries followed the EU post-Chernobyl regulation. Infant foods, milk, and milk products, and animal foods are among the most widely regulated commodities in this regard. The EU has also established a regulation setting maximum permitted levels for radioactive contamination of foodstuffs, which could be applied in the event of a future nuclear accident or other radiological emergency potentially involving radionuclides other than cesium.

Yuk-Kwan Choi, Robert Z. Qi, in Methods in Enzymology The solutions are mixed gently and allowed to sit at room temperature for 3 min. Each sample is diluted to 1 mL with chilled BRB Further dilution may be required to obtain a concentration of nucleated microtubules that is optimal for microscopic examination. Samples 0. Solutions are aspirated carefully, and the cover slips are withdrawn and mounted on glass slides.

Microtubules are examined under an inverted fluorescence microscope Axio Observer ZI, Carl Zeiss and counted in 20 random fields to determine the average of microtubules per field. The 40 Ar— 39 Ar process is based on the same idea as that of the K—Ar method, except that neutron activation is used to estimate the amount of 39 K by activation to 39 Ar.

The in-growth of 40 Ar is a variant of the in-growth eqn [4]since there is actually a branching decay to 40 Ca and 40 Ar from 40 K:. K—Ar dating has many applications, particularly during the Quaternary to the dating of bones and tephras. Krzysztof Duda, Jerzy A. Zoladz, in Muscle and Exercise Physiology The measure of the total amount of potassium in the body [total body potassium TBK ] is based on the activity of the natural 40 K isotope with 1.

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According to the formula Eq. Moore — in the midth century see Section 1. Bethan J. The technique is based on the radioactive decay of 40 K to 40 Ar. This allows for smaller single aliquot samples Briner, ; Jull, ; Wijbrans and Kuiper, It is usually applied to K-rich volcanic rocks. In Hawaii, K-Ar dating of lava flows that underlie and overlie glacial sediments provided bracketing ages for the glacial advances Porter, Potassium 40 40K undergoes a branching decay to both 40Ar and 40Ca, so it is important to revise the simple radioactive decay equation to take into the decay via different processes.

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About this. View chapter Purchase book. Strauss, in Encyclopedia of Food Sciences and Nutrition Second EditionTotal-Body Potassium The naturally occurring radioactive isotope of potassium, 40 K, is present in a known, constant, very low percentage of total potassium. Dating Archeological Materials M. Aitken, in Encyclopedia of Materials: Science and Technology2. Body Composition D. Stansfield, in Encyclopedia of Food Sciences and Nutrition Second EditionRadioactivity Natural radioactivity occurs in all foods, arising from the radioactive isotopes carbon and potassium Qi, in Methods in Enzymology4.

Zoladz, in Muscle and Exercise Physiology1.