Опасная зона

Опасная зона

Sunday, January 12, 2014

Twinkle, Twinkle, Little Light

At Amazon you can buy tritium. Really.

Tritium is an isotope of hydrogen, the nucleus consists of 2 neutrons in addition to the positively charged proton. That makes it unstable, and tritium decays converting one of its neutrons into a proton and an electron i.e. beta-minus decay.

In other words, tritium is radioactive.

To my knowledge, tritium is rather expensive and in fact even in laboratories a rare sight. Most prominent uses is for thermonuclear weapons and fusion reactors, as it is the easiest reaction to fuse tritium with another hydrogen isotope called deuterium (one proton and one neutron) while releasing energy.

So, at Amazon you can buy tritium gas lights which are small glass ampules filled with tritium gas and a radioluminescent material.
Medium sized tritium gas light from FireFly.

The electrons released from the beta decay are stopped in the radioluminescent material which in turn will release the energy in form of photons in the visible spectrum (...what a fancy way of saying "to shine").
This is similar to an old-fashioned TV-tube, where an electron beam (electronically accelerated to about 15 keV) hits the screen coated with radioluminescent colors and makes it shine. The electron energy released by decaying tritium is lower, only 5.7 keV, which is so low they are easily stopped even in air.

The half-live of tritium is about 12.3 years, so in 12.3 years the source will be half as bright. This is rather amazing, the gas light shines and shines and keeps shining... no batteries, no recharging.

Several different radioluminescent materials can be added to produce different colors.


I bought a couple of green ones, as well as blue, white and something in between white and blue. The green ones clearly have the best yield, probably also because the eye is most sensitive in the green region.

How bright are they? Well, clearly in normal ambient light, they are not noticeable. In the picture below I have three tritium gas lights in three sizes. The two smaller ones are green, the largest one is white.

Three tritium gas lights.
Wie Sie sehen, sehen Sie nichts.
But as the lights are dimmed and the eye adapts...

This is pretty much how the human eye sees them whey you are in complete darkness.

More or less as the adapted eye sees them. Light parts a bit weaker, and the dark parts brighter, the dynamic range of the camera is a bit narrow.
The gas lights are bright enough that you can recognize things in a complete dark room, like position of a bed, cupboard etc. The white one is not bright enough to see colors when the eye is in night vision mode, but still bright enough to read texts a few cm from the light.

Just to brighten it up, I made a long-term exposure (5 mins, iso 100) of the set.

Shine bright like a diamond.
It should be added that some of the sources which are sold on Amazon may be illegal to posses without a permit. The smallest one has < 1 GBq. The white large one has < 15 GBq.
In Germany rules are that closed tritium sources below < 1 GBq are allowed without any permit. Any sources above 1 GBq can be fined with up to 50.000 €. Similar rules are in Denmark, so these sources are kept at my University which has the necessary permits. (Used for teaching BTW, I am surprised how many students never have heard about tritium.)

Nonetheless, there is no way any dangerous radiation can escape the glass ampule within the perspex holder in normal operating conditions. Tritium behaves as hydrogen, so if one would break them open, burn the gas, condensate the resulting super heavy water vapor, and be silly enough to drink that super heavy water, you might receive a dose, possibly in the mSv region. (Most likely it will be automatically diluted before any intake can happen, so one really has to be very dedicated to be that foolish.)

Tritium is a waste product from water moderated nuclear reactors when in normal operation. Following the wikipedia article on tritium, over 41000 curies of tritium was released in 2003 which would be enough to make 1.5 million of the smallest tritium gas light shown above.

Anyway, I can recommend the small ones as geeky gifts for geeky physicists. There is some strange fascination in seeing this light with no batteries, or phosphorescence pre-activation - just radioactive decay and radioluminescense. One cannot help but getting philosophical contemplating the light. Where does the energy come from... and 1 billion beta decays per second (for a fresh 1 GBq source ). Think 1 - 0.5 billion decays per second for 12.3 years and then you have used up half of the tritium. Amazing.

Oh, and I submerged one into the bottom of my little aquarium. Green light shining on the ocean floor at night. :-)

[link to photo album]