Nuclear+Methods+in+Materials+Dating

Summary: This contribution provides a necessarily brief summary of radioactive dating techniques which can produce dates (ages) ranging from tens to thousands through millions to billions of years often with assumptions not universally accepted, - especially those involving the assessments of half-lives and radioactive decay constants.

INTRODUCTION
Radioactive dating methods involve use of radioactive isotopes of various elements. Of the more than 2000 presently known nuclides, over four-fifths are unstable although most do not occur naturally on Earth, because of their very rapid rates of radioactive decay. To date rocks and minerals, naturally occurring radioisotopes are used that continued to exist long after the so-called Big Bang because of their extremely slow rates of radioactive decay. These include 238 U, 235 U, 232 Th, 87 Rb and 40 K. Some result from the radioactive decay of long-lived, naturally occurring radioactive parents and among these are 234 U, 230 Th and 226 Ra. Others may have been created by natural nuclear reactions and these include radiocarbon (14 C) and tritium (3 H). Today, artificial radio¬nuclides have been introduced into the environment as a result of thermonuclear testing and the operation of nuclear fission reactors and particle accelerators.

Whatever its sources, radioactivity is significant as regards geochronology and radioactive dating began in an attempt to determine the age of the Earth once the phenomenon was discovered and uranium and thorium minerals were found to emit radiation. Two new elements were identified, namely polonium and radium, and the word 'radioactivity' was derived from the latter. Ever accelerating progress has been made in determining the ages of minerals, rocks, archeological and historical objects and so on to such an extent that numerous approaches have become routine. Major methods for achieving these data are presented alphabetically in abbreviated form in order to provide a brief but useful perspective of what is in fact a vast and continuously expanding subject. The techniques include four radiation damage techniques, namely electron spin resonance (ESR), fission track dating (FTD), pleochroic haloes and thermoluminescence.

Great advances resulted from Hans Oeschger's development of the Oeschger Counter that has a lower background than other such instruments. It became the leading instrument for measuring the activity of a variety of naturally occurring radionuclides such as tritium, 14 C, 26 Al, 37 Ar, 39 Ar, 81 Kr and 85 Kr. Using this method, Oeschger and his team were able to quantify exchange processes between different components of the Earth System. There is a final caveat to make here and this refers to the words 'assuming', 'presuming', 'implying' and 'inferring' appearing occasionally in the text regarding certain dating methods and in a number of cases dubiety apropos half-lives and radioactive decay constants are mentioned. From this, it is apparent that there is still some way to go to improve and, if possible, perfect many of the relevant techniques.

In this Chapter the following short-hand notation will be used. [<span style="font-size: 75%; vertical-align: super;">A X] denotes the number of atoms/concentration of nuclide <span style="font-size: 75%; vertical-align: super;">A X, where X stands for the chemical symbol of some element and A is the mass number of one of its isotopes. (<span style="font-size: 75%; vertical-align: super;">A X) is the activity (either absolute or specific) of nuclide <span style="font-size: 75%; vertical-align: super;">A X. Symbols like [<span style="font-size: 75%; vertical-align: super;">A X/<span style="font-size: 75%; vertical-align: super;">B Y] actually mean atomic ratios of nuclides <span style="font-size: 75%; vertical-align: super;">A X and <span style="font-size: 75%; vertical-align: super;">B Y, i.e. they represent the ratio [<span style="font-size: 75%; vertical-align: super;">A X]/[<span style="font-size: 75%; vertical-align: super;">B Y]. Similarly, the ratio of activities (<span style="font-size: 75%; vertical-align: super;">A X)/(<span style="font-size: 75%; vertical-align: super;">B Y) will be abbreviated sometimes as (<span style="font-size: 75%; vertical-align: super;">A X/<span style="font-size: 75%; vertical-align: super;">B Y). An asterisk as a right superscript like <span style="font-size: 75%; vertical-align: super;">A X* or [<span style="font-size: 75%; vertical-align: super;">A X*] etc. is to stress that the nuclide in question or the specified number of them is of radiogenic origin.