Cloud Chamber

Cloud chamber workshop during the OpenDay @ CERN

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The Cloud Chamber


At CERN they used a plastic aquarium of about 40cm x 20cm x 20cm - felt was attached to the bottom of the aquarium, which was soaked with pure 100% isopropyl alcohol (96% will do too).

Then the black iron plate was placed above the dry ice plates (the plates were about 1-1.5cm thick).

The channel around the black painted iron plate (see photo below) was filled with isopropyl to ensure an air-tight placement of the aquarium.

Then the aquarium was placed upside down (the alcohol soak felt is now at the top of the chamber) on the iron plate.

After a few minutes rain-like mist was visible, falling down to the iron plate from about 5cm above of the plate to the bottom. Some minutes after, first cosmic rays (muons) and alpha or beta particles produced straight fuzzy lines in the supersaturated alcohol vapour just above the iron plate. You have to look carefully to discover the first condensation tracks, but then you should see one each second.

How it works: Alcohol vapor is emerging from the soaked felt at the top of the chamber. Due the temperature gradient in the container (the top of it is at room temperature, but the bottom is very cold - dry ice has a temperature of -78 deg.C.) the vapor is falling down, cooled and it starts to condense a few centimeters above the iron plate. You can see this condensation by using a flash light or office halogen lamp - tiny droplets are raining down. Due the steep temperature gradient, the alcohol vapor reaches a point above the plate where it usually can't exist at this temperature. This condition is called supersaturation.


Now when a cosmic ray (muon) or alpha/beta particle is crossing thru the supersaturated mist above the iron, the ionization caused by the crossing particle causes condensation in form of a fuzzy white line. Make sure your light is well placed to illuminate the bottom of the chamber and after some time you will see a track every second.

The black iron plate: Just a regular iron plate, painted black, but with a channel for the plastic container. The black adhesive tape was just used to lift the plate into the dry ice box, but if you don't want to press down the container all the time to ensure air-tightness, use adhesive tape to attach it to the plate.		The cloud chamber in action. The flash lights were probably a bit too directional - better use a sort of halogen office desk lamp.
I tried to take photos of the tracks, but that's more difficult than it may seem. In this photo I've captured two small tracks, just to give you a basic idea of how it looks like.		I also tried to build a chamber with a mixed pickles glass. In my experience such glasses have a way to big distortion to see inside of it clearly. Furthermore it is absolutely necessary to have a black bottom. Paint the bottom plate with black, water insoluble paint. Some use black tape, but in my opinion black paint is better.

Most of the time you will just see straight tracks, but sometimes you will also recognize a track that bends off. Then most likely the muon decayed into an electron which track is still visible and a neutrino (maybe two), which can't be detected this way. Or in other cases, you may see a track which duplicates. Then the incoming muon hit an electron of an atom, which is now producing a new track in your chamber. In the third case, you may see a zigzag track - this is multiple scattering of a low energy muon, which bounced off of one atom to the next.

Be very careful when handling high voltages!
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