Question

How is carbon-14 is produced in the upper atmosphere?

Answer 1

Carbon 14, also known as radiocarbon, is produced in the upper atmosphere as a bi-product of cosmic ray collisions.

Explanation:

Cosmic rays are high energy particles traveling through space. They are emitted by stars, supernovas, black holes, etc, and can be problematic for astronauts who spend extended amounts of time in space.

Cosmic ray collisions with particles in our atmosphere, are similar to collisions that take place in particle accelerator labs. They produce a spray of high energy particles, many of which decay into daughter particles, some of which may decay even further.

Typically, high energy neutrons are emitted by these collisions. When these neutrons collide with nitrogen (`""^14N`) atoms, which are abundant in the atmosphere, a proton is emitted, resulting in the formation of radiocarbon (`""^14C`).

`n + ""^14N -> ""^14C + p`

Once `""^14C` is produced, it reacts the same as stable `""^12C`, quickly bonding with oxygen to create `CO_2` molecules. The `CO_2` is then introduced into the food chain via photosynthesis.

The amount of `""^14C` inside of a plant or animal is continuous as long as it's alive because the same ratio of `""^14C` to `""^12C` is constantly being cycled through it. Once the creature dies, however, no new `""^14C` is introduced, and the existing `""^14C` decays with a half life of `tau ~~ 5000 "yrs"`. That's why radiocarbon dating can be used to approximate the age of a fossil.