Avr 082011

J’ai remarqué qu’a chaque fois que l’on souhaite parler d’un truc radioactif dans un film, bd, jeu video etc… On le représente toujours sous la forme d’une boue poisseuse vert fluo qui brille dans le noir.
Je suppose que cette idée viens des peintures fluorescentes au radium mais il se trouve qu’en temps normal, la radioactivité est complètement invisible.

suivent des extraits d’un article trouvé sur le net

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. But does radioactive material actually glow? In general no.

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. 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.

The first highly concentrated radioactive material to be avaliable for study was radium-226, which was isolated by Pierre and Marie Curie in 1898. 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.

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.

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.

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 an exception to this: The 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.

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