When half of these neutrons have been emitted this is called the half life of the element, and thus, dictates what range it can date. Assume that this giving off neutrons occurs at a steady rate. There is a divergence about 3000 years ago. Before that the calibration has come from Uranium of ancient coral reefs. But note the areas where they cannot separate out dates.
Potassium going to Argon-goes back farthest in time.
Very slow decay of radioactive K40 to the inert gas Ar40 in volcanic rock. Its half-life is 1.3 billion years. Measure the amount of Ar 40 locked into a rock. Often used to date tephra, volcanic tuff.- hominid studies across Africa in particular. Dates things older than about 50,000 years.