10. Cloud Chamber

A continuous cloud chamber shows tracks of charged particles. Advance notice is needed to obtain the dry ice necessary to operate the chamber. Thoron gas (thorium emination, Rn 220, half-life = 1 min.) can be blown into the chamber to produce alpha particle tracks. Since the daughter nucleus Po 216 with a half-life of 0.15 sec. is also an alpha-emitter, two pronged tracks will be seen in the chamber. A needle with Pb 210 also produces a-tracks. Two to five students look at this demonstration at once so it is best to arrange a little time at the beginning or end of class for them to come down and look.

Cloud Chamber

Methanol evaporates from the trough, and the vapor falls toward the cold dry ice (-100 F = -73 C). In the process the vapor is super cooled; that is, cooled below its normal condensation point. When a high speed charged particle from a radioactive source or from a cosmic ray passes through the super cooled vapor, it ionizes the air and methanol atoms along the way; i.e., it strips electrons from these atoms. These ions and electrons serve as condensation centers for the methanol vapor, which condenses out in tiny droplets along the track of the charged particle outlining its path.

The charged particles from the radioactive source are typically helium nuclei (alpha particles). This source is "license free", meaning it is too weak to be considered dangerous by governmental regulatory agencies. Charged particles from cosmic rays are typically protons and muons.