In the first place, recall that one of the potential problems with the K-Ar method is that it requires two different samples, one to measure the potassium and the other to measure the argon; if the two samples had different chemical compositions when they first formed then this will introduce an error.
However, in Ar-Ar dating the two isotopes of argon are both measured from the same sample, and so at least one potential source of error is eliminated.
The reader should be thoroughly familiar with the K-Ar method, as explained in the previous article, before reading any further.
So now we know J, and we have measured the R-value of the sample we're actually interested in dating, so we can use these data to solve the equation for t, giving us the age we're looking for.
You will note that this means that we have to be able to date some rocks accurately using some method other than Ar-Ar, so that we can find a standard to use for the determination of J; fortunately we can do this, and geologists have put a lot of effort into identifying rocks which can be accurately dated and used as standards.
Therefore when the rock first forms, some of the minerals in it will have more potassium in and some less, but all the minerals will have the same initial ratio of But what is J?
J is a factor which depends on the nature of the neutron bombardment.
Sources of interference with the ideal mechanism are investigated and with the exception of argon produced from isotopes of calcium, are shown to be negligible.