Consequently, the amount of it found in rocks is negligible — unless you subject them to an artificial neutron source.
A crucial point to note is that because K are isotopes of the same element, they have the same chemical properties.
Now the bad news is that there is no way we can somehow manipulate this data to give us a correct date for the sample.
But the good news is that we do know that there's a problem; whereas if we'd analyzed the same rock using the K-Ar method, then it would have supplied us with a date and there'd have been no sign in the K-Ar data of anything wrong with it.
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.
Or if we consistently get one date for the steps below (for example) 400°C, and consistently get another date in the steps above 400°C, then it seems as though argon loss occurred as a result of metamorphism at a temperature of about 400°C, with the younger date representing the date of the metamorphism, and the older date representing the formation of the rock; and we can investigate this clue further by looking for other evidence of the metamorphic event.
And if the dates we get are all over the place, then we are probably looking at excess argon.
However, the Argon, a noble gas, constitutes approximately 0.1-5% of the Earth's present day atmosphere.
However, if you put it near the core of a nuclear reactor, so that it is bombarded by neutrons, then this will convert it into Ar.
This isotope of argon is quite unstable, having a half-life of only 269 years.
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.