The idea that radioactive elements glow in the dark is a common trope in popular culture, often depicted in movies and comics as an eerie, greenish light emanating from substances like uranium or plutonium. However, the reality of glowing radioactive materials is more complex and less visually dramatic.
Why Some Radioactive Elements Glow in the Dark
Radioactive elements glow due to various mechanisms, some related to radioactivity and others not:
- Ionizing Air: Radioactive elements that release charged particles or sufficient electromagnetic energy ionize nearby air particles, causing a faint glow. This is not the element itself glowing, but the air around it. Ionizing oxygen in air typically produces a blue glow.
- Excitation of Atoms: Radioactive decay sometimes provides enough energy to excite atoms in a material’s own crystal lattice, leading to the release of light as those atoms return to their ground state.
- Cherenkov Radiation: This is a blue light produced when charged particles (like those emitted by radioactive decay) move through an insulating medium (like water) at speeds greater than the speed of light in that medium. This is a blue glow that is often observed in nuclear reactors.
- Heat: Some elements glow because they release a lot of heat through radioactive decay. For example, plutonium glows with a red to orange heat.
- Pyrophoric Behavior: Some radioactive materials spontaneously ignite in air at or below room temperature. The glowing comes from oxidation (burning) and heat.
- Fluorescence with UV Light: While not a direct result of radioactivity, some radioactive materials fluoresce when exposed to ultraviolet light, emitting visible light in the process. Others release energy that causes fluorescence in fluorescent phosphors.
- Phosphorescence: Similar to fluorescence, phosphorescence involves the absorption of energy (which could be from radioactive decay) and the subsequent release of light over a longer period. The glow associated with tritium and radium mainly comes from the light released by phosphors, not the element’s themselves.
Each of these mechanisms contributes to the glow associated with radioactive materials, but it’s important to note that not all radioactive materials exhibit visible glowing.
Radioactive Elements That Glow
Here is a list of radioactive elements ordered by atomic number, with details on their potential to glow, the color of light, and the mechanism responsible:
- Hydrogen (H): Atomic number 1: The tritium isotope of hydrogen is radioactive. While it does not glow on its own, it emits electrons via beta decay that produce phosphorescence in various phosphors. Tritium radioluminescence occurs in any color of the rainbow.
- Technetium (Tc): Atomic number 43: Technetium and its compounds glow faint blue. However, the claim that technetium makes skeletons glow comes from its absorption by bones and release of gamma radiation. While invisible to human eyes, detectors image the gamma signature just fine.
- Promethium (Pm): Atomic number 61: Promethium salts glow with a blue or green light due to ionization of the medium.
- Polonium (Po): Atomic number 84: The decay products from polonium ionize surrounding air, giving the element a blue glow.
- Astatine (At): Atomic number 85: Astatine vaporizes into a dark purple gas that glows with a blue light from exciting molecules in air.
- Radon (Rn) – Atomic number 86: Radon gas only emits a blue glow when you gather enough of it to make the ionization of air visible. Cooling radon produces a clear liquid and eventually a yellow and finally orange-red solid that glow with a blue light. Due to the color range of the solid, the glow sometimes appears blue-green or lilac.
- Francium (Fr) – Atomic number 87: Extremely rare and highly radioactive; it decays too quickly for observation. It likely has a blue glow in air.
- Radium (Ra) – Atomic number 88: Radium is a self-luminous, silvery-white metal. The radioluminescence is a pale blue-green reminiscent of an electric arc. The light comes from excitation of nitrogen molecules and ionization of oxygen. It readily activates phosphors, which traditionally were green, but could be any color.
- Actinium (Ac) – Atomic number 89: Actinium is a silvery radioactive metal that glows blue from ionizing air.
- Thorium (Th) – Atomic number 90: Thorium and its decay products release alpha and beta particles and gamma radiation that cause a faint glow in the air due to ionization. Like most radioactive elements, it does not glow on its own.
- Protactinium (Pa) – Atomic number 91: Protactinium ionizes air for a blue glow. It readily reacts with water or oxygen in air, glowing red from incandescent heat
- Uranium (U) – Atomic number 92: Uranium releases a faint blue-green luminescence. Uranium glass fluoresces under UV light, producing a greenish, yellow, or blue hue.
- Neptunium (Np) – Atomic number 93: Neptunium produces a blue glow from ionizing air and Cherenkov radiation.
- Plutonium (Pu) – Atomic number 94: Plutonium glows multiple ways. Its high rate of decay releases so much energy that it glows red-hot to orange from heat. It burns in air, producing a dull red surface glow. It also ionizes air and exhibits Cherenkov radiation, resulting in a blue glow.
- Americium (Am) – Atomic number 95: The alpha decay from americium self-damages its internal structure, making it self-luminescent. It also stimulates phosphors so they glow.
- Curium (Cm) – Atomic number 96: Curium is a self-luminescent metal that glows deep pink (red) or purple.
- Berkelium (Bk) – Atomic number 97: Berkelium emits low-energy electrons and does not glow visibly under normal conditions.
- Californium (Cf) – Atomic number 98: Some californium compounds are self-luminescent and emit green light from the intense radioactivity exciting f-electrons.
- Einsteinium (Es) – Atomic number 99: Einsteinium is a silver metal that is warm to the touch and glows blue from energy released by radioactive decay.
- Elements 100-118: So little of these manmade elements exist that they have not really been observed. They likely ionize air and produce Cherenkov radiation, glowing blue.
Is Radiation Green?
Radiation can be green, but it can also be any other color of the spectrum or invisible. Technically, green light is green electromagnetic radiation, after all. But, blue light is blue radiation and gamma radiation is outside of the range of human vision.
The misperception that radioactive materials glow green traces back to a combination of historical artifacts, pop culture depictions, and the properties of certain radioactive substances. Mainly the misperception comes from the color of the light released by radium-based paint. Radiation from radium excites electrons in copper-doped zinc sulfide and produces a green glow. Even though we don’t use radium in everyday products anymore, the green phosphor maintains its popularity due to its please color and brightness.
As far as radioactive elements go, they release ionizing radiation that produces a blue glow in oxygen, air, or water. If radiation had a “color”, it would mostly be blue!
References
- Haire, R. (1986). “Preparation, properties, and some recent studies of the actinide metals”. Journal of the Less Common Metals. 121: 379–398. doi:10.1016/0022-5088(86)90554-0
- Jüstel, Thomas; Möller, Stephanie; Winkler, Holger; Adam, Waldemar (2012). “Luminescent Materials”. in Wiley-VCH Verlag GmbH & Co. KGaA (ed.). Ullmann’s Encyclopedia of Industrial Chemistry. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. ISBN 978-3-527-30673-2. doi:10.1002/14356007.a15_519.pub2
- Lide, David R., ed. (2006). Handbook of Chemistry and Physics (87th ed.). Boca Raton: CRC Press, Taylor & Francis Group. ISBN 0-8493-0487-3.
- Muller, Richard A. (2010). Physics and Technology for Future Presidents: An Introduction to the Essential Physics Every World Leader Needs to Know. Princeton University Press. ISBN 978-0-691-13504-5.
- Zelenina, E. V.; Sychov, M. M.; Kostylev, A. I.; Ogurtsov, K. A. (2019). “Prospects for the Development of Tritium-Based Solid-State Radioluminescent Light Sources”. Radiochemistry. 61 (1): 55–57. doi:10.1134/S1066362219010089