How Radiation in Smoke Detectors Keeps You Alive

Radiation so often gets a bad rap. Obviously, no one wants to swallow a chunk of uranium, but some radiation isn't just harmless, it can keep you alive. Hopefully, most of you are sitting not-too-far from a piece of decayed plutonium right now - your smoke detector.

Many of us have reason to fear plutonium. It's radioactive. It makes bombs. It's named after the god of death. There's a lot of sinister stuff going on with that element. On the other hand, a specific descendant of plutonium has probably saved thousands of lives.

The trouble-making plutonium is the isotope 239, with 94 protons and 145 neutrons. There's another isotope of plutonium, Plutonium-241, with two more neutrons. It starts a process which leads to safer homes everywhere. Under the right circumstances, plutonium can actually climb up the periodic table. One of its neutrons undergoes beta-decay, otherwise known as the spontaneous ejection of an electron and an antineutrino. This turns the neutron into a proton, and changes the Plutonium-241 into Americium-241.

How Radiation in Smoke Detectors Keeps You Alive

Now armed with 95 protons and 146 neutrons, Americium-241 goes down another decay pathway. Every so often it shoots out an alpha particle - a grouping of two protons and two neutrons. Basically, an alpha particle is a shaved helium atom, free of electrons. As anyone who has worked with balloons knows, helium isn't a fearsome element. As long as a material that emits alpha particles isn't swallowed, it can't do much damage to a body. The particles can't even penetrate the top layer of dead skin cells. What it can do is knock the electrons off existing atoms. The atoms become positive ions, and free electrons.

If these ions and electrons are in an electronic device, and they feel a voltage across two metal plates, they can move between those two plates, creating an electric current. So a hunk of Americium-241 can keep a current going in, for example, a smoke detector. That is, it can keep it going until smoke gets in the detector. Smoke grabs hold of the ions, stopping their movement between the two metal plates, and cutting off the current. A smoke detector will then start beeping away, letting people know that they've either burned the roast, or that they need to scramble out of the building as soon as possible. So while radiation isn't always a source of joy, under the right circumstances, it can keep you alive.

[Via EPA, The Particle Adventure, The Naked Scientists, World Nuclear Association.]