By carbon 14 dating
Radiocarbon then enters animals as they consume the plants (Figure 1b).
So even we humans are radioactive because of trace amounts of radiocarbon in our bodies.
The radiocarbon half-life or decay rate has been determined at 5,730 years.
Next comes the question of how scientists use this knowledge to date things.
Since each beta particle represents one decayed carbon-14 atom, we know how many carbon-14 atoms decay during a month.
Chemists have already determined how many atoms are in a given mass of each element, such as carbon.4 So if we weigh a lump of carbon, we can calculate how many carbon atoms are in it.
We can measure in the laboratory how many carbon-14 atoms are still in the skull.
If we assume that the mammoth originally had the same number of carbon- 14 atoms in its bones as living animals do today (estimated at one carbon-14 atom for every trillion carbon-12 atoms), then, because we also know the radiocarbon decay rate, we can calculate how long ago the mammoth died. This dating method is similar to the principle behind an hourglass.6 The sand grains that originally filled the top bowl represent the carbon-14 atoms in the living mammoth just before it died.
Through photosynthesis carbon dioxide enters plants and algae, bringing radiocarbon into the food chain.The standard way of expressing the decay rate is called the half-life.5 It’s defined as the time it takes half a given quantity of a radioactive element to decay.So if we started with 2 million atoms of carbon-14 in our measured quantity of carbon, then the half-life of radiocarbon would be the time it takes for half, or 1 million, of those atoms to decay.These excited neutrons then collide with nitrogen atoms in the atmosphere, changing them into radioactive carbon-14 atoms.CARBON-14 IS ABSORBED (Figure 1b): Plants absorb this carbon-14 during photosynthesis.