Geiger-Muller Counter
Geiger-Muller Counter The Geiger counter may be a particle sight designed to detect radiation, like alpha and beta additionally as a gamma-ray (although with considerably lower sensitivity than other types of detectors). It was invented by the German physicist Geiger and later improved by his student Muller, therefore the name Geiger-Muller counter. It is probably one of the most famous radiation detectors, a clue to its simplicity.
Principle Of Geiger-Muller Counter
Because of the high positive voltage of the central wire, the electrons will be attracted to it while the positive ions will be attracted to the cathode wall. The high voltage will accelerate the positive and negative charges, they gain more energy and collide with more atoms to release more electrons and positive ions; the process escalates into a Gas Multiplication or Avalanche which produces an easily detectable pulse of current.
Geiger-Muller Counter Construction
A typical GM. tube essentially consists of a cylindrical cathode in the form of graphite the coating on the inner wall Of a glass envelope or copper cylindrical cathode and an anode within the form of fine metal wire that stretches among and on the axis of the tube. The tube has 1cm to 100cm of length and 0.30m to 100m of diameter. It is filled with a mixture of inert ' gas (argon or neon) at a partial pressure of about 10cm of mercury and a quenching gas (halogens or organic vapors) .at a pressure of 10cm of mercury. To allow ionizing particles to enter the tube, a window covered with a thin sheet of mica is provided at one end of a tube mic has generally preferred 1n the construction of Geiger-Muller tubes mostly because it is able to detect alpha particles, as opposed to glass, but it is also much weaker than glass and can be easily damaged. Although it's aforesaid that Geiger counters sight radiation, most of them are not sensitive to neutrons. Nevertheless, the Geiger-Muller tube can be modified accordingly by coat g it with boron or by using boron trifluoride gas or helium-3.
Working of Geiger-Muller Counter
In the Geiger-Muller (GM) region, gas amplification 15 increased to the point where any single ionizing event will produce so many secondary ions that a very large pulse is produced. A single beta produces a few in pairs; these ion pairs produce more ion pairs until literally millions of ion pairs are produced.
This result is named avalanching and is that the result of the high voltage potential across the positive and negative electrodes. When ionization occurs, the negatively charged ions (free electrons), which are much smaller than the positively charged ions, move quickly to the positive electrode. The positively charged ions, which are larger, move much more slowly. In fact, they tend to form a cloud of ions that moves gradually to the negative chamber wall. The cloud of positively charged ions effectively forms a second ‘positive electrode’, which actually divides the large voltage potential into two smaller voltage potentials one between the highly positive central electrode and the positively charged cloud and another between the positively charged cloud and the negatively charged walls.
Because these 2 tinier voltage differentials take the place of the one giant voltage differential in result 2 small sub-detectors area unit made.
The voltage potential in either of those sub-detectors is well below the applied voltage of the Geiger Mueller region, so the avalanching stops. When avalanching occurs, it is so great
that the heartbeat size doesn't depend upon the number of particle pairs made by the radiation coming into the detector.
The pulse size is the same, no matter what type of energy radiation caused the ionization. For example, a small beta particle would produce a much smaller initial effect than a large alpha, but the avalanche would become so large that it would be impossible to tell what started it. Therefore, in the Geiger-Muller region, it is possible to tell that radiation is present, but it is not possible to determine the type of radiation.
In the gm region, the number of ion pairs collected is always the same, no matter how many ion ' pairs were originally produced by the radiation. If a second particle was to enter the detector during the avalanching period, it would not produce a pulse large enough to be detected.
A second pulse couldn't be detected until the results of the avalanche of secondary ions had been cleared out.
The time during which the second pulse could not be detected IS called dead time and IS equal to approximately 10/1000000 seconds.
After the first pulse dies out, another full-sized pulse can be produced.
The time from the beginning of 1 full-sized pulse till a second full-sized pulse will begin is.called the resolution time. A typical resolving time is about 10/10000 seconds
The dead time and the resolving time are really dependent on the time it takes for the positively charged cloud of ions to move toward the chamber walls. As we know that, after the avalanching occurs, there are two smaller voltage potentials present, like two sub-detectors, both operating well below the GM regiOn.
This means that the other particle of radiation interacting in either of the sub-detectors are affected by a far lower voltage, and the number of secondary ion pairs
produced 'will be very small in comparison with the millions of ion pairs already produced. Thus, the new pulse would go undetected, and this would be the dead time. As the positively charged cloud moves closer to the chamber walls, one large voltage potential and one smaller voltage potential sub-detector are present.
The larger voltage potential will cause enough gas amplification to supply a replacement pulse.
However, the larger voltage is not yet as large as it should be, so the new pulse is not full-sized, and it will not be detected by the outside electronics. When the positively charged cloud reaches the chamber walls, there is only one large voltage potential, and the detector is once again operating in the Geiger-Muller region.
Now a particle of radiation might cause a replacement avalanche of ions.
Again, the time before a full-sized pulse could be produced is the resolving time. Geiger-Muller tubes are the detection instruments used in the Geiger-Muller region. Most of those detectors use argon gas within the chamber as a result of argon gas is well ionized. However, there is a problem with argon gas. When the positive argon ion is collected on the conductor, item its photons within the style of UV.
The photons can undergo a photoelectric effect interaction with the walls of the chamber, producing more electrons and starting the avalanche all over again. To stop the continual avalanche, another gas, called a quenching gas, is mixed with the argon. The quenching gas is usually a halogen ' gas such as bromine or chlorine.
Applications of Geiger-Muller Counter
- It is very useful for counting B-particles. It can also be used for measuring the intensity of y-rays, X-rays, and electrons.
- It is also used to record the events involving high energy particles and cosmic rays.
- It can detect even small activities, i.e. it is sensitive to the production of even a single ion-pair.
- It can detect all types of radiation.
- It has a long insensitive time to allow entry of each particle.
