Why Does Radioactive Stuff Glow Green? (Or why do people think it does)
April 22nd, 2008
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Whenever you see radioactive material represented in a cartoon or a similar fictitious depiction, it always seems to have a lime-green glow that has become nearly iconic. You also see this illustrated in nuclear protests and anti-nuclear groups who depict things like nuclear waste or those exposed to radiation as being glowing, usually with a green glow. Even those who protest cell phone towers have carried signs that state “hell no, we won’t glow,” which is funny because it’s not even referring to radioactivity, so it’s wrong twice over!
But does radioactive material actually glow?
In general no. The image you see to the right is an ingot of weapons grade plutonium. Although plutonium is the kind of material which is commonly portrayed as a glowing goo, you’ll notice that it simply looks like a chunk of metal. Plutonium-239 is quite radioactive. Although it has a relatively long half-life of twenty four thousand years, it produces high energy alpha particles and has numerous radioactive daughter products. However, there’s no visible glow.
The same is true for most radioactive materials. In general there’s no actual glowing at all. On the occasions that radioactivity does generate a visible light glow, it’s also not usually green. (But more on that later.)
How it got started..
The first highly concentrated radioactive material to be avaliable for study was radium-226, which was isolated by Pierre and Marie Curie in 1898. (Previously polonium had been isolated by the Curies but it was short lived and only could be concentrated in the tiniest quantities). Radium therefore became the de facto general purpose radioactive source for decades until safer and more useful artificial radioisotopes became avaliable. To anyone prior to the 1940’s, “radioactivity” was synonymous with “radium,” which was seen as something of a wonder material.
One of the first things that was noticed by Marie Curie after producing a test-tube of concentrated radium was its “fairy-like glow.” It did not glow green, however. It was said to be a bright blue color. Reportedly she even kept a vial of it next to her bed as a kind of night light. But the glow of the radium in the tube was not actually coming from the radium itself. Radium is a highly radioactive material and emits numerous alpha and beta particles as well as gamma rays as it decays. These particles have the effect of ionizing the material around them, and when some materials are ionized they emit a visible light glow. Materials which emit light when excited by light or charged particles are known as “fluorescent” or “phosphorescent” – The difference being that the former only emits light when initially excited and the later will continue to emit light for a period of time after. As it turns out, the compounds of radium which Marie Curie had in her vial, radium bromide and uranium chlorides had mild fluorescent properties, so when they were irradiated by the radium they contained, it actually caused the compounds to glow.
The property of fluorescence was already known before radioactive materials were isolated. It had been observed with X-rays and was used to create “fluoroscopes” which allowed x-ray images to be viewed in real time by use of a screen, treated with material which glowed when exposed to x-rays. It was also known that fluorescent compounds could be stimulated by cathode rays (electrons) and it was this effect which would later be employed in creating television tubes. Fluorescent lights also use this property to create visible light by exciting fluorescent compounds with UV light which is produced by passing electricity through mercury vapor.
This effect is also used in a certain type of radiation detector called a scintillation detector. In this type of detector, a substance which produces a glow in the prescience of ionizing radiation is employed as the detection medium. When a charged particle or gamma ray photon strikes the material it creates a brief pulse of visible light. The light from the scintillation material, often called a “crystal” is picked up by a highly sensitive light detector called a “photomultiplier.” This produces a pulse which is registered on a counter or spectrometer. On some occasions, a liquid scintillation medium is used for analysis of radiation.
Radiolumonescent Products:
It was recognized early on that the glowing of radium compounds could be very useful in order to provide a reliable source of light which would not be dependent on electricity or being “charged” by a light source and then glowing for only a limited period of time. However, radium was simply too expensive to use as the primary material in a light source. It cost thousands of dollars per gram, so only a few milligrams or less could be economically used in an end product.
The solution to this was to use a more potent phosphorescent material. In 1908 a paint was developed which used zinc sulfide doped with copper which could produce a visible light glow when bombarded with charged particles. The addition of a tiny amount of radium to the paint provided these particles and that assured a continuous glow of the paint. The glow came primarily from the beta particles emitted from radium and its daughter products and had a very recognizable green glow, not that unlike modern “glow in the dark” products which require exposure to light to produce a glow.
The color of the light could be changed to nearly any color by using different formulas and adding dyes, but green was by far the most common because it was one of the more effecient (and therefore bright) color formulas avaliable and because the human eye is more sensitive to green than any other color. Thus nearly all radiolumonescent products were green in color. The use of radium-based paints became very popular in the first half of the 20th century. They were used for watches, alarm clocks, aircraft instruments, radio dials and other such needs. They were phased out in the late 1960’s, but radiolumonescent produces continue to be manufactured, only using safer isotopes like tritium or promethium-147. Radium, on the other hand, has a high biological uptake and emits high energy alpha and gamma particles. Thus the paint is highly toxic and was responsible for the death and illness of some of the painters who used no precautionary measures and sometimes tipped the brushes with their tongues!
So for most people their experience with radioactive material was in the form of glow products such as clocks or watches. The green glow became iconic of radium and therefore radiation. But in general, radioactive material does NOT glow.
There are, however a few other exceptions to this:
Cerenkov Radiation: This effect occurs when a high energy beta emitter is submerged in a dense medium such water. High energy beta particles are able to pass through water at a speed greater than light can pass through water – although not greater than the normal speed of light in a vacuum. As the beta particles pass through the water they alter the magnetic field and displace electrons in the water. The electrons realign themselves back to the ground state as a beta particle passes. In doing so a photon is emitted from each electron. Normally these photons tend to cancel each other out and no light is seen, but when the beta particles exceed the speed of light the photons are emitted with a slight lag, allowing them to escape without interfering with each other. Most Cerenkov radiation is in the ultraviolet spectrum, but part of the energy is visible light and can be seen as a blue glow. Normally this is only visible when there is very intense radiation, such as an operating pool reactor, or a large amount of a powerful beta emitted. Photos which show the effect on spent fuel rods or reactor vessels are time exposures and would not appear the same to the naked eye.
Spectral Emission: This is somewhat speculative, but there have been some who have reported a glow in the air around extremely radioactive material or radiation emissions. It is difficult to document this effect because most of those who would observe it would probably be dead, but it has been reported in criticality accidents that a glowing region of air was produced. It is believed that this may be the effect of ionization of the oxygen and nitrogen in the atmosphere. This same effect is what produces the Aurora Borealis and is why sparks of electricity have a blue glow. It has been observed in ion beams from particle accelerators. Being composed primarily of nitrogen and oxygen, the atmosphere at sea level tends to produce a dim, light blue glow if ionized.
Incandescence: This effect actually has nothing to do with radioactivity and everything to do with heat. When a material is heated to a certain point (around 1000 Kelvin) it begins to emit a red visible light glow. As the temperature increases the glow becomes brighter and it begins to emit higher frequency wavelengths of light. This is where the term “red hot” and “white hot” come from. It is how traditional light bulbs work and is also the reason why hot irons from a furnace or lava flows emit a red glow. Few radioactive substances generate enough energy to actually produce enough heat for incandescence to occur, but a few high energy particle emitters, such as plutonium-238 produce a large amount of heat from the deceleration of decay particles. Pu-238 is used for heaters and power generation in deep space probes. A naked tablet of plutonium-238 will generate enough heat to glow red.
Note, however, that in none of these cases is the typical color green.
This entry was posted on Tuesday, April 22nd, 2008 at 1:03 pm and is filed under Bad Science, Culture, Good Science, History, Nuclear. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
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April 22nd, 2008 at 1:58 pm
You mean that radioactive stuff is not all a green glowing goo? Wow. The Simpsons got it totally wrong. Also, I think there are quite a large number of people who use the Simpsons as their source of info on nuclear energy.
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April 22nd, 2008 at 2:43 pm
Buzzo: couldn’t have said it better myself. Nice job. I tell folks I use radioactive materials and I routinely get the question, “So, do you glow in the dark?” My standard answer is, “My wife thinks so!”
You might have noted that Pu-238 is worthless for weaponry (Pu-239 is used in bombs).
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April 22nd, 2008 at 3:02 pm
Yeah, yeah, yeah. So the “plutonium rod” doesn’t glow green.
But what about “Blinky” the three-eyed fish?
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April 22nd, 2008 at 3:04 pm
Atomic lights were always a staple of pre-war science fiction. Why any one would want such a thing was never discussed.
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April 22nd, 2008 at 3:22 pm
DV82XL said:
They work well for times when you want reliable glow without having to expose it to light and then have it fade within a short period of time. Tritium wrist watches are good and so are compasses and safety markers and gun sites. I think I read tritium tube lights were first installed in submarines as reliable non-electric lighting for emergencies if the power failed so it was not pitch dark.
I assume you’re not talking about that though. they’re usually too dim to be very good for general lighting anyway and I don’t know why you wouldn’t use electric lights if you could. You can’t turn them off either which would be annoying.
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April 22nd, 2008 at 3:50 pm
Brian said:
Depends on the species. Some fish are bioluminescence.
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April 22nd, 2008 at 7:22 pm
I don’t think you have the right picture of plutonium as I am very sure it is more of a liquid or gel and plutonium is not a metal. Plutonium I think is green but I don’t know if it glows or not. I have seem plutonium on television and I think that it does not look like that. You just show metal.
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April 22nd, 2008 at 8:04 pm
Fernando said:
Plutonium is a metal with a dull luster. It is often alloyed to make it easier to work with. It’s not a green goo.
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April 22nd, 2008 at 8:28 pm
I always wondered about the radioactivity/glow thing. Very interesting.
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April 22nd, 2008 at 9:15 pm
Fernando said:
I think you may have seen too much SciFi with a lot more Fi to it than Sci. That’s just how stuff is always shown in cartoons and stuff. It doesn’t look a thing like that!
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April 22nd, 2008 at 9:51 pm
Fernando said:
There is this thing called the periodic table. Here’s one:
http://www.dayah.com/periodic/
I don’t know about you, but I see gases, liquids, solids, nonmetals and metals. No category for green glowing goo. Also, look, there’s Plutonium, element PU, right there in the Actinoids row, two over from uranium, clearly a metal. I don’t know about you, but I have never seen any green metals. Guess the TV was wrong, or should watch the Science Channel rather than the Simpsons.
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April 22nd, 2008 at 11:00 pm
I have to admit I didn’t know this at all. Fascinating information. I had always wondered about the “radioactive glow” thing.
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April 23rd, 2008 at 5:15 am
Fernando said:
Nope – it’s a solid, grey metal, exactly as has been described. It’s just a metal, like iron or lead.
Aside from radioactivity and nuclear fissionability in certain nuclides – neither of which are unique properties – it’s just a metal. (With some really complex, unique and cool metallurgical and chemical properties!)
Here are some more pictures:
http://www.physorg.com/newman/gfx/news/2006/Puingot309×232.jpg
http://www.srs.gov/general/news/photos/button.jpg
http://upload.wikimedia.org/wikipedia/commons/0/0f/Plutonium_ring.jpg
However, some compounds of plutonium – Pu(III) hydroxide, I think, and maybe Pu(IV) fluoride, are kind of green appearing under certain conditions – see the greenish oxidation in that first photo?
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April 23rd, 2008 at 2:09 pm
That is extremely interesting. I didn’t know that radium painted clocks and meters did not need to be exposed to light. That’s much better than the most of the glow in the dark stuff we have today where you have to charge it with light and then it fades after a short while. I guess I can see why they added it to the paint then.
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April 23rd, 2008 at 3:34 pm
Very interesting read. I feel enlightened. I didn’t actually know any of this very well. I did know that uranium and plutonium and those kind of things don’t glow though.
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April 24th, 2008 at 12:16 pm
I mentioned before that I had previously done research into the longterm stability of nuclear fuel and spent fuel of different compositions. I’ve seen plutonium myself, although only small amounts of it in the metallic form. I’ve seen it plated onto rods from a solution as well as in the form of small buttons or ingots, about the size of two US quarters stacked on each other. More commonly it’s been in the form of an oxide or in MOX fuel assemblies.
But I can testify to you right now that it is not a “Green Goo.” It does not glow either. It’s a metal which occasionally has a bright silvery luster but has a tendency to get a dull patina very quickly on the surface. When plutonium is used in the metalic form it is almost always alloyed with a small amount of other material (often gallium). This helps improve the stability and workability of the material. Plutonium alone can be somewhat brittle and it can get worse over time due to the alpha irradiation.
There’s also something else which is important to know about plutonium: Plutonium metal is highly reactive and will tarnish quickly but it can also react with water or moisture in the air and form plutonium hydrate. Plutonium hydrate will occasionally auto-ignite if the conditions are right and plutonium is flammable under the right conditions. It can burn relatively easily when in a granulated form, similar to aluminum. For this reason it is handled with precautions and often in an inert atmosphere when it is being molded or modified.
The risk of fire from solid plutonium alloys is small but the results can be a big problem. In the 1960’s there were a couple of nasty fires in US weapons production plants. As you can imagine, burning plutonium is not the kind of thing you want to deal with. It’s also why it’s an oxide when used in fuel in general. It is much more stable in that form.
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April 5th, 2009 at 1:25 am
Kim said:
Whoosh!
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April 5th, 2009 at 9:54 pm
The green color of radioactive material that suggests “radioactivity” is grounded in reality, but has nothing to do with “radioactivity” and everything with uranium: There was a time when uranium ores were used in the process of glass making (no joke). And the color of this uranium glass is a spectrum from lightish yellow to intensive “radioactive” green. The glass is not radioactive (ok, a little bit, but the natural radiation from the sun etc. is far more intensive) but when the right light shines on it, it looks really nice (just google for images).
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April 5th, 2009 at 10:53 pm
Daniel said:
Yes I have a small collection of marbles, acquired in my youth that are made of uranium glass. Transparent in ordinary light, they do glow green under UV. Some are milk-white and glow yellow when exposed to ultraviolet. EBay is awash with uranium glass marbles and beads, some made within the last few years.
Uranium glass has utilitarian as well as decorative applications. Its coefficient of thermal expansion happens to make it suitable for graded seals and glass-to-metal seals in glass apparatus, such as vacuum tubes and vacuum capacitors.
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February 27th, 2010 at 4:09 pm
Have you seen images of the Chernobyl reactor? There was enough ionizing radiation to cause a glow in the core of the reactor. Not green, but red or blue. Sure looks like Hell though.
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February 27th, 2010 at 4:50 pm
The glow in what was left of the Chernobyl core was not due to ionizing radiation; rather, it was due to radiation of a more mundane kind: blackbody thermal radiation.
Graphite, when it becomes hot enough, begins to emit sufficient blackbody radiation in the visible spectrum to be seen by the naked eye. That is, if you heat it hot enough, it will glow red, just like other materials do, such as iron or steel. Heat the graphite to even higher temperatures, and it will begin to glow white hot.
The red glow that was seen at Chernobyl was simply red-hot graphite, which was heated to such high temperatures by the decay heat of the fission products in the core.
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February 27th, 2010 at 5:02 pm
There was also a raging fire that took at least a few days to extinguish. That would make it even more difficult to distinguish any glow from just plain old flames.
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March 1st, 2010 at 1:12 am
Early glow-in-the-dark toys I had were all made of an off-white translucent plastic. And when they were taken into the dark they glowed a greenish glow. Im pretty sure thats where the cartoons get the ‘green glow’ from. Early glow in the dark toys. Thats what Ive always assumed anyhow.
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June 25th, 2010 at 3:47 am
this is why. green radium glass that seems to glow under regular light then dose glow under a black light
http://www.google.com/imgres?imgurl=http://z.about.com/d/chemistry/1/5/p/9/1/uranium-glass-fluorescence.jpg&imgrefurl=http://chemistry.about.com/b/2010/03/15/uranium-glass-see-the-glow.htm&usg=__tSsF1ke6RLKFjdmmUMT0aSiTTu0=&h=300&w=400&sz=45&hl=en&start=1&um=1&itbs=1&tbnid=xnVCmk_KbUunbM:&tbnh=93&tbnw=124&prev=/images%3Fq%3Dgreen%2Buranium%2Bglass%26um%3D1%26hl%3Den%26safe%3Doff%26client%3Dfirefox-a%26hs%3DAWk%26sa%3DN%26rls%3Dorg.mozilla:en-US:official%26tbs%3Disch:1
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