Geiger-Muller Counter: Principles, Construction & Applications
Geiger-Muller Counter (G.M. Counter)
The Geiger-Muller counter, named after its inventors, evolved from the ionization chamber and is a very efficient and accurate device for counting individual particles. Because the detecting portion is generally in the form of a tube, it is also called a Geiger-Muller tube or G-M tube.
Construction of a Geiger-Muller Counter
- It consists of a hollow metal case enclosed in a thin glass tube. This hollow metal case acts as a cathode.
- A fine tungsten wire is stretched along the axis of the tube and is insulated by ebonite plugs. This fine tungsten wire acts as an anode.
- The tube is evacuated and then partially filled with a mixture of 90% argon at 10 cm pressure and 10% ethyl alcohol vapors at 1cm pressure.
- The fine tungsten wire is connected to the positive terminal of a high-tension battery through a resistance R, and the negative terminal is connected to the metal tube.
- The direct current voltage is kept slightly less than that which will cause a discharge between the electrodes.
- At one end of the tube, a thin window of mica is arranged to allow the entry of radiation into the tube.
Working Principle
When an ionizing particle passes through the tube, it ionizes the gaseous mixture, thereby releasing a number of ion-pairs. The positive and negative ions so produced are drawn towards the appropriate electrodes, and the resulting pulse of current through the tube produces a voltage pulse (as small as 10 µV) across R. An electronic pulse amplifier accepts these small pulse voltages and amplifies them to about 5 to 50 volts. This amplified output is then applied to a relay-type recorder or to a counting rate meter. This G. M. counter is used for the measurement of β- and γ- rays.
Characteristics of a G.M. Counter
The characteristic shows the plot of count/min as a function of voltage.
- For voltages less than 1000 Volts, there is no discharge and hence no counts.
- Between 1000 – 1200 Volts, the number of counts increases linearly with the applied voltage. This region is called the proportional region.
- Above 1200 Volts up to 1500 Volts, the count rate shows the least variation, almost constant. This region is called the plateau region or Geiger region or operating region.
- If the voltage is applied above 1500 volts, a continuous discharge will take place, and the count rate increases rapidly due to the discharge of Argon gas, which is undesirable.
Applications of the Geiger-Muller Counter
Geiger counters have many applications in radioactivity detection. Here are a few examples:
- To detect radioactive rocks and minerals in the course of mineral prospecting.
- For fire responders to make an initial determination of radiation risk.
- For hazard management personnel in checking for radiation danger in an emergency situation.
- To check for environmental levels of radioactivity near a nuclear power facility.
- To test for danger amidst a nuclear accident or leakage of radioactive coolant.
- To check for radioactive contamination of clothing and shoes in your workplace.
- Radiation detection in the scrap metal processing business.