- Makes use of the ionising ability of the three types of radiation to detect them
- Consists of a metal tube(cathode) sealed with argon gas and bromine vapour at about 1/10 of the atmospheric pressure.
- Anode is the rod lying along the axis of the tube.
Steps to detecting ionising radiation
- A potential difference of about 400V to 500V is applied between the cathode and anode.
- When an ionising particle enters the tube through the thin mica window, the particle causes ionisation inside the tube. The negative electrons are attracted to the anode and the positive ions to the cathode. The high potential difference between the cathode and anode accelerate the ions and they collide with other atoms.
- Finally, an avalanche of ions is produced. When this avalanche of ions reaches the electrodes, a pulse of current flows through the G-M tube. This pulse of current is counted by a pulse counter whose reading goes up by one. The process in which a few ions initially created by the ionising particle finally give rise to an avalanche of ions is known as gas amplification.
Bromine vapour is used in the tube to act as a quenching agent.
- The bromine molecules absorb the energy of the positive ions moving towards the cathode.
- In this way, secondary pulses produced by the ejection electrons from the cathode surface when energetic positive ions strike its surface can be eliminated.
- Hence, the G-M tube will only give one pulse of current for each ionising particle that enters the tube and an accurate pulse count can be obtained.