Spark chamber at Brookhaven National Laboratory. Credit: BNL.
Up to 100 parallel metal plates are stacked in the chamber. connected alternately to the positive and negative terminals of a very high-voltage supply (with a voltage of some thousand volts between each plate). The spaces between the plates are filled with gas, usually neon or helium. The ionizing radiation leads to spark formation between the plates along the path of radiation through the chamber. The spark trace can be registered photographically or electronically.