Theory
Scattering
Atoms
αβγ decay
Decay Series
Law
Half-life
Geiger
Carbon-14
Fission
Chain Reaction
Nuclear Energy
Nuclear reactors
Fusion

The prototype Geiger Counter was used in the alpha-scattering experiment.


Click on the symbol to listen to a geiger counter

The Geiger Counter ia a gas filled detector. When radiation enters it, the gas is ionized.

Geiger Counters are usually eqyipped with a speaker. A clicking noise indicates radiation.

How does a Geiger Counter work?
The Geiger counter is a refinement of the ionization chamber. The principle of operation of the Ionisation Chamber is set out below.

 Click on the symbol to listen to a geiger counter

, and-rays all cause ionisation. We can use this property to measure the count rate of a sample.

The ionisation chamber consists of a metal cylinder with a metal rod along its axis. The rod is insulated from the cylinder and maintained at a positive potential with respect to it. The cylinder contains a gas usually air, and the radiation is allowed to enter through a thin mica window, which keeps out stray ions.

If an -particle enters the chamber, it ionises the air and the positive ions travel to the cathode and negative ions to the anode. A single -particle might produce 10 to the power of 4 ions which would be registered as a pulse of current, and be recorded as one count on the meter.

If the pot difference is too low, the ions might recombine before reaching the electrodes and so the pulse would be too small to be detected. If the pot difference is too high, the ions would be accelerated to very high speeds and so could cause further ionisation by collision with the gas molecules, thus resulting n an ‘avalanche’ of ions travelling to the electrodes. Thus an important prerequisite for accurate use of a Geiger counter is to plot a Geiger plateau. The plateau is the range of voltages where the counter gives a consistent count rate, where every count corresponds to exactly one or particle entering the detector.