Geiger counter |
| The traditional Geiger counter responds to the passage of an ionising particle through air with a small click in a speaker. The radiation enters a chamber through a thin aluminum window and leaves a path of ionization. The ions are then quickly swept out of the tube with an electric field. The small pulse of current gives the click. |
 Hand-held Geiger counter - the background in 214 is ~15 counts per minute. |