We cannot see, hear or feel radioactivity, so we need detectors. Two methods you need to know are photographic film and the Geiger-Müller tube.
Photographic film is darkened by radiation in the same way that it is darkened by light. The more radiation that hits the film, the darker it becomes. People who work with radiation wear film badges. The film is developed afterwards and its darkness shows how much radiation the worker has been exposed to.
A Geiger-Müller tube, or GM tube, is a sealed tube filled with a low-pressure gas. When radiation enters the tube it ionises the gas, which makes a brief pulse of electric current. A counter connected to the tube clicks or displays the number of pulses. The more pulses each second, the higher the count rate, which shows how much radiation is arriving. For example, 600 pulses counted in 20 seconds is a count rate of 30 pulses each second.
