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Teachers who use cloud chambers with their students most often use them for the sole purpose of observing radiation trails. The phenomenon is quite impressive, because the emanations go on at a constant rate with no apparent decrease in activity, despite lengthy observations of the chamber. However, we would like to suggest that the cloud chamber also can be used by students as an experimental tool, allowing them to perform their own investigations of radiation. This active experimentation on the students' part will help them to understand some aspects of the behavior of radiation through first-hand experience and is much more interesting for them than the traditional simulations of nuclear experimentation.
For anyone unfamiliar with the apparatus, the cloud chamber is used to "see" radioactivity. A student model cloud chamber is usually a short cylindrical box made of transparent plastic. See Figure 1. It has a removable lid, a black bottom surface and an absorbent paper wrapped around the sides. The paper has a few windows cut into it so that a bright light may be shined into the chamber. A small amount of ethanol is used to soak the absorbent paper. The lid is set firmly in place and the chamber is placed on dry ice. A source of alpha or beta particles is placed inside the chamber by insertion through a hole in the side of the chamber. As the radioactive emissions from the source streak around the chamber, they leave ionized gas particles in their wake. Super-cooled alcohol vapor then condenses on these charged nucleation points. The path of the radiation is thus seen as a fog trail of condensed alcohol.
Cloud chambers may be purchased from many science supply companies. A typical cloud chamber, with a radiation source, costs about 20 dollars. The series of experiments that we present here works best when the students have access to a number of cloud chambers. As such, we have found it much more economical for the students to construct their own chambers from less expensive materials. In addition, because the entire chamber is constructed by the students, alterations in design to accommodate different experiments are more easily made. One added benefit for the students is the opportunity to construct their own experimental equipment....