Radium watch dials put the idea of glowing green radioactive elements in the public consciousness, but radium doesn’t actually glow. If you want to see the glow of radiation, take a look at the blue light coming off actinium.
Technically, actinium, or more specifically actinium-227, doesn’t glow on its own. What it does is emit massive amounts of electrons in a process known as beta decay. In doing so, it puts an odd kink in the chain of radioactive decay. Elements like uranium don’t emit radiation and then stop. Many of them decay into “daughter products” that are also radioactive. In this case, uranium decays to thorium-232, which has 90 protons and 142 neutrons. Thorium-232 decays by kicking out a helium nucleus, two protons and two neutrons, becoming radium-228. One of those neutrons decays, sending out an electron and turning into a proton. The element climbs up the periodic table, becoming actinium-228, and the star of this particular article. Actinium-228, which has 89 protons, pulls the same trick, kicking out an electron again. This makes it climb up the periodic table to what it originally was – thorium. The new thorium doesn’t have the same amount of neutrons it once did, but it did reconstitute itself. (It will later decay more, slipping farther and farther down the periodic table once more.)
So, what does this decaying particle’s brief time as actinium yield? Actinium is, according to physicists, “about 150 times as active as radium, making it of value in the production of neutrons.” It’s sending off more electrons than radium – so many that it’s disturbing the air around it. The blue glow is the energy given off by the oxygen that actinium briefly ionizes. So, technically, if you put the element in a void, it wouldn’t glow. But if you ran across it in a sewer, and saw a bunch of baby turtles wallowing in it, it is one of the few elements that would let you know, just by looking, that they were being bathed in radiation.
Image: Pieck Darío.