Invented in 1911, cloud chambers like this one were among the first devices used to study high-energy particles called cosmic rays, believed to come from exploding stars called supernovas.
As cosmic rays travel through the air, they collide with air molecules and break them up into charged fragments called ions. Meanwhile, inside the cloud chamber, a tray of alcohol is heated to form alcohol vapor. The alcohol vapor in the cloud chamber condenses around the trail of ions created by a cosmic ray, a trail you see as a visible streak of tiny droplets.
Most of the tracks you see—roughly 4 out of 5—are made by unstable elementary particles called muons. Muons have a negative charge like electrons, but they are 207 times heavier. The rest of the tracks are electrons; these tracks are shorter and curlier than muon tracks. Occasional heavier tracks are made by protons or by natural radioactivity in the air or in the exhibit itself.
Today, cloud chambers have been replaced by tracking chambers and scintillators, which track particle paths by producing an electric signal, rather than a visible trail.
When cosmic rays from outer space reach the Earth’s atmosphere, they crash into air molecules and knock free many more particles. These particles go on to crash into more air molecules and create even more particles. Thus a single particle from outer space—called a primary—can create a shower of particles from the Earth’s atmosphere—called secondaries. Primaries (which are actually protons) rarely reach the Earth’s surface; almost all of the tracks in this exhibit are from secondaries (muons and electrons).
Artist’s rendering of showers of particles produced by cosmic rays. (click image to enlarge)
Before he founded the Exploratorium, Frank Oppenheimer used cloud chambers while working as a physicist at the University of Minnesota. In this 1948 photo, Frank and his colleagues loaded various sensors—including a small cloud chamber—into a sphere to be lofted 100,000 feet into the atmosphere by balloon, to document the interactions of cosmic rays. Photo courtesy of Lawrence Berkeley Lab, University of California, Berkeley. (click image to enlarge)