Page 12 - Curriculum Visions Dynamic Book

Measuring radioactivity
When Henri Becquerel first detected radioactivity, it was because
it had caused a photographic plate to become fogged. It was quite natural, therefore, that Becquerel should use photographic film as a radiation detector. By seeing how much a film had become fogged, he was able to get a rough guide to the intensity of the radiation.
This technique is still used to measure the amount of radioactivity received by hospital or laboratory workers. The small badge that such people wear contains a film that is developed on a regular basis.
Geiger counter
One of the early workers in the field of radioactivity was Hans Geiger. He developed a sensitive radiation detector using a gas-filled tube.
The Geiger–Müller tube (Geiger counter) is a metal tube fitted with a thin metal wire at its centre. The tube is filled with gas (usually a mixture of neon and bromine gases) at very low pressure. A large voltage is applied between the central wire and the outer casing.
Normally nothing happens. But in
the presence of radioactivity, radiation causes some of the gas atoms to break up (they ionise). This allows the gas in the tube to conduct electricity. The flow of electrical current can be detected
on an electrical meter. In many Geiger counters the flow of electricity goes into a special circuit that makes clicks in a loudspeaker. The greater the intensity of the radiation, the more often current flows and the more clicks
are heard each second.
Each of the types of radiation has
a different power of penetration,
so cylinders can be designed with “windows” – different thicknesses of body wall – in order to measure each type of radiation.
 A film badge measures the dose and type of radiation its wearer receives. Like the Geiger counter, badges have different “windows” (thicknesses of shielding over different parts of the film) for this purpose.
 A modern Geiger counter.
12
12