Imagine a piece of plastic that glows in the dark for 12 years without any external power:

- Litroenergy – New Light Source Material

- Flexible light source for 24/7 lighting (photos)

That’s the claim. How would it do it? It uses “self-luminecent micro particles”. What this means is that it is taking a very old nuclear technology that is still used in some watches today, encapsulating it in tiny beads to try to make it safer, and then mixing the beads with paint, plastic, adhesives, etc. to make them glow. This article offers a nice description:

Tritium paint on watches is a mixture of tritium and phospor. Tritium is naturally radio-active and needs no external source of light or charge to work. Tritium does not glow. As it decays, tritium emits beta radiation, which is a bunch of excited electrons that in turns excite the electron in the phosphor atoms making them emit photons, or light, as they return to their ground (non-excited) state: the phosphor GLOWS. Phosphor can also be excited by UV light from the sun or other light sources. Thus, the tritium paint relies on tritium radioactivity to make the phosphor glow in the dark, not any charge from external light source.

Tritium, has a half life of 12.3 years, a half-life is simply the time it takes HALF of the tritium to decay. So, as long as you have enough tritium in your paint, the watch will glow in the dark for years, not hours or days, without any need of charging. If your watch stops glowing after an hour in the dark, it means that the glow came from the light exciting the phospor atoms, not from the tritium. In other word, most of the tritium in your watch is GONE! This is quite possible even with a fresh coat of tritium paint if that paint has been sitting around in the watchmaker’s shop or supplier’s shelf for years. The 12.3 years of half-life starts from the second the tritium is born (i.e. freshly produced), not from the time the paint is applied to your watch.

Is it dangerous? This article offers the following perspective:

For a short time during the 1970s, tritium containing gas tube light sources (GTLS) were used to backlight liquid crystal display (LCD) watches. At that time, a typical activity for GTLS was 200 mCi. The sealed tubes, made of borosilicate glass, were 0.5 to 1.0 cm long, and one or two millimeters in diameter. The inside wall of the tube was coated with the luminescent material while the tritium was present as a gas. Since then, similar but smaller tubes attached to the hands and dial faces of analog watches have been used. I don’t have much information about the activities in these watches but it seems that they range from about 25 to 100 mCi.

When incorporated into radioluminescent paint, much lower activities (2 mCi) are employed. The tritium, which is really hydrogen, is incorporated into a polymer that also contains the luminescent material (usually ZnS).

The dose to wearers of tritium-containing radioluminescent watches is due to the leakage of tritium from the device and the tritium then being absorbed through the skin. The betas emitted by tritium are of too low an energy to penetrate the watch case and contribute to the dose. Measured leakage rates from watches employing tritium containing paint were up to 83 nCi (0.083 uCi) per day in one study, and 1 to 370 pCi per minute in another (0.0014 to 0.5 uCi per day). The typical release rate from watches using tritium sealed in glass tubes (e.g., GTLS) is less than 0.00025 uCi per hour. As such, the H-3 activities in GTLS might be higher than those in paint, but the resulting doses are somewhat lower.

In other words, if you wear a watch all the time you might be getting a dose of less than 1 mrem per year. This calculator gives you a great way to get a perspective on what that means. It points out that “The average dose per person from all sources is about 360 mrems per year.” That dose comes from things like cosmic radiation, the earth itself, brick, TV, x-rays, etc.

For more on cosmic rays see: Where do cosmic rays come from?

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