7.8 How do we know ... how to determine half-lives and decay rates?
Fig 7.23
40K-40Ar is actually two separate decay reactions as shown in Fig. 7.23. The measurable range by this method is 1.25 billion years. These rates are determined by measuring the specific energy emitted by the decays in a unit of time. This is done numerous times and, as shown, is repeated by many researchers, to get an averaged, accepted value.