It Don’t Necessarily Glow, Bro!

16 July 2008
Posted by Chris Smith.

Radioactive Sign"Simpsons" fans will know only too well the opening sequence to the cartoon in which Homer discovers, during his commute, that he's taken some of his work home with him - in the form of a radioactive fuel rod from the nuclear power plant!

Unsurprisingly, the lump of material he subsequently throws out of the car window is glowing an ethereal green colour. But therein lies multiple myths of atomic-powered proportions, because most radioactive substances don't really glow at all, let alone light up green!

The basis of this belief stems from the late 1800s and early 1900s when the Polish pioneer of radioactivity, Marie Curie, working in Paris with her husband Pierre, discovered the element radium, which they named after the Latin word "radius", meaning a ray.

Once they began to isolate the metal in reasonable amounts, one of the first things the Curies noticed was that it appeared to emit an attractive blue glow. Marie described it as "fairy-like", and even kept a tube of radium beside her bed where it presumably functioned as an attractive radioactive night light!

But contrary to popular belief, the glow from the tube wasn't the radiation per-se, but rather the effect it was having on other chemicals that were also present. This is because radium releases alpha particles. These are positively charged and consist of two protons stuck to two neutrons. When they pass through a material they pull negatively-charged electrons away from the nearest atoms. Most of these excited electrons subsequently fall back to their starting positions, releasing as they do so the extra energy they gained but this time in the form of light, some of which is in the visible part of the spectrum.

Chemicals that behave like this, by emitting visible light when they are excited, are referred to as being "fluorescent". Marie Curie, circa 1898And as it turns out, Marie Curie's bedside sample contained the compounds radium bromide and uranium chloride, both of which are slightly fluorescent. So when they were excited by the radioactive decay of the radium itself, the compounds glowed, producing the fairytale blue light.

So where does the idea that radioactive chemicals glow green actually come from? The most likely source was the development in 1908 of a paint containing zinc sulphide, doped with a small amount of copper. This mixture glowed bright green when it was exposed to radioactivity, and adding a small amount of radium to the paint was a cheap way to provide a long-lasting supply of radiation capable of keeping the paint illuminated. Over-night, quite literally, reliable glow in the dark products were born.

Scientists subsequently tinkered with the chemicals to produce a range of different colours, but the greens were the easiest for the eye to see, so those were the ones that stuck, together with the myth that radiation glows in the dark!

Thankfully, today, safer alternatives to radium have been found, together with better materials that can soak up the energy in natural light and release it very slowly to produce glow-in-the-dark products that shine for much longer. But radiation-powered fluorescence still has its place however, mainly on the hands of certain expensive watches and in military equipment, although these tend to use the safer substance tritium to achieve what radium once did...

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