Thursday, 14 July 2011

Make your own cloud chamber

I recently had the pleasure of spending a Saturday with about 25 teachers from around Oxford and the UK (one had even come over from Poland!) for the APPEAL-2 programme run by the John Adams Institute at the University of Oxford.

The day was packed full of interesting lectures about particle physics, cosmology and particle accelerators. After lunch, the teachers descended to the basement of the physics building into the undergraduate laboratories where I held a workshop making small cloud chambers out of (mostly) household materials.

When I was developing the workshop, a colleague from Oxford (Andrew) was kind enough to film a short video about cloud chambers with me. He did an amazing job managing to film the chamber working and even finding a really snazzy thermal camera to show the temperature difference in the cloud chamber.

The thing I find so amazing about these simple devices is that they allow us to see tiny charged particles that we otherwise wouldn't know were there. The particles are called muons and are created when high energy cosmic rays from outer space interact with atoms in the Earth's upper atmosphere. These collisions produce particles called pions which very quickly decay into muons.

Muons are like electrons but they are 200 times heavier and are unstable. Normally, muons decay (into an electron & 2 neutrinos) in 2.2 millionths of a second (!) and wouldn't even last as long as it takes to get from the atmosphere to Earth. The fact that we observe them at all is really strong evidence of time dilation in Special Relativity.

Because muons are heavier than electrons, they don't interact as easily with matter, so can pass straight through the air, the buildings we're in, and tens of metres into the Earth. This is another reason why some particle physics experiments have to be buried deep underground, so that they don't see these cosmic ray muons!

Interestingly, about half of the muons we observe in a cloud chamber are positive and half negatively charged - we could find that out by bending the particles using a magnetic field, positive particles would go one way and negative the other. The negatively charged ones are 'matter' particles, but the positive ones are actually anti-matter. I think it's pretty amazing that we can observe antimatter with such a simple experiment!

I'm glad to say that 9 out of 10 of the chambers that the teachers constructed really worked, even if we were running a little short on dry ice towards the end of the session!