Radioactive dating-the clocks in rocks (http://earthsci.org/fossils/geotime/radate/radate.html)

Introduction to the topic (Wikipedia)

Radioactive dating (also called radiometric dating) is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates.

It is the principal source of information about the absolute age of rocks and other geological features, including the age of the Earth itself, and can be used to date a wide range of natural and man-made materials.

Path to discovery (http://myweb.cwpost.liu.edu/vdivener/notes/radiometric_dating.htm )

In 1896 Henri Becquerel and Marie Curie discovered that certain isotopes undergo spontaneous radioactive decay, transforming into new isotopes. Atoms of a parent radioactive isotope randomly decay into a daughter isotope. Over time the number of parent atoms decreases and the number of daughter atoms increases. Rutherford and Soddy (1902) discovered that the rate of decay of a radioactive isotope depends on the amount of the parent isotope remaining. Later it was found that half of the parent atoms occurring in a sample at any time will decay into daughter atoms in a characteristic time called the half-life."

Fundamentals of radioactive dating

Radioactive decay

All ordinary matter is made up of combinations of chemical elements, each with its own atomic number, indicating the number of protons in the atomic nucleus. Additionally, elements may exist in different isotopes, with each isotope of an element differing in the number of neutrons in the nucleus. A particular isotope of a particular element is called a nuclide. Some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will spontaneously transform into a different nuclide. This transformation may be accomplished in a number of different ways, including radioactive decay, either by emission of particles (usually electrons (beta decay), positrons or alpha particles) or by spontaneous fission, and electron capture.(Wikipedia)

Example of a radioactive decay chain from lead-212 (212Pb) to lead-208 (208Pb) . Each parent nuclide spontaneously decays into a daughter nuclide (the decay product) via an α decay or a β − decay. The final decay product, lead-208 (208Pb), is stable and can no longer undergo spontaneous radioactive decay. (On the left side)

Types of radioactive decay

Alpha decay

In an alpha decay, the atomic nucleus emits an alpha particle(helium 4 nucleus) and thereby transforms into an atom with a mass number 4 less and atomic number 2 less. For example:

23892 U

→ 23490 Th

+ 42 He 2+ [1]

which can also be written as:

238U

→ 234

Th + α

Beta decay

It is of two types.

Electron emission(beta minus)

The atomic nucleus converts a neutron into a proton emitting an electron.

Eg. See in the base module

Positron emission(beta plus)

In positron emission,the atomic nucleus converts a proton into a neutron emitting a positron.(a particle that has the same mass as that of an electron but with a positive charge)

Eg.see in the base module

Electron capture

In an Electron capture (also called Inverse Beta Decay) a proton in the atomic nucleus captures an inner electron (that is, an electron in an inner shell) and forms a neutron. As a result, the number of protons within the nucleus decreases by one unit and the atom of the "parent" element is transformed into that of another ("daughter") element. The number of nucleons (protons plus neutrons) within the atomic nucleus remains unchanged.

Gamma decay

See base module

The nucleus that undergoes decay is the parent nucleus and the nucleus formed after the decay is called the daughter nucleus. (Base module)

Half life

While the moment in time at which a particular nucleus decays is unpredictable, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-life.it is the time taken for half of the number of atoms in a radioactive sample to disintegrate. It is usually given in units of years when discussing dating techniques. (Wikipedia)

In general, the half-life of a nuclide depends solely on its nuclear properties; it is not affected by external factors such as temperature, pressure, chemical environment, or presence of a magnetic or electric field. (For some nuclides which decay by the process of electron capture, such as beryllium-7, strontium-85, and zirconium-89, the decay rate may be slightly affected by local electron density; therefore these isotopes may not be as suitable for radiometric dating.) But in general, the half-life of any nuclide is essentially a constant. This predictability allows the

relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclide(s) into a material to the present.

Mass spectrometer used in radiometric dating

The age equation

The mathematical expression that relates radioactive decay to geologic time, is

D = D0 + N (t) (eλt − 1)

Where

t is age of the sample,D is number of atoms of the daughter isotope in the sample,D0 is number of atoms of the daughter isotope in the original composition,N is number of atoms of the parent isotope in the sample at time t (the present), given by N (t) = Noe-λt, andλ is the decay constant of the parent isotope, equal to the inverse of the radioactive half-life of the parent isotope times the natural logarithm of 2.

The equation is most conveniently expressed in terms of the measured quantity N(t) rather than the constant initial value No.

The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below its closure temperature. This is well-established for most isotopic systems.

Ale's Stones at Kåseberga, around ten kilometers south east of Ystad, Sweden were dated at 600 CE using the carbon-14 method on organic material found at the site.

Modern dating methods

Radiometric dating has been carried out since 1905 when it was invented by Ernest Rutherford as a method by which one might determine the age of the Earth. In the century since then the techniques have been greatly improved and expanded.

1. Uranium-lead dating method

Uranium-lead dating is often performed on the mineral zircon (ZrSiO4), though it can be used on other materials, such as baddeleyite. Zircon and baddeleyite incorporate uranium atoms into their crystalline structure as substitutes for zirconium, but strongly reject lead\Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event.

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 700 million years, and one based on uranium-238's decay to lead-206 with a half-life of about 4.5 billion 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.

2. Radiocarbon dating method

An organism acquires carbon during its lifetime. Plants acquire it through photosynthesis, and animals acquire it from consumption of plants and other animals. When an organism dies, it ceases to take in new carbon-14, and the existing isotope decays with a characteristic half-life (5730 years). The proportion of carbon-14 left when the remains of the organism are examined provides an indication of the time elapsed since its death. The carbon-14 dating limit lies around 58,000 to 62,000 years.

Benefits of Radioactive Dating

Researchers trust radioactive dating for two reasons. First, the half-life of radioactive elements is well-established and recorded. If half of the carbon-14 element in a fossil has changed, the age of the fossil would be 5,730 years old because that is the element's half-life. Second, the half-life of these elements does not change because of environmental effects. The half-life does not increase in hot weather, for instance. That means the measurement remains stable and reliable regardless of the environmental changes taking place at different times in the Earth's history.

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Advantages include the ability to date an object without destroying it, having many different techniques to choose from, and the ability to procure a relatively accurate age of objects that are hundreds of thousands, millions, or even billions of years old.

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applications of radioactive dating

One of the most familiar applications of radioactive dating is determining the age of fossilized remains, such as dinosaur bones. Radioactive dating is also used to authenticate the age of rare archaeological artifacts. Because items such as paper documents and cotton garments are produced from plants, they can be dated using radiocarbon dating. Without radioactive dating, a clever forgery might be indistinguishable from a real artifact. There are some limitations, however, to the use of this technique. Samples that were heated or irradiated at some time may yield by radioactive dating an age less than the true age of the object. Because of this limitation, other dating techniques are often used along with radioactive dating to ensure accuracy.

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One very interesting and timely application of radiometric dating in paleontology was the recent discovery, by researchers at the University of Alberta in Canada, of a fossilized dinosaur bone in New Mexico that is 64.8 million years old.

Another recent example is a 2011 study, by an archaeological team from the University of Tubingen in Germany, that found stone tools 127,000 years old from a site near the entrance to the Persian Gulf.

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artifact

Rutherford

Frederick soddy

henry Becquerel

Alpha decay

gamma decay