Positronium - seriously

PET scanners love positronium

Positronium, you just couldn't make this stuff up if you tried. It reminds me of Unobtainium, which is pretty close.

Talking of which, was it just me or was the Unobtainium in Avatar actually a room temperature superconductor?

The difference here of course is that positronium, unlike Unobtainium or Handwavium, is actually real, sort off. I say sort off because its life time is so unimaginable short. If something exists you get the idea that you could take a look at it, give it a prod and so on, but not with positronium.

Positronium comes in a couple of variations. It is always an electron/positron pair, but, just like an atom, it can exist in different energy states. The energy state determines just how long positronium survives. The problem is that the positron (first postulated by Dirac) is the antiparticle of an electron and as soon as the electron and positron get close enough they annihilate each other completely.

This annihilation happens very quickly after the positronium has formed. The longest surviving state is the 2S state and lasts for about 1 micro second, that is 1 millionth of a second. A very short time. In 1 millionth of a second light can travel about 300 meters because the speed of light is so fast. The shortest surviving form of Positronium is known as para-positronium and lasts for about 125 picoseconds.

125 picoseconds is an extremely short period of time. A picosecond is a millionth of a millionth of a second. So using the speed of light again, in 125 picoseconds, light would travel 37.5 mm, which is about 1.5 inchs, about the length of your thumb. Not very far because 125 picoseconds is not very long at all.

Now, you may be thinking to yourself, fair enough, it has a cool name and all, but what use is something that only hangs around long enough for light to travel the length of my thumb? Well, you may be surprised to find that it does have some use in the real world. See, when the electron and the positron annihilate each other they give off high energy photons, known in the trade as gamma rays and have a clearly defined energy.

PET scan of a brain

By measuring the gamma rays that result and by using some my numbingly clever analysis you can actually build up a picture of the inside of a person. This process is known as Positron Emission Tomography  and the device for taking the measurements is often known as a PET Scanner.
PET scanners have a number of uses particularly in oncology for obtaining images of tumors as well as the detection of brain diseases. It is also used in cancer research.

In medicine it is possible to introduce positronium into a human being using something called a radionuclide (a radioactive material). The radionuclide is often a glucose based material such as flourine-18 floourodeoxyglucose (FDG). Once the FDG enters a cell it becomes trapped until it decays. (When it decays it gives out a positron which interacts with an electron to form positronium. The positron and electron then annihilate each other to produce gamma rays.) This means that tissues that have a high uptake of glucose, the brain, liver and most cancers can be clearly seen.


While positronium is of interest from a physics point of view it has also been used in some really clever practical applications.

It is amazing to think that we have been able to make use of something that lasts only long enough for light to travel the length of your thumb and from this we can actually determine brain diseases and cancers!

How cool is that?