Hans Geiger and Ernest Rutherford invented the original Geiger counter in 1908 to detect alpha particles. Geiger and Walther Mueller revised it in 1928 to detect other forms of radiation as well. The Geiger counter's sensor is a central metal wire anode surrounded by a thin metal cathode tube filled with neon, argon and a halogen gas that detects radiation by how much the gas inside the tube is ionized.
Turn on the Geiger counter to apply an electrical charge to the anode wire. The counter will click or flash about 10 to 20 times per minute as it detects background radiation.
Pass the sensor, called a Geiger-Mueller tube, over the material to be evaluated with the thin mica window facing the material. Radiation from the material, if any, will pass through the window and ionize the gas inside the tube.
Study the readout, whether a needled meter, flashing LED or audible clicking. If this is higher than the level of the background radiation, the material is radioactive.
Count the number of clicks or flashes or read the attached meter to determine how radioactive the material is.