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In 1928, Geiger and Muller in Germany developed a ‘Particle detector’ for measuring ‘ionizing radiation’. They named it ‘Geiger Muller Counter’. It has been one of the most widely used nuclear detectors in the early days of Nuclear physics. It is a gas filled counter that operates in the Geiger region. The main difference between ‘proportional counter’ and GEIGER-MULLER Counter is that in the former, the avalanche is formed only at a point whereas it forms in the central wire in case of Geiger Muller Counter. The amplification does not therefore depend on the initial ionization produced by the ionizing particle.
The construction of a typical Geiger-Muller Counter may be almost identical with that of the proportional counter and is shown in figure.
It consists of a metallic chamber with a thin central tungsten wire insulated from the outer chamber. The central wire is at positive with respect to the outer chamber and hence the central wire acts as anode while the outer serves as cathode. If the outer chamber is made out of glass, then its inner surface is wanted with some conducting material to serve as cathode.
Geiger-Muller Counter is usually filled with noble gases such as argon, neon etc. The velocity of propagation of the discharge is about and as a result of this spread of the avalanches, the charge available for collection by the wire has a constant value. Thus the output pulse observed, of the order of few volts, is independent of the magnitude and location of the initial ionization. When the electrons from the avalanches have been collected acts as a partial electrostatic screen and reduces the field at the wire below the below the value required for ionization by collision so that the discharge should case. This simple expectation is, however, modified by the fact that the positive ions can liberate electrons from the cathode when they reach it. Since the avalanches may once more occur and a single ionizing event can, therefore, lead to multiple discharges. The discharge, therefore, has to be quenched. There are two ways in which the discharge can be quenched:
- Externally-by-lowering the voltage with some suitable electronic device or
- More simply in an internal way by adding a politic gas, such as ethyl alcohol vapours, to the argon gas.
The counter is then said to be self-quenched. The alcohol performs the ‘quenching’ action in a manner described below.
The ionization potential (11.3 eV) is lower than that argon (15.7 eV). As a result the positive ions moving out towards cathode consist mostly of alcohol ions. The positive argon ions on their journey to cathode undergo many collisions with the complex alcohol molecules. The argon positive ions, therefore, are neutralized by acquiring electrons from alcohol molecules. The ions reaching the cathode give up the ionization energy by dissociating themselves into simple molecules or atoms rather than by the initiation of new electrons. The discharge is thus quenched.
The counting life of such counters is apparently unlimited. The counting starts at some particular voltage, Vg known as threshold voltage, and then shows a sharp rise with increase in applied voltage. It then settles into an almost flat region known as plateaus.
Main features of a GEIGER-MULLER counter:
- Constant output pulse size, independent of initial ionization.
- A long intensive time to allow entry of each particle. The insensitive time is usually made definite by decreasing the applied voltage with the help of suitable design of external electrical circuit; it is then called the ‘Paralysis Time’.
- Sensitive to the production of even a single ion pair.
- Ability to detect V and other cosmic rays.
When a GEIGER-MULLER tube operates in Geiger region, the secondary electrons increase the current pulse by further ionization of gas molecules. The object of counter is to produce a single pulse due to entry of a particle. The tube should not then give any succeeding spurious pulses but should recover as quickly as possible to be in the state to record the next entering particle. Therefore, it is desirable to prohibit the production of spurious pulse following the main required plus due to a single particle entry. The process of prohibiting the undesirable secondary pulse is called quenching. There are two method of quenching known as internal and external quenching.
In this method a suitable poly atomic vapour such as ethyl alcohol or hydrogen gas is introduced along with the inert gas filled in counter tube. The quenching gas must have the ionization potential lower than that of inert gas. The quenching action may be understood as follows. The ionization potential of alcohol (11.3 eV) is lower than that of argon (15.7 eV). As a result the inert gas ions get neutralized by transferring their charge to alcohol ion during their long path towards cathode. The alcohol ion produced, capture electrons from the cathode and are neutralized. Hence there is no multiple pulsing and the discharge is quenched. The cathode is coated with graphite to avoid chemical attack.
In this method a large series resistance is used in the counting circuit. Due to secondary emission of electrons a large current pulse is produced. This current produces a large voltage drop across R, thus lowering the potential difference between the electrons of the counter, so that further gas ionization is avoided. This technique is not usually employed as it requires large recovery time (known as deal time of counter) for the tube to operate for next count.
- The operating voltage should correspond to the midpoint of flat plateau region.
- In case the continuous discharge is produced, the voltage should be lowered.
- The applied voltage must be relatively stabilized.
- Introduction of light should be prevented to avoid photo electric effect.