Put in some positrons
If we want to find out about a material’s internal structure, we can start by introducing positrons into the material. Positrons are high-energy particles: they rush around at random, colliding with other particles in the material. With each collision, each positron loses energy and slows down.
Positrons and electrons form positronium
As each positron slows down, it becomes more likely to pair up with an electron in the material, and form positronium. If we can detect how long the positronium survives before the positron and electron annihilate, that can tell us a lot about the structure of the material.
Not every pair of electron and positron will form positronium. Some will just annihilate straight away, so positronium doesn’t form evenly throughout an object.
Positrons last longest in empty spaces
Positrons have a positive charge, so they are repelled by the positive charges of the nuclei in the material. This is one of the reasons why positrons tend to drift into any open spaces in the material.
In an empty space there are fewer electrons for each positron to annihilate with, so positrons can survive for longer.
Empty spaces delay annihilation
A positron is more likely to pair up with an electron, to form positronium, and thus delay its annihilation, if it finds itself in an empty space.
So we know that the regions where annihilation is most delayed are the emptiest. Similarly, the regions where annihilation happens first are the densest.
How do we know when annihilation occurred, and where?
If an electron and its antiparticle (a positron) come together and annihilate, they are both entirely converted into energy, in the form of two gamma rays.
The gamma rays are always given off in opposite directions, and they always have an energy of 511 thousand electron volts each.
Whenever we detect a matching pair of gamma rays, we know their speed and the direction they came from. This tells us exactly where and when the positron-and-electron pair annihilated.
So just by introducing positrons into a material, and detecting where and when they annihilate with electrons, we can build up a picture of the material’s density, and any spaces or flaws.
We can detect spaces of about 1 nanometre in size. That’s one millionth of a millimetre.
Holes of this size can have important effects at the human scale. They can affect how porous the material is, or how well it conducts electricity.
Finding imperfections before they do harm
This process can even show up areas where material is becoming worn or degraded, long before they are visible to the human eye.