If you're interested in how to do it, here's the setup we used: We took a Styrofoam cooler and cut off the bottom so that it would fit nested inside the top. We then filled the bottom with dry ice, and stuck a customized aluminum baseplate with a rim on top of that (aluminum makes the awfullest noise when placed in direct contact with dry ice as it cools to the −109.3 °F temperature of the ice. Sort of a screechy metallic shriek). On top of the aluminum, we placed our radiation source, an americium pellet from a cannibalized smoke detector. [Side note: I just did a little research on americium, and wow, I knew americium was highly radioactive, but I didn't know it was 3.5 times as active as radium. Or that it emitted gamma rays. I thought it was mostly relatively benign alpha particles (helium nuclei). I will keep that in mind when we run the experiment again...]
Anyway, after that, we placed a plastic container to be the actual cloud chamber upside-down on top of the aluminum plate. The container had strips of felt that had been wetted with 99% pure isopropl alcohol stuck to its bottom, which became the top of the chamber when it was inverted. On top of that, we placed a hot water bottle.
Since a picture is worth a thousand words, here you go:
I should probably explain the principle behind a cloud chamber, for those of you who aren't familiar with it. In theory, the hot water bottle pressed against the felt-soaked isopropl alcohol would cause it to evaporate, whereupon it would cool and sink towards the bottom of the chamber. At the bottom, it would be much cooler because of the dry ice just beneath the chamber floor. The alcohol would enter a supersaturated state, forming a cloud such that the slightest nudge from equilibrium would trigger condensation into tiny droplets. The alpha particles emitted by the americium, which are really just doubly-ionized helium nuclei, would provide such a trigger, allowing us to watch where individual atomic nuclei are going. Which is pretty amazing when you stop to think about it.
Now, after having told you all this, I hate to break it to you that our experiment didn't work because we were unable to initiate the formation of a cloud. We have several theories, which I won't bore you with, as to why it didn't work, but the upshot is that we just don't know. Apart from that, we discovered a few other hurdles with our setup, all of which simply gives us more to tweak and tinker with as we try to get it going. When we do, rest assured you shall have pictures.
On the lighthearted side, we had a little dry ice left over after the experiment, so to amuse myself I had fun dropping little bits into any standing water I could find around the house. Doing so made me thirsty, so I indulged in some of the official drink of mad scientists everywhere:
(Pellets of dry ice in blue Mountain Dew makes a most refreshing cold beverage)