For 10 points, name this device that tracks the paths of particles via the formation of small air pockets.
They have been used as detectors in the SIMPLE and PICASSO experiments, and are now being used by the COUP in their attempts to detect WIMPs.  A major drawback of them was an inability to measure energy directly, but they could infer momentum based on the radii of magnetically induced helical paths.  A piston is moved to expand the volume of a fluid with low surface tension, causing it to become superheated, so that a tiny amount of additional energy will cause the desired phenomenon, the density of which is proportional to the ionization of a particle. For 10 points, name this device that tracks the paths of particles via the formation of small air pockets.
They have been used as detectors in the SIMPLE and PICASSO experiments, and are now being used by the COUP in their attempts to detect WIMPs.  
bubble chamber