Geiger Muller Counter | Geiger Muller Tube | GM counter

Habib Khan | P.G Jahanzeb college Geiger Muller Counter (GM counter): Hans Geiger developed the device in 1908. In 1928, Geiger and Walther Muller improved the counter so that it could detect more types of ionizing radiation. The GM tube is one of a variety of radiation detectors that take advantage of the fact that charged particles lose energy in a gas by creating electronion pairs. The particles are usually beta and gamma rays, but certain models can also detect alpha particles. It is perhaps one of the world’s best-known radiation detection instruments

Construction: A Geiger-Muller tube consists of a sealed metallic tube filled with argon or another noble gas mixed with a small amount of alcohol vapour or bromine gas. The argon gas is called the detecting gas whereas the bromine gas or alcohol vapours are referred to as the quenching gas. The gas mixture inside the tube is at a pressure below atmospheric pressure. A thin metal wire runs through the center of the tube. An electric potential of up to 1 kilovolt is maintained between the metal wire (the anode) and the cylinder (the cathode). In the absence of any radiation no current flows between the wire and the cylinder.

Operation:When a radioactive particle enters the tube it ionizes an argon atom. The resulting electron is accelerated towards the metal wire or anode. As the electron approaches the metal wire it experiences an increasing electric field strength which in turn applies a greater accelerating force on the electron.

The accelerating force becomes so strong that on collision with other argon atoms the electron can ionize them. The electrons from these ionizations can go onto to generate a cascade of further electrons, an effect called the avalanche effect. The ionization by one particle can result in millions of electrons striking the metal wire.

This migration of electrons inside the tube results in an electric discharge. This gives a measurable voltage pulse in the external circuit of the Geiger-Muller tube. The counter registers the number of pulses and converts them into sound signals or displays them as a measure on the screen.

Where

is the linear absorption coefficient, and => =

Quenching Gas:

â„Ž

=> =

is initial intensity

When first pulse is detected by hitting the electrons to the anode, it emits secondary electrons. These secondary electrons may generate false counts. The quenching gas will slow these electrons and make the detector ready for next pulse.

The purpose of the quenching gas is to absorb the positive argon ions as they accelerate to the cathode.

Without the quenching gas these positive ions will be neutralized at the cathode in an exited state or could even also dislodge electrons from the cathode. These dislodged electrons or excited atoms could trigger further ionization creating a further voltage discharge giving inaccuracies in the measure from the device. When the quenching gas migrates to the cathode it recombines at ground state and so does not present the potential to cause any further ionization.

Dead time: After a count has been recorded, it takes the G-M tube a certain amount of time to reset itself to be ready to record the next count. The resolving time or "dead time", T, of a detector is the time it takes for the detector to "reset" itself. Since the detector is "not operating” while it is being reset, the measured activity is not the true activity of the sample.

Advantages:   

They are inexpensive They are durable and easily portable.

They can detect all types of radiation.

Disadvantages   

They cannot differentiate which type of radiation is being detected.

They cannot be used to determine the exact energy of the detected radiation.

They have low efficiency.

Applications:    

For the detection of alpha and beta particles To detect radioactive rocks and minerals in the course of mineral prospecting or as a mineral collector To check for environmental levels of radioactivity For Fire and Police first responders to analysis for making an initial determination of radiation risk.