Nuclear radiations emitted by disintegrating nuclei cannot be sensed directly. Indirect methods are employed to detect them. The Geiger Muller (GM) Counter is a radiation detector that is used for detection and measurement of alpha, beta, and gamma radiation. The other radiation detectors are Ionization chamber and scintillation counters.
The GM tube consists of anode & cathode. The anode is a fine wire in the middle of the cylinder. The cathode is the long metal cylinder. A small amount of an inert gas is added to the tube.
Basic Principle of Operation of Geiger Muller Counter
Radiation ionizes the gas through which they pass and produces few ions. If the applied voltage is strong enough, these ions produce a secondary avalanche and small voltage drop is recorded across the load. This voltage is amplified so that the counter can record it. The Geiger Muller Counter works as follows
- The battery voltage is kept below the ionization potential of gas.
- When radiation enters the GM tube through the mica window, it ionizes some of the gas atoms.
- Ionizations creates positive and negatives ions.
- Positive ions go towards the cathode and negative ions towards the anode.
- During this process, ions collide with some of the gas atoms, causing further ionization of the gas.
- This process continues till the whole of the gas atoms are ionized.
- This will cause a pulse of current to flow through the resistance, RL.
- The resulting voltage drop across RL is the output voltage.
- This voltage drop is sufficient to reduce the anode voltage below the ionization potential and therefore ionization stops.
- Now, gas is not ionized, no current flows through RL. There will be no output voltage.
- The output pulses across RL is amplified using vacuum tube amplifier and registered by a counting device.
- The number of particles in a particular interval of time entering GM tube is proportional to the number of pulses.
- Using GM tube, alpha, beta, and gamma radiation can be easily detected.
image reference: sites.google.com/site/puenggphysics/home/unit-iii/gm-counter