A process of determining the
age of a substance by analyzing the
decay rates of radioactive nuclides. This dating process relies on several principles of
atomic physics and
radioactivity.
Certain chemical isotopes are stable, in that they will not change in structure until they are affected by an external force. However, there are also unstable isotopes which will undergo a process of radioactive decay within a given period of time. As explained in the radioactivity node, there are three basic types of decay. The half-life of an isotope is defined as the time it takes for half of the radioactive ions to go through the process of decay. This measurement is not dependent on time, in that a sample of U-238 that is one hundred years old will have the same half-life as a sample that is one thousand years old.
Therefore, since mathematical formulas can be created to analyze the rate of decay by analyzing the number of breakdowns per second in a sample, all that is needed to determine the date of the sample is the amount of the isotope originally present and compare it to how much remains now. If the amount originally present cannot be determined, than the substance is not eligible for radiometric dating.
In order to have cases where the original amount can be known, an isotope that is part of a larger mineral compound is needed. This is due to the fact that chemical processes similar to those involved in the formation of mineral substances cannot distinguish between different isotopes of the same element. If a substance has more than one isotope, and a mineral forms in a magma melt that includes that element, the various isotopes within the original substance will appear in the newly formed mineral in the same ratio that they were found in that environment. When a substance undergoes a process of decay, the chemical behavior and structure of it changes due to a shift in the makeup of its election shell.
Methods
The principles explained above are central to the various specific practices of radiometric dating. Below are four basic types of this technique.
- Carbon: The most basic principle of C-14 dating, or radiocarbon dating, is that all subjects of it must be organic. Of carbon atoms, almost 99% are C-12 and stable along with the stable C-13 isotope. C-14 occurs in very small proportions which are formed in large part from Nitrogen-14 in the higher levels of the atmosphere. Due to the necessity of carbon for life, C-14 exists in all terrestrial organisms in the same proportions as it does in the atmosphere. With a half life of 5,730 years, this type of radiometric dating is ideal for archaelogical pursuits. More information can be found on this technique in the carbon dating node.
- Uranium-238/Uranium-235/Thorium-232 Radioactive Series: These three isotopes all follow a specific radioactive series through their decay. The series are as follows:
- U-238:
U-238 => Th-234 + α
Th-234 => Pa-234 + β
Pa-234 => U-234 + β
U-234 => Th-230 + α
Th-230 => Ra-226 + α
Ra-226 => Rn-222 + α
Rn-222 => Po-218 + α
Po-218 => Pb-214 + α
Pb-214 => Bi-214 + β
Bi-214 => Po-214 + β
Po-214 => Pb-210 + α
Pb-210 => Bi-210 + β
Bi-210 => Po-210 + β
Po-210 => Pb-206 + A
- Th-232:
Th-232 => Ra-228 + α
Ra-228 => Ac-228 + β
Ac-228 => Th-228 + β
Th-228 => Ra-224 + α
Ra-224 => Rn-220 + α
Rn-220 => Po-216 + α
Po-216 => Pb-212 + α
Pb-212 => Bi-212 + β
Bi-212 => Tl-208 + α
Tl-208 => Po-212 + γ
Po-212 => Pb-208 + β
- U-235:
U-235 => Th-231 + α
Th-231 => Pa-231 + β
Pa-231 => Ac-227 + α
Ac-227 => Fr-223 + α
Fr-223 => Th-227 + γ
Th-227 => Ra-223 + α
Ra-223 => Rn-219 + α
Rn-219 => Po-215 + α
Po-215 => Pb-211 + α
Pb-211 => At-215 + γ
At-215 => Bi-211 + α
Bi-211 => Tl-207 + α
Tl-207 => Po-211 + α
Po-211 => Pb-207 + α
It is easy to determine the half-lives of all of these elements in the radioactive series. In so doing, it is easy to see that all of the intermediate elements have very short half-lives, and therefore will not exist except as a by-product of reactions such as these. For example, taking the first series, it is possible to determine the normal distribution of Pb by analyzing a sample without any uranium in it. Then, looking at the sample containing the uranium and comparing the two, any excess lead distributed in the sample must be a result of the radioactive decay. Taking the amount in excess of lead, the current quantity of uranium-238, and the half-life of U-238, it is then possible to solve for how long the element has been decaying and how old the formation is. This same process is applicable to the U-235 and Th-232 series, which also yield forms of lead.
Unfortunately, this technique is flawed because of the possibility of chemical changes in the rock being analyzed which could alter results. It is important that the rock remain a closed system in terms of uranium and thorium for the results to be accurate. A more consistently accurate testing technique can be found in the next method, where contamination is not as likely a problem.
- Potassium-Argon: The K-40 isotope of Potassium is the only radioactive one, and it is different from the other isotopes looked at so far in that it breaks down in two different ways. It can decay into Calcium or Argon, the noble gas. Due to the structure of Argon's electron shell, it does not form compounds with other elements easily. Therefore, any pure Argon found inside a rock sample containing K-40 is most likely from the decay of that isotope. However, contamination still can occur in this dating process, still leaving the potential for flawed results.
- Rubidium-Strontium: This is a highly complex yet far more accurate form of radiometric dating. From Jonathon Woolf's essay:
"Yet a fourth method, rubidium-strontium dating, is even better than potassium-argon dating for old rocks. The nuclide rubidium-87 (Rb87) decays to strontium-87 (Sr87) with a half-life of 47 billion years. Strontium occurs naturally as a mixture of several nuclides. If three minerals form at the same time in different regions of a magma chamber, they will have identical ratios of the different strontium nuclides. (chemical processes can’t differentiate between nuclides). The total amount of strontium might be different in the different minerals, but the ratios will be the same. Now, suppose that one mineral has a lot of Rb87, another has very little, and the third has an in-between amount. That means that when the minerals crystallize there is a fixed ratio of Rb87:Sr87. As time goes on, atoms of Rb87 decay to Sr-87, resulting in a change in the Rb87:Sr87 ratio, and also in a change in the ratio of Sr87 to other nuclides of strontium. The decrease in the Rb87:Sr87 ratio is exactly balanced by the gain of Sr87 in the strontium-nuclide ratio. It has to be -- the two sides of the equation must balance."1
The techniques of radiometric dating are often contested by Creationist geologists. While certain assumptions operated under could be wrong (mostly in terms of contamination of samples), certain verifications of the practices themselves through such things as plate tectonics show that the techniques themselves are valid under the laws of atomic physics.
1 Jonathon Woolf. An Essay on Radiometric Dating. As given to me in a class lecture.
* All radioactive series courtesy of reference to my old chemistry/archaeology notes.