Tuesday, April 12, 2011

"Barely Bad Bananas" or "Where Does Background Radiation Come From???"

The totally normal banana that this totally normal monkey is eating is radioactive, but so is the monkey!

Radioactive Monkey Eating Radioactive Banana, from Arne Bevaart

But that is so confusing!  If radioactivity/radiation = scary, and bananas/monkeys = not scary, how can bananas/monkeys = radioactivity/radiation?????  Well, it turns out that whether we like it or not, we live in a radioactive world, as so wonderfully illustrated:

via xkcd, click image to enlarge

For a brief reminder about what nuclear radiation is and its relationship to other kinds of radiation, see this previous post.  In this post I will describe how/why living on earth means we are necessarily exposed to (small amounts of) high energy radiation, starting with bananas and monkeys.  I won't keep repeating it throughout, but keep in mind that just because something is radioactive or emits radiation does not necessarily mean it is harmful.

Bananas are known for their high concentrations of potassium, an element that monkeys, humans, and almost all other living things require for the normal functioning of their cells.  Most potassium on earth is not radioactive, but 0.01% of it is!  This radioactive potassium was around when the solar system formed out of ancient starstuff.  It is still around today because it releases its radiation very slowly, with a half life of over one billion years.  As the solar system is ~4.6 billion years old, that means that when the earth was formed only around 0.08% of the potassium on it was radioactive.

Another element that has been around since the beginning of our solar system is uranium--the same stuff that is used in nuclear power plants.  Uranium is found in low concentrations throughout the earth's crust, and this is the reason that living in or visiting stone, brick, or concrete buildings exposes you to radioactivity.  In fact, the concentration of uranium is so high in the granite walls of the US capitol building, that they release more radiation than is legally allowed for the walls of a nuclear power plant!  We are also exposed to radiation from uranium (and thorium) released into the environment by coal-fired power plants (though that is among the least worrisome of the health risks posed by these plants).  Uranium and thorium occur naturally in coal.

Mega huge granite mine, in Graniteville, Vermont (definitely click to see high-res).    Check out that insane staircase!!!

When a uranium atom undergoes radioactive decay, it becomes an atom of a different element, which is usually also radioactive.  One of those elements is radon, which seeps out of the ground and into our atmosphere (or our basements) then into our lungs.  Another radioactive uranium-decay product is the element radium.  We encounter radium when we eat brazil nuts, which have the dubious distinction of being the most radioactive food in the world.  These nuts contain 1000x more radium than most other foods of their size.

A Brazil Nut Tree, which due to the HUGE size of its root system, is able to concentrate relatively high levels of  radium in its nuts (tee hee).

All of the background radiation sources described so far come from radioactive elements naturally present on earth since the very beginning.  The other type of background radiation we are exposed to comes from space!!!  Most of the energy hitting earth from space comes from the sun, and most of that energy is in the form of electromagnetic radiation in the visible range (that's why we evolved to see it!!!).  The sun also sends us some higher energy electromagnetic radiation (i.e. gamma rays and X-rays), but lucky for us, that radiation is blocked by our atmosphere.  The cool picture below shows what kinds of electromagnetic radiation make it through our atmosphere.

Only visible light and radio waves make it through our atmosphere very well.  click image to enlarge.

Even though our atmosphere blocks most of the harmful (ionizing) electromagnetic radiation coming from the sun, that's not the only thing the sun sends our way.  It also bombards us with very fast moving tiny particles known as cosmic rays.  Cosmic rays are essentially little parts of the sun, as they consist primarily of protons and alpha particles (a.k.a. hydrogen and helium atoms without their electrons).  Some of the energy released when these particles slam full speed into our atmosphere can be seen near the northern and southern poles of earth in the Aurora Borealis and Aurora Australis:

Aurora Australis.  Created when charged cosmic rays are funneled to the south pole by the earth's magnetic field where they slam into oxygen and nitrogen atoms in our atmosphere.

Even though the cosmic rays are slamming into our atmosphere (and not our bodies), we are still exposed to the energy that results from this collision.  When a single cosmic ray hits our atmosphere, as many as a billion (!!!!) other particles can be created (including radioactive carbon atoms, which is why carbon dating works).  In addition, high energy electromagnetic radiation like gamma rays can be created by these collisions low enough in our atmosphere that the gamma rays actually reach us down on the surface of the earth.  It's these particles and gamma rays that contribute to a significant portion of the background radiation to which we are exposed.
Brief nerd-out: in fact some cosmic rays are sooooo energetic, traveling at near the speed of light, that there is no way they came from our sun, and scientists aren't yet sure exactly where they come from at all!
When you go higher in the atmosphere, there is less of its protective oxygen and nitrogen atoms to block the radiation coming from space.  Therefore you receive radiation at faster rates when you are in the mountains (~2x compared to sea level), flying on planes (~10x), or in the international space station (~100x).  In fact, it was first established that cosmic rays were coming from space by bad-ass scientist Victor Hess, who in 1912 carried a radiation counter up in a balloon and detected increasing levels of radiation as he rose to over 17,000 feet.  Now that's what I call science!

Victor Hess, bad-ass scientist.

Bananas, brick houses, brazil nuts, bedmates, and our beautiful sun... they all expose us to radiation.  Again, not generally enough to worry about, but that doesn't mean its not totally crazy!!!

Baby ape going ape about how cool background radiation is.

Related Post:
What is Nuclear Radiation and How Can It Hurt Me?