We are targets. Every millisecond, hundreds of neutrinos blast through us as if we were not here. And, to a neutrino, we're not. Cosmic rays, high-energy particles created by such events as supernovae, also pass through us, though at much smaller numbers. They are, however, more hazardous, in that their considerable mass (relative to a neutrino) means they are capable of causing damage in sufficient quantity. To those of us on the Earth's surface, cosmic rays are not a serious problem. To inhabitants of the International Space Station, however, there is reason for interest, and perhaps even concern.

Before the advent of sophisticated and sensitive particle detectors, researchers employed an apparatus known as a cloud chamber to observe the properties and behaviors of particles created as a result of radioactive decay. The chamber, also known as a Wilson chamber (after Charles T. R. Wilson, who went on to earn a Nobel Prize for his invention), operated on the principle that air which is completely saturated withan evaporant is very sensitive to any disturbance, and any particles within it that can act as a condensation nucleus will be readily evident. In the case of a cloud chamber, the evaporant is alcohol. An electrically-charged cosmic ray, in passing through alcohol-saturated air, will create an ionized trail, which will cause the alcohol vapor to condense in its wake. It's not at all unlike the contrail left by a jet at altitude, which may often widen to hundreds or thousands of times the width of the original craft as high-level moisture condenses in its wake.

Contemporary researchers have abandoned the cloud chamber for more powerful and exacting instruments, but it is still amazingly satisfying to be able to construct a device which will make visible the presence of particles thousands of times smaller than a virus. It's both easy and inexpensive, and the result is that you can not only explore the behavior of subatomic particles with no special equipment more advanced than a slide projector, you can also be a witness to some of the most violent events in the galaxy, all in the comfort of your own home.

For instructions on how to build a simple cloud chamber which will allow you to observe cosmic rays at the rate of about one per second, visit Andy Foland's Cloud Chamber page at http://www.lns.cornell.edu/~adf4/cloud.html

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