The principle of Geiger-Mueller counter
Geiger-Mueller counter is a gas field detector developed by Geiger and Mueller in 1928. It works in the Geiger region or plateau region of the I-V characteristic of a gas-filled detector. At voltages below the plateau region i.e. in the proportional region the output pulse height depends on the number of primary ions. Which is produced in the gas as well as the applied potential difference. In this region avalanche due to gas, multiplication occurs only at one point where primary ionization takes place.
In the Geiger region due to an increased electric field, the avalanche spreads along the whole length of the anode (central wire). As a result, the dependence of output pulse height on primary ionization and applied voltage disappears and it becomes constant. The output pulse is thus independent of the nature of ionizing particles and their energy, so it cannot distinguish the type of particles detected and cannot measure their energies. It is primarily used as a counter of charge ionizing particles and the large output pulse size obtained obviates the need of electronic amplifiers.
The Geiger-Mueller counter consists of a glass tube with a thin axial wire along its axis as shown in Fig(7.5-1). The thin wire made from tungsten acts as an anode and a copper cylinder surrounding in acts as a cathode. A high voltage between 800-2000 V corresponding to the operating point of the Geiger region or plateau region, is maintained between the wire and the surrounding cylinder.
- The electric field in the vicinity of the thin central wire is always very high. The tube is filled with argon gas at a pressure of about 10 cm mercury column. A small quantity (nearly 10%) of the ethyl alcohol is introduced into the tube as a quenching agent to increase its resolving power. The glass tube is provided with a window of very thin mica foil or cellophane or glass so that particles of small penetrating power such as β-particles or slow β-particles can enter inside it.
For the detection of fast β-particles, instead of taking a copper cylinder as cathode it is often made of a thin layer of colloidal carbon deposited on their inner wall of the glass envelope. In this way, β-particles can enter the tube from all directions. A resistance R is connected between the electrodes in the counter circuit so that the current pulse produces a voltage across it. The voltage pulse is then amplified and counted by an electronic counter.
When counter operates in the Geiger region, the ionizing particle passing through the tube ionizes the gas. And the electrons released by ionization is accelerated towards the axial wire (anode). This electron acquires a very high velocity and produces a large number of electron-ion pairs by repeated collisions with the atoms of the gas. The secondary electrons so liberated are also accelerated and more electron-ion pairs and produced. This multiplication action is very rapid and an avalanche results all along the length of the axial wire. Thus a large pulse of ionization current is produced. And it is independent of the number of primary electron-ion pairs. Which is generated by the incident particles as well as applied voltage.
When the positive ions produced in this process are accelerated towards the cathode, due to their larger masses their velocities remain very small as compared to the velocity acquired by the electrons. Therefore positive ions do not contribute much to the avalanche. But after the avalanche set in some of the positive ions with considerable kinetic energy collide with a cathode. And secondary electrons emitted and these electrons start a new avalanche. This results in the lengthening of the current pulse and continuous discharge.
If ionizing particles are received in quick succession, then they cannot be properly counted. Thus for the detection of successive incident particles by GM counter, it must region in shortest time. Its original state after the discharge starts. In another word, the time of discharge must be very small. The process of limiting continuous discharge called quenching. A slow-moving positive ion takes about 100 µs to reach the cathode. Within this time the counter remains insensitive and if any other ionizing particle enters the tube, the counter is unable to detect that particle. Because of the potential difference between the electrodes decreases to a low value during this time due to avalanche at the anode. This time is called dead time τd of the counter.
After dead time counter also take additional time nearly 100 µs to restore its initial working conditions. This time is called recovery time τr of the counter. The sum of these time (τd+τr) is called paralysis time. This counter will respond only to another ionizing particle. After the time τp from the time of entrance of earlier ionizing particle.
Ordinarily, to reduce the paralysis time of the counter self-quenching method is generally used. In this method, a small percentage (about 10%) of vapour like ethyl alcohol or methane is added in the tube as a quenching agent. This agent prevents the release of secondary electrons from the cathode surface.
As the positive ions move towards the cathode, they get neutralized after taking electrons from ethyl alcohol molecules during collisions with these molecules and by emitting ultraviolet photons. They are thus prevented from reaching the cathode. Thus only alcohol ions reach the cathode. They acquire electrons from the cathode and absorb emitted ultraviolet photons and become alcohol molecules. These excited molecules lose energy through dissociation. Thus the possibility of continuous avalanching by emitting photons is prevented.
In this article we have discussed about Geiger-mueller counter in detail. We have also discussed its construction and working in most easiest way.
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