Do radioactive substances glow?

It really is a widespread myth that radioactive substances glow in the dark, or even in broad daylight if "power" is high enough. So much so that, in the hospital, more than one patient has doubted the operation of the radiotherapy accelerator because they did not see a bright beam coming out of the equipment. I guess people picture a beam of ionizing radiation resembling a lightsaber like the ones Yoda, Obi-Wan, or Luke Skywalker used in their fight against the Dark Side of the Force.

Another of the great icons of our time and my generation, my beloved Homer J. Simpson, also contributes his bit to the general belief that radioactive substances shine. One only has to look at the opening sequence of the series, when Homer is handling a uranium rod (which he finally takes with his clothes and throws it out the car window).

Radioactivity is, in the vast majority of cases, imperceptible, so we can affirm without too much fear of being wrong that if a substance, object or material is radioactive, it does not shine, even in the dark. If that were the case, and since the rocks or ourselves contain traces of radioactive material, the earth, animals, and plants would glow in the dark, at purest Mr. Burns style after dozens of years working at his power station.

The reality is very different: by the mere fact of being radioactive, the energy of the emitted radiation is so high that it becomes invisible to the human eye. To understand it, all you have to do is turn to the electromagnetic spectrum. The visible section (by the human eye) of the spectrum is only a small part of it, and it is concentrated in the low energy zone. Ionizing radiation, on the contrary, is located in the high energy part, beyond the ultraviolet and, therefore, not visible. It is precisely because it is so energetic that it becomes dangerous for humans and, contrary to popular belief, it is further away from human visibility the more energetic it is.

Now, the myth of the glow of radioactive substances, like almost all popular beliefs, has a historical basis. So where does the idea come from that radioactive substances are glowing, or that if you ingest something radioactive you're going to glow in the dark, like Radioactive Man? There are two physical effects responsible for this belief being so widespread: radioluminescence and Cerenkov radiation. In this post we are going to try to shed some light (pun intended) on both effects to help understand why certain glows are produced in the presence of which radioactive substance.


At the end of the 19th century and the beginning of the 20th, the Curies discovered an unusual fact (and which, according to them, violated Carnot's principle): the radiation emitted by polonium and radium compounds made platinum-cyanide slightly luminescent. of barium. Therefore, the issue of the glow of radioactive substances has been present in popular belief since the very discovery of radioactivity itself.

This effect of producing light in a material by bombarding it with ionizing radiation is known as radioluminescence and can be used as a low-level light source to illuminate certain objects, clocks or road signs without the need for external power sources. . On a physical level, the incident radiation particle interacts with an atom of the white material, exciting an orbital electron. Said electron returns to its original state of lower energy by emitting a photon with the “excess” energy, which depends on the material. In general, this photon is outside the visible range, but by choosing a suitable material, such as phosphor or zinc, photons visible to the human eye can be produced, thus releasing the material a certain color glow.

The popularity of the “green glow of radioactivity” spread like wildfire when it was discovered that if you added a little bit of radium to certain types of paints, they would take on an extraordinary greenish luminosity, even in the dark. This brightness penetrated society so deeply that even today more than one large paint company calls the brightest green in its color palette “radioactive green”. So someone had the idea of decorating clocks and walls with this strange mixture (patented by the US Radium Corporation as Undark paint) to bring out the vividness of the colors. Furthermore, if manganese was added, the luminescence was orange, thus creating a whole wide palette of “radioactive” hues. However, what glows is not the radioactivity of the radium as such, but a reaction (radioluminescence) that occurs when the radium is mixed with copper and zinc sulfide contained in the paint itself.

Unfortunately, far from remaining a mere luminous anecdote, this invention led to the death of more than a hundred women, the so-called "radio girls", workers who applied layers of this paint in watch factories, and who died after suffering kidney, bone or facial cancer problems since they ingested a large amount of radium by moistening the brush with which they worked with their own tongue. What's more, ignoring the real problem of ionizing radiation, some used leftover paint chips to highlight their nails and lips. What was really dangerous was not the bright green light emitted by zinc sulfide (completely harmless) but the invisible radiation generated by radium.

Don't be scared if you have some radioluminescent object at home that glows in the dark, like bracelets or watches. Rest assured, you are not radiating without realizing it. Currently, the use of radium to produce phosphorescence is prohibited, with tritium (an unstable isotope of hydrogen) being the only radionuclide legally permitted to be used as a radioluminescent light source. Tritium emits beta particles that interact with the phosphor molecules that coat the interior of the container to emit a greenish light and it is used because it does not present a radiological risk to the user since the energy of its electrons is insufficient to pass through the glass tube in which it is contained, being, in any case, incapable of passing through human skin. In addition, its relatively high half-life (12.3 years) makes the useful life of the radioluminescent source very high. But it should be remembered, once again, that what shines is not the tritium, but the excitation of the phosphor, which is obviously not radioactive. Tritium as such (like radium) is not shiny if no specific substance is added to it.


As such, we could spend hours and hours explaining the Cerenkov effect, but since it is not the original idea of this post, we will simply say that the Cerenkov effect or radiation is visible electromagnetic radiation produced by the passage of electrically charged particles in a medium at speeds greater than the speed of light of that same medium. Something similar, and perhaps more easily understandable, happens with the sonic boom: a shock wave is produced in the air when a certain object (such as Concorde or military fighters) exceeds Mach 1 speed.

Don't panic, we are not throwing Einstein's relativity to the ground, the speed of light is still a physically insurmountable limit IN A VACUUM (the famous figure of 300,000 km per second). In other media this speed is lower. To give an example, in water (at 20 ºC), the speed of light is approximately 225,000 km per second which, even though it is a very high value, corresponds "only" to 75% of the speed of the light in a vacuum and, therefore, it may be the case that a certain particle can travel at a higher speed in said medium. Of course, not any particle (it has to be charged) or any medium (it has to be dielectric) is valid.

This radiation, in addition to earning Pavel Cerenkov the 1958 Nobel Prize in Physics (together with Frank and Tamm who provided theoretical support for their discoveries), is the cause of a beautiful bluish glow typical of nuclear reactors, both in swimming pools , in the open vessels of light water or in the deactivation pools of said reactors. Indeed, the theoretical formulation of Cerenkov radiation predicts that Cerenkov radiation is most intense in the ultraviolet and blue range of the spectrum, as is observed in nuclear pools.

Spent fuel rods from nuclear power plants are at a very high temperature, which is why they are stored in pools called SFPs (Spent Fuel Pools), which serve as a coolant for the residual heat produced by the disintegration of uranium. and the rest of its radioactive chain, whose half-life is very high. These pools are what emit that characteristic bluish light, which leads us to think that the radioactivity is bright, but what is shining is not the radioactivity as such: the particles emitted by the fission products (mainly electrons) move faster than the light in the medium that covers them, which is usually water, producing, as we have said, Cerenkov radiation. That is, what shines, properly speaking, are the water particles themselves. If we were to extract a bar of uranium from the pool and dry it properly (I recommend you not to do so) the pellet would stop glowing, although, logically, it would continue to emit radiation that would again be undetectable to the human eye.

The bluish glow of Cerenkov radiation, apart from producing a really beautiful effect, is extremely useful on different occasions, such as, for example, to measure the speed of a charged particle in a certain medium or as a tracer in certain particle detectors. One of these detectors (and perhaps the most famous) is the Super-Kamiokande, a heavy water neutrino detector.

This effect also gives its name to a type of telescopes (Cerenkov telescopes), which take advantage of the Cerenkov radiation produced by the entry into the atmosphere of very high-energy cosmic gamma rays from space. Although gamma rays as such do not have an electrical charge, the interactions they produce with atoms in the Earth's atmosphere produce a cascade of charged particles that initially travel at a speed greater than that of light in this medium, producing Cerenkov radiation, that can be detected by this type of telescope, such as the HAWC (High Altitude Water Cerenkov) or the “Spanish” MAGIC (Major Atmospheric Gamma-ray Imaging Cerenkov), located on La Palma.

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