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The Question

(Submitted October 30, 2006)

Can you see cosmic rays? My dad told me about a science project he saw when he was in school that involved dry ice, a fish tank, and an iron, and maybe something else. With this set-up you can actually see cosmic rays. I found this very interesting and want to learn more about it.

The Answer

Thanks for your excellent question. Cosmic rays are generally single atoms or particles that are produced from supernovae events when a star dies. These tiny particles get to very high energies (fractions of the speed of light) and occasionally some of them reach the Earth. Most of these particles are deflected by the Earth's magnetic field or scatter in the Earth's atmosphere, but particles called muons can reach the ground. Hundreds pass through your body per second! You can read more about cosmic rays at our website:

http://helios.gsfc.nasa.gov/

Now almost 100 years ago, scientists discovered that muons leave a trail in a super-cooled, super-saturated mist (water vapor) that can be observed. The device that was first used to produce these trails is called a cloud chamber, see:

http://en.wikipedia.org/wiki/Cloud_chamber

The trail that is formed when a charged particle (muon) passed through the cloud chamber is likely what your Dad observed. We don't actually see the particles themselves (much too small), but we can see the trail of ionized gas that is produced as muons pass through the gas chamber. In fact, you or your classroom can actually build a cloud chamber without too much difficulty, see:

http://www.lns.cornell.edu/~adf4/cloud.html

Hope this helps,
Georgia and Mike
For "Ask an Astrophysicist"

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