Cosmic Rays Cloud Chamber Construction Details

Principle of Operation
The crucial idea is that we build a sealed chamber in which there are two phases of alcohol (in addition to some air):  Liquid, particularly small droplets of condensation, and vapor. The chamber has a temperature gradient from warm at the top to very cold just above the metal plate at the bottom. Descending alcohol vapor cools to below its condensation temperature ('supercooling') so that it is highly susceptible to condensing into its liquid phase.  All that is required is a trigger of some sort to induce this condensation. The prevailing idea is that a small particle, for example a bit of ash or dust, can behave as a nucleation site where condensation can begin. This is the trigger that allows supercooled alcohol vapor to condense into liquid droplets. These fall to the bottom of the chamber.

Even when 'nothing is happening' particle-wise (cosmic-ray-wise), there is enough condensation occurring to produce a fine mist of visible droplets that are always drifting slowly downwards, quite pretty. When 'something happens'--that is, some sort of particle goes flying through the chamber, it bangs into and ionizes gas molecules along its path. Hence suddenly there is a long string of ions that act as nucleation sites. That is, along the particle's path there is sudden condensation of many many alcohol droplets, 'marking the way'.


cloud chamber schematic

The picture below is the first version of the cloud chamber. It is not particularly well-constructed but works well enough to produce results. Referring the picture below to the diagram above: You can see the alcohol-soaked black felt through the wall of the chamber, the edge of the metal plate clamped via a frosty C-clamp to the bottom and some of the dry ice below the plate. The plate rests on the ice and hence the bottom of the chamber is very cold.

The band of felt at the top of the chamber (inside) is also visible. The books are used to weight down the plastic container onto the metal plate; the idea is to make a good seal so as not to allow warm air inside. (I think the books are Sherlock Holmes and the collected poems of Carl Sandburg; the device might work with other books but I'd have to check.)

cloud chamber photo

Construction details
A large shallow storage container--wide open--is the base for everything.

Sitting inside the base is a big tablet of dry ice, solid carbon dioxide, which slowly sublimates away at room temperature over the course of several hours. The tablet should be greater than or equal in size to the base of the cloud chamber.
    Dry ice runs $1/pound at the grocery store.
    It might be cheaper from the chemistry stockroom at a local university.
    I'm using about a 10-pound block which will last six hours.

Next is the cloud chamber base, a piece of sheet metal with felt on one side (the top side).
    This has been C-clamped to the edges of the cloud chamber.

Inverted over the sheet metal is a semi-clear plastic box with no top (i.e. no bottom since its inverted).

Once the chamber is ready (the internal felt having been soaked with alcohol and the metal bottom sheet clamped on) it is weighted down onto the dry ice.
    I've had my best seal (plastic to metal sheet) using a large bucket full of water as top-weight.
    The top-weight also pushes the metal plate flat onto the dry ice.
    In turn this pressure-ablates the dry ice until it makes a good flat thermal contact to the metal plate.
    For the metal plate I used 26-gauge steel.
    Good thermal contact and temperature stabilizing might take up to 40 minutes.
    Water-ice will condense here and there.

Inside the cloud chamber the top of the sheet metal is covered with alcohol-soaked black felt.
    The alcohol to use is 99% isopropanol, not 70%. This is a subtle but important point.
    I've found that denatured alcohol also works fine.
    The black felt came from a craft store "Michaels".
        It has an adhesive back; very easy to apply and not alcohol-soluble.
    You can avoid the (clunky) C-clamps by carefully applying felt to the base.
        Do this so the felt fits inside the perimeter of the covering box.
        Once the cloud chamber is set up the felt will keep the covering box from sliding across the plate.

An additional band of adhesive felt is attached around the top inside of the chamber.
    This band goes all the way around the inside perimeter.
    It could be extended to the inside top of the chamber, the ceiling.

As described, both the inside-top felt band and the felt on the metal sheet are thoroughly saturated with alcohol.
    The chamber is then sealed up and set in place on the dry ice with the top-weight added.

The system is now sealed and has a very cold base.
    Again: It will take 20--40 minutes for everything to settle down and equilibrate.

The alcohol-soaked felt will continuously pump alcohol vapor into the chamber.
One alcohol soak keeps the chamber running for several hours.

Observing
A strong light source, e.g. a slide projector, shone through the chamber wall will help immensely in illuminating the particle tracks. I find some trial-and-error is involved in coming up with a good illumination geometry.

Using a steel sheet for the base is a good means of securing magnets in the interior. Most tracks I saw were closer to the bottom of the chamber. The very strong gold-coated NIB magnets shown in some of these pictures can be bought here.

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