If you were radioactive, would you glow?

Dear Cecil:

After watching a campy mid-1950s science fiction movie recently, I was left wondering: how radioactive must something be to begin glowing? And could a living creature become that radioactive and survive, even briefly?

Cecil replies:

Radioactive stuff doesn’t glow, muchacho. Hollywood screenwriters just think it does. (Frail creature that I am, I admit to having helped perpetuate this myth.) High-energy radioactive particles sometimes cause other stuff to glow, but that’s the exception, not the rule. For example:

Cerenkov radiation. Perhaps you’ve seen depictions of the eerie blue glow emanating from spent nuclear fuel that’s stored underwater. That’s Cerenkov radiation. It occurs when beta particles (electrons) travel faster than the speed of light.

You reply: Say what? I thought nothing could travel faster than the speed of light.

Not exactly — nothing can travel faster than the speed of light in a vacuum, c. However, in translucent media, notably water, light travels much slower, at maybe 75 percent of c. A beta particle traveling through air, say, moves considerably faster than light traveling through water.

Now suppose a beta particle enters the water. What happens? It throws up a shock wave of photons, much as a boat plowing through water creates a bow wave or a jet creates a sonic boom. It gets a little complicated after that, but basically constructive interference between wave fronts generates visible light. Note that the radioactive stuff isn’t what glows, nor does the water glow once the radioactive stuff is removed.

Fluorescence. When certain compounds are struck by radiation, they glow. For instance, glow-in-the-dark watch dials used to be painted with a mixture of radium and zinc sulfide. Radiation from the former caused the latter to fluoresce. There’s nothing magical or dangerous about fluorescence; it can be caused by lots of things, including ordinary sunlight.  The radioactive material itself emits no visible light. Madame Curie, who discovered radium, talked about watching the stuff glow in the dark, but the light was emitted by minerals mixed up with the radium, not the radium itself.

Bremsstrahlung. When a charged particle speeds up, slows down, or changes direction, it emits bremsstrahlung radiation. Typically bremsstrahlung consists of invisible X rays, but I’m told that under certain circumstances it can be visible, making it the closest thing to a glow arising from radioactivity itself. Even so, an intervening medium is generally required to speed/slow/divert the charged particle.

None of these phenomena is going to make you or any living creature glow. If you were to tarry near a spent fuel canister bathed in Cerenkov radiation, you’d receive a lethal dose in seconds. You wouldn’t glow, though; you’d just die. Tragic, but at least we’d put this silly misconception to rest.

Send questions to Cecil via cecil@straightdope.com.

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