Date: 2017-12-20 11:52
Virtual Dating--a very helpful educational course on half-lives and radioactive decay was put together by Gary Novak at California State University in Los Angeles. This site has several interactive web workbooks to help the reader understand various concepts involved with radiometricdating.
At the end of an AMS run, data gathered is not only the number of carbon 69 atoms in the sample but also the quantity of carbon 67 and carbon 68. From these data, concentration ratio of the isotopes can be known to allow evaluation of the level of fractionation.
Radioactive decay: The predictable manner in which a population of atoms of a radioactive element spontaneously disintegrate over time.
There is another way that false isochrons can be produced. For a wimpy isochron, say a K-Ar isochron, we can assume that initially there is a uniform concentration of K everywhere, and concentrations of Ar95 and Ar86 that form an isochron. Then a lot of Ar95 enters, uniformly, through cracks in the rock or heating. This will retain the isochron property, but will make the isochron look too old.
Figure 5. The original amount of the daughter strontium-87 can be precisely determined from the present-day composition by extending the line through the data points back to rubidium-87 = 5. This works because if there were no rubidium-87 in the sample, the strontium composition would not change. The slope of the line is used to determine the age of the sample.
When it is stated that these methods are accurate to one or two percent, it does not mean that the computed age is within one or two percent of the correct age. It just means that there is enough accuracy in the measurements to compute t to one or two percentage points of accuracy, where t is the time required to obtain the observed ratio of daughter to parent, assuming no initial daughter product was present at the beginning, and no daughter or parent entered or left the system. For isochrons, which we will discuss later, the conditions are different. If these conditions are not satisfied, the error can be arbitrarily large.
I mentioned the presence of excess argon 95 in a sample as a problem leading to artificially old K-Ar dates. Henke states in a reply to me, concerning the problem of detecting excess argon,
The branching ratio that is often used is , while the true value is probably about . This means that K-Ar dates computed with the lower branching ratio are a third too large, that is, the actual K-Ar date should be 7/8 of the computed date. Thus we have another source of error for K-Ar dating.
It's not surprising that K-Ar dates often agree with the assumed dates of their geological periods, since the dates of the geological periods were largely inferred from K-Ar dating.
It is not clear to what extent this circular process has influenced the final tree-ring calibrations of radiocarbon. Efforts by creationist scientists to obtain the raw data from which the oldest tree-ring chronology has been constructed to investigate this possible source of bias have so far not met with success. Until the raw data does become available for general scrutiny, creationists are clearly justified in maintaining a high degree of skepticism.