Do radioactive things glow in the dark?
The short answer to your question is "no," radioactive things do not glow in the dark - not by themselves anyway. Radiation emitted by radioactive materials is not visible to the human eye. However, there are ways to"convert" this invisible energy to visible light. Many substances will emit visible light if "stimulated" by the ionizing radiation from radioactive material. These materials are known as "fluors" or "scintilators." So, by mixing some radioactive material with such a fluor, you can make a substance that glows. This kind of material has been used in things like the faces of clocks, watches, and instruments on ships and airplanes to make them visible in the dark. This is why most people think of glowing things when they think of radioactive materials.
It is also possible to "trick" radioactive material into creating visible light. This is called Cherenkov radiation. This happens when the radiation from the radioactive material goes into a material such as glass or water. Because the speed of light in this material is relatively slow (compared to the maximum speed of light in a vacuum), the radiation is actually traveling faster than light can travel in that material, and so it gives off light as it slows down. But to actually see this glow, it usually takes something which is very radioactive, such as the internal parts of a nuclear reactor. Weak Cherenkov light can be made from smaller amounts of radioactivity. Usually, sensitive devices have to be used to detect it.
Keith Welch, Radialogical Controls Group (Other answers by Keith Welch)