Why Cooling Radon Makes It Glow

Illustration for article titled Why Cooling Radon Makes It Glow

Radon is dangerous mostly because we don’t notice it. We can buy detectors, but we’re not equipped with any of them. The gas is colorless, odorless, and tasteless. You can’t see it... because it’s too hot. As radon cools down, it starts to glow. And it’s tough to say why.

When radon is cooled into a liquid, which happens at -62 degrees celsius, it takes on a glow. The glow has been described as blue, blue-green, or even lilac. It’s probably due to a phenomenon called Cherenkov radiation. We say that nothing goes faster than the speed of light, but truly, nothing goes faster than the speed of light in a vacuum. When light goes through media, like air, it can slow down considerably. It can even slow down so much that, when radioactive elements send out particles, those particles travel faster than the speed of light in that medium. This sets off cascades of photons.

But radon isn’t done. When it becomes solid at -71 degrees, it changes. The glow it gives off turns yellow, and then, as it cools, dark red. Apparently “characterization” of this glow is difficult because of the hazards and complications of working with a material that needs to be very cold and that is very radioactive.


Still, the best way to get any given quantity of radon to glow is to cool it down. And the best way to switch off the light is to heat it up until it becomes a gas.

[Source: Descriptive Inorganic Coordination and Solid State Chemistry, Noble Gases]

Image: Darren Hester.

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Older SF stories had throwaway lines about atomic light bulbs - I think there’s one in “Foundation”. Now we know how that could actually be done! If you kept it under pressure, it could be liquid at room temperature, although its half-life is unusably short. The SF authors were probably thinking of something powered by radium, though. A radon-powered bulb would be a lot more dangerous still...