A friend of mine was telling me about building a cloud chamber while he was in graduate school. As I understand it, this allows you to "see" interactions caused by high energy particles going through the cloud chamber. This has fascinated me, and I would like to build one with my daughter, but I want to make sure I am able to explain it to her when the eventual questions come. Can someone help me out please? How would I explain a cloud chamber to someone who is a freshman in high school?
Answer
Feynman used to say - if you can't explain something in simple words, such that a child could understand, then you don't understand it either. So here's my take:
A cloud chamber is nothing more than a box where mist is about to form, but not quite yet. There's vapors of stuff (either alcohol, or water, or something else) in it, and the temperature is such that the vapors are almost about to produce mist (or "clouds"). Imagine wetlands or marshes on a cold autumn morning, it's kind of like that - fill a box with that kind of "cold wet air".
Now a charged particle (such as Alpha radiation from a chunk of radioactive ore) zips through the chamber at high speed. It bumps into water (or alcohol) molecules and ionizes them - it creates a trail of ionized molecules marking its path.
Now, the vapors are such that they really want to produce mist; any tiny disturbance is enough to push them over the edge. The trail of ionized molecules is enough to do that - the ions attract a bunch of molecules, the resulting clumps attract even more, and before you know it a droplet of water is formed, then another, and another. Voila, a trail of mist follows the particle.
I could try to describe the construction, but this Instructables page will do it much better:
http://www.instructables.com/id/Make-a-Cloud-Chamber-using-Peltier-Coolers/
Basically, you evaporate some alcohol and let it run over a very cold area (cooled by the Peltier elements). Like breath coming out of your mouth in the cold air of winter, the alcohol vapors will tend to produce mist, so some vapors will turn to mist anyway. But the process happens a lot faster when a charged particle zips through the chamber - so, if you place a tiny bit of radioactive material nearby, tiny white trails will seem to come from it and traverse the chamber, because mist tends to form that much better around the ionized trails left by the radiation in its wake.
More designs:
http://www.lns.cornell.edu/~adf4/cloud.html
http://www.nothinglabs.com/cloudchamber/
http://www.bizarrelabs.com/cloud.htm
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