Introduction to Radiometric Dating Techniques
The third of three pages designed to give the reader a background in the concepts of Radiometric Dating Techniques
If you are having problems understanding terms such as half-life, Isotopes, Nuclides, nucleon, mass defect, Nuclear Binding Energy, and various Atomic Symbols See the Atomic Structure Page.
If you are having problems understanding concepts such as Average Nuclear binding Energy and nuclide stability; What is it that drives fission; fusion; and other nuclear reactions; Types of radioactive decay, alpha, beta, gamma, positron, and a summary of characteristics; Nuclear reactions; Nuclear equations; The use of nuclide charts to visually chart out nuclear reactions; The U238 decay series shown on a nuclide chart. See the Nuclear Reactions Page.
Half-life of radioactive nuclides
Since Radiometric Dating Techniques are based on the half-life phenomena of radioactive nuclides, lets explore it a little.
Radioactive atoms are unstable so they decay into a something else. The rate that atoms decay or break down is not constant. The rate changes and it is dependent on how many radioactive atoms are in a sample.
If all radioactive atoms have the same chance of breaking down we might expect that the more atoms present, the more atoms would be breaking down at any one time. This is exactly what happens.
However something interesting happens. It doesn't matter how much radioactive material we start with, if we stick with the same radioisotope, such as Carbon 14, it will always take the same amount of time for one half of the radioactive material to turn into something else. It's a first ordered reaction which means that it doesn't matter how much material we start with, we always will have the same half-live. So we will have half of what we started with when that half-life is reached.
The Half-life is defined as the amount of time required for one-half of a sample to decay to a new substance. For Carbon 14 it is always 5730 years. For Carbon 15 it is always 2.25 seconds. For Uranium 238 it is always 4,500,000,000 years. Each different nuclide has a different half-life but the half-life of each specific nuclide stays constant and as far as we can tell, it never changes.
The Chart (left or above) shows what happens to one gram of Carbon over a greater amount of time than just one half-life. The effect is compounded. After one half-life there is 1/2 present, but after two half-lives 1/2 of 1/2 (or 1/4) is present, and after three half-lives 1/2 of 1/2 of 1/2 (or 1/8) is present, and so on. So as we count the half-lives in time we see the amount C14 decline from the original gram of material to 1/2 gram, to 1/4 gram, to 1/8 gram, to 1/16 gram, etc. The loss of C14 is high initially but than slows down thus allowing the half-life rule to work throughout the whole time period. Every 5730 years, half of the C14 that was left, is lost.
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Some nuclides emit more than one type of radiation
As we can see, every isotope or nuclide that is radioactive in that it gives off alpha, beta, gamma, or even positron has its own half-life. There are some nuclides that give off more than one type of particle. In the graphic below, look for potassium. It is the first top two nuclear reactions listed in the graphic below.
Potassium can either give off a beta particle or a positron particle. Actually, it also has electron capture (e.c.) which produces the same change as a positron decay. So we can see that potassium 40 gets converted over to argon 40 or calcium 40 depending on the particle that is released. There are two outcomes: beta decay to produce Calcium 40; or positron decay (and electron capture) to produce argon 40.
Also notice that the half-life is different for these two processes. Potassium 40 breaks down to Calcium 40 a little faster then Potassium 40 breaks down to Argon 40. We can determine this be seeing that the half-life for Potassium 40 to Calcium 40 is a shorter time them the half-life for Potassium 40 to Argon 40.
Potassium 40 breaks down to calcium 40 almost ten times faster than it breaks down to Argon 40. This is because the half-life for the production of Argon 40 is almost 10 times longer in time.
Half-lives of Radio-nuclide Geochronometers
There are certain nuclides in nature that are of special importance when it comes to dating techniques. Scientist use them to date the rock surrounding fossils because they seem to work well, producing numbers that they expect (more on this topic later).
The potassium argon determination is extremely important in that it is widely used. Most of the others listed bears mention since they are also used, however they are less important. On the other hand, the rubidium strontium determination is used for rocks that seem to be very old, like the basement rocks of Earth, Moon rocks and meteorites.
The first four examples in the graphic to the left or above, are mostly a single beta decay. The other examples, (Lead-210, U238, U235, and Th232) are a series of nuclear reactions (decays). They are chain reactions like dominoes that kids play with. Often kids will set dominoes up so that when the first one is knocked down all the others will be pushed over in a chain reaction, in a series of events. The two next graphics below illustrate these four series.
The graphic to the left or above shows three series. Both U235 and U238 are labeled. Pb210 (lead-210) is actually within the U238 series. Look for it.
If you look at these two series, U235 and U238, you can easily identify which steps are releasing an alpha particle, and which are releasing a beta particle. As was discussed in previous pages, the steps that go up and to the left are beta decay reactions. The steps that go to the left and down two places are alpha decay reaction.
Looking at the graph, you can see that there are a number of steps that have two different daughter products. Both alpha and beta decay occur with the same nuclide. We can see that there is one such nuclide in the U235 series, Ac-227. For the U238 series, we can see that there are three nuclides that produce both alpha and beta particles. In spite of this phenomena, we can see that no matter which branch a reaction goes in the series, it still gets to the same end product (daughter product).
The graphic to the left or above, shows the Thorium-232 series. Again we can easily determine which are the alpha and which are the beta reaction steps. There is only one nuclide that produces both alpha and beta particles, and that is Bi-212.
A nuclide chart is a great way to view a decay series because in a single glance we can visualize the whole chain reaction. In addition, it is easy to determine which step is an alpha reaction and which step produces beta particles.
Click on the K/Ar Dating button to go to the next Page.