Living organisms are constantly incorporating this C-14 into their bodies along with other carbon isotopes.
When the organisms die, they stop incorporating new C-14, and the old C-14 starts to decay back into N-14 by emitting beta particles.
But I have news for you: your characters and your plot make less sense when you take these shortcuts. This is what is known as the “cosmic ray air shower”.
Radiocarbon dating can easily establish that humans have been on the earth for over twenty thousand years, at least twice as long as creationists are willing to allow.
Therefore it should come as no surprise that creationists at the Institute for Creation Research (ICR) have been trying desperately to discredit this method for years.
Creationists often criticize radiocarbon dating in the context of discussions of the age of the earth.
But, as is clear even from the very brief discussion in the previous paragraph, radiocarbon dating can say nothing one way or the other about whether the earth is many millions of years old, since such dates are far beyond this method's range of resolution.