Dating by decay

Ammo acid racemization (Section 27 2) A method for dating archeological samples based on the rate at which the stereo chemistry at the a carbon of ammo acid components is ran domized It is useful for samples too old to be reliably dated by decay... 

C22-0081. Uranium deposits are dated by determining the ratio of U to its final decay product,... 

Dating with Radiocarbon. The important information held in a sample to be dated by radiocarbon is its present radiocarbon concentration comparing this concentration to that of radiocarbon in the atmosphere, which is considered to be constant (however, see discussion below), yields the conventional radiocarbon date of the sample. All that is required to establish the age of a sample, therefore, is to determine the present-day relative amount of radiocarbon in the sample. Once this has been determined by either the conventional radiocarbon decay counting or by the AMS method (see Fig. 63), a number of internationally established conventions and assumptions are used to calculate the age of a material or object ... 

We were quite elated, and it appeared that it was a rich field. Now, fifty years later, I must say that it wasn t as rich as we thought. But we have over the years discovered half a dozen natural radioactive elements, and two of these, the samarium-147 with its decay to neodymium-143 and rhenium-187 with its decay to osmium-187, prove to be of use in Nuclear Dating. The importance of rhenium is that it is iron soluble while the other radioactivities are insoluble in metallic iron. In fact, the best half life we have for rhenium-187 was obtained by measuring the osmium-187 to rhenium-187 ratio in iron meteorites which had been dated by other methods. This work was started many years ago by Dr. Herr and others in Germany. The half life is 43,000,000,000 years. 

This is exploited in the technique of uranium-thorium dating, which involves measuring the amount of thorium-230 that has accumulated in a substance by decay of uranium. If the object contained no thorium at all when it was formed, the ratio of remaining U to accumulated °Th is a measure of the age. The object being dated must not have had access to sources of fresh uranium that could reset the clock. This is true, for example, of coral left stranded on fossil beaches when sea levels recede, or of... 

The most famous cosmogenic radionuclide is C (t j = 5730 a), which is produced by the interaction of cosmic ray neutrons via an (n,p) reaction with nitrogen [ N(n, p) C], whereas the radioactive decay of C takes place by decay to form the stable N isotope. C is the most important cosmogenic radionuclide for dating (see Section 9.7.5) in archaeology and can be analyzed using isotope sensitive accelerator mass spectrometry. Extremely small isotope ratios C/ C = 10 in nature can be measured by means of AMS. ... 

However, once such processes cease, the amount of radiocarbon decreases by decay as measured by the isotope s half-life. Thus, a radiocarbon date is based on the measurement of the residual contained in the sample. However, for a radiocarbon age of a sample to be equivalent to its real or calendar age, certain fundamental parameters and assumptions must... 

The laws of radioactive decay are the basis of chronology by nuclear methods. From the variation of the number of atoms with time due to radioactive decay, time differences can be calculated rather exactly. This possibility was realized quite soon after the elucidation of the natural decay series of uranium and thorium. Rutherford was the first to stress the possibility of determining the age of uranium minerals from the amount of helium formed by radioactive decay. Dating by nuclear methods is applied with great success in many fields of science, but mainly in archaeology, geology and mineralogy, and various kinds of chronometers are available. 

In practice, two approaches are used - independent determination of N2 and N or simultaneous determination of N2 and N by mass spectrometry (MS). The second approach is not applicable if the properties of the mother nuclide and the daughter nuclide are very different, e.g. in the case of dating by the " °K/" °Ar method or by measuring " He formed by radioactive decay. Both methods require additional determination of the unknown number but in special cases A can be neglected. 

In the early stages of dating by nuclear methods, the measurement of He formed by a decay in the natural decay series (9, 6 and 7 He atoms in the uranium series, the thorium series and the actinium series, respectively) has been applied. The preferred method was the U/He method which allows dating of samples with very low concentrations of U of the order of 1 mg/kg. Helium produced by a decay is driven out by heating and measured by sensitive methods, e.g. by MS. However, it is difficult to ensure the prerequisites of dating by the U/He method neither " He nor a-emitting members of the decay series must be lost and no " He atoms must be produced by other processes such as decay of Th and spallation processes in meteorites. 

The development of more sensitive ways to measure levels of radioactive substances has allowed scientists to take advantage of the decay of nuclides other than carbon-14. For example, chlorine-36 can be used to date ground water, marine sediments can be dated by measuring levels of beryllium-11 and aluminum-26, and krypton-81 has been used to estimate the age of glacial ice. 

The half-lives of radioactive isotopes have been used as atomic clocks to determine the ages of certain objects. Some examples of dating by radioactive decay measnre-ments will be described here. 

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