Carbon-14 decay is the best example of a top-only hourglass. Carbon-14 is constantly being produced in the upper atmosphere from atoms of nitrogen-14 being struck by neutrons that had their origin in [COSMIC RAYS]. The reaction is written:
¹⁴N + neutron → ¹⁴C + proton
Radioactive decay then follows, with a half-life of 5800 years[9] for the ¹⁴C.
¹⁴C → ¹⁴N + electron ([BETA PARTICLE])
The radiocarbon emits an electron and changes back into nitrogen.
As far as anyone can tell, ¹⁴C was produced at a constant rate above the earth for at least 50,000 years before the first atomic bomb was exploded. In other words, the ¹⁴C cycle is like an hourglass in which the sand in the upper part is replenished as fast as it runs out through the hole in the waist. A process of this sort, where production equals decay, is called a [SECULAR EQUILIBRIUM].
The newly produced ¹⁴C soon is evenly mixed with the carbon dioxide in the air, is taken up by all living plants, and then finds its way into all living animals. In effect, all carbon in living organisms contains a constant proportion of ¹⁴C. If any of this carbon is taken out of circulation—when a tree branch is broken off, for instance, or when a shellfish dies in the ocean—no more new ¹⁴C is added to that particular system, but the old ¹⁴C continues to run out. In effect it now starts measuring time as an hourglass should.
To illustrate secular equilibrium, one must imagine an hourglass in which the sand in the top bulb is continuously replenished—as fast as it runs out through the hole in the waist and disappears.
When we find a piece of charcoal in a cave or a piece of wood in some ancient structure, for example, we can measure the amount of carbon in it, determine how much of it is ¹⁴C, and then calculate back to the time when the radioactivity from the ¹⁴C was the same as we now find in living wood. In other words, if we assume that we know from the observed secular equilibrium how much ¹⁴C originally was present in living material, then we can calculate the time of death of any similar but ancient material. That is the basis of the ¹⁴C method of age determination.