Monday, September 24, 2012

Alcohol and clouds in my institute


Already two weeks ago a new "toy" arrived in my institute. It was the first time that I was seeing a working cloud chamber and I spent most of the afternoon looking for the different tracks, trying to guess the origin, wondering how much radioactive material there is around my office :)
I then thought that it would have been nice to start a new current in this blog talking about the main detectors that we use nowadays in particle physics....and what can be better then the cloud chamber to start this list?
Cloud chambers played a prominent role in experimental particle physics from 1920s to the 1950s, until the advent of the bubble chamber. In particular, the discoveries of the positron in 1932 (acknowledged by a Nobel Prize in Physics in 1936) and the kaon in 1953 were made using cloud chambers as detectors.


Cloud chamber photograph by C.D. Anderson of the first positron ever identified.
Positrons were discovered by Anderson and it was the first evidence of antimatter. They were discovered making use of a cloud chamber were cosmic rays were able to pass through. The whole detector was surrounded by a magnet so that, due to the Lorentz force (which is the force due to an electromagnetic fields on a point charge), charged particles are bended in opposite directions depending on the sign of the charge itself.
The track left by the positron, shown in figure, proves that the positron must have come from above since the lower track is bent more strongly in the magnetic field indicating a lower energy (a 6 mm lead plate separates the upper half of the chamber from the lower half). Moreover the curvature is such that the particle is perfectly compatible with the mass-to-charge ratio of an electron, but the direction indicates that the charge is positive!!
For that reason Anderson decided also to call this new positive electron as positron.

In its most basic form, a cloud chamber is a sealed environment containing a supersaturated vapor of water or alcohol. When a charged particle (for example, an alpha or beta particle) interacts with the mixture, it ionizes it. The resulting ions act as condensation center, around which a "cloud" will form. The high energies of alpha and beta particles mean that a trail is left, due to many ions being produced along the path of the charged particle.

The diffusion cloud chamber was instead developed in 1939 by Alexander Langsdorf. This chamber differs from the cloud chamber because it is continuously sensitized to radiation, and because the bottom must be cooled to a rather low temperature, generally as cold as or colder than dry ice. Alcohol vapor is also often used due to its different phase transition temperatures.
This is actually the cloud chamber that my institute bought, which can be find also here.

But then one could ask me...Why did you wait so long before to post this news? The answer is simple...there was not enough alcohol :) What I mean is that I wanted to make a video of the cloud chamber but we finished the alcohol just the day after it arrived in my institute!! But today I saw that it was working again so...here you are, in the beginning of this post, the video of the tracks passing through the chamber! You can try to guess yourself which particle was causing the condensation: each track have a distinctive shape so, for example, an alpha particle's track is broad, more energetic and shorter than the others, while an electron's (beta particle) is thinner and longer. Enjoy it :)
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