Out of every trillion normal carbon atoms in your body, there’s a single atom of carbon 14. And while that chemically acts like normal carbon, it’s a bit radioactive.
Carbon 14 is created when cosmic rays – mostly high-speed protons zooming here from distant parts of our galaxy – strike nitrogen in our atmosphere. This changes it to “heavy” carbon, an unstable atom with two extra neutrons. As your body’s carbon 14 decays, it gives you a radiation dose of one millirem per year. That’s about 1/360th of the natural radiation that you receive annually: nothing to worry about.
It’s less than what you get from eating bananas. The potassium radioactivity of a single banana fully equals one percent of your daily natural exposure. It’s enough that the radioactivity from a truckload of bananas has set off alarms when passing through Radiation Portal Monitors, now routinely used at US ports to detect nuclear material. Still, you’d need to gobble down 70,000 bananas to equal the radiation from one CT scan.
But let’s stick with radioactive carbon. There’s normally 50 tons of it in our atmosphere. When you dine on plants or animals or both, you’re continually taking into your body the same ratio of carbon 14 that is found in the air and everywhere on Earth. But testing of atom bombs in the 1950s significantly increased atmospheric carbon 14; for about eight years the air had 51½ tons of it. Thus, if you were born in the ’50s, you have much more carbon 14 in your tooth enamel and elsewhere in your body than the rest of us. So even if you want to keep your age a secret, “they” could find out by testing you.
Half of any batch of carbon 14 decays in 5,740 years. When an animal or plant dies, it stops breathing and therefore stops replenishing its carbon 14. A corpse’s normal carbon 12 lasts forever, but its carbon 14 continually diminishes. After 5,700 years half of the carbon 14 is gone, and after 23,000 years only one-eighth of it remains. Thus, measuring a body’s ratio of normal carbon to carbon 14 lets you ascertain the age of anything that was once alive, including clothing. Instead of having to rely on theories, we have a scientific way of dating things.
An additional wrinkle is that supernova explosions hurl additional cosmic rays in our direction, which briefly create extra carbon 14 in our atmosphere. So as a bonus, we can tell how often stars explode by paying attention to strange spurts of increased carbon 14 in plants and animals that died long ago.
This whole business incorporates an unlikely blend of biology, physics, weaponry, medicine and astronomy. It illustrates, once again, that science is really cool – don’t you think?