GM counter working and construction
What is GM counter
A Geiger Muller (GM) counter is used to detect the direction and measurement of all types of radiation like α, β, and γ radiation.
GM counter Construction
A Geiger Muller counter consists of a fine wire (usually made of tungsten) mounted along the axis of a cylindrical cathode made of glass or metal with a metalized inner coating and filled with a suitable gas mixture.
The gas mixture usually consists of the organ (90%) at 10 cm of Hg and ethyl alcohol (10%) at 1 cm of Hg. The diameter of the cylindrical cathode or tube varies from 1cm to 5 cm and its length varies from 2cm to 100cm depending on the purpose for which it is to be used.
The circuit to which the Geiger Muller tube is connected to detect the radiation is shown in the figure. A high voltage is established across the cylinder and the wire.
When ionizing radiation passes through the gas in the Geiger Muller tube, the ionization of gas molecules takes place. The electrons produced in the process of ionization are attracted towards the central wire and counted as a single voltage pulse.
The wire (i.e anode) is surrounded by the slow-moving positive ions sheath which in turn reduces the electric field. As a result of this decreased electric field, the discharge stops. The discharge continuous to cease till the positive Ions around the wire move away from it. Since it is positive and takes enough time to move away from the anode, so during this period of time, the Geiger Muller counter remains insensitive or dead.
This time period is known as dead time. Thus, the dead time of a Geiger Muller counter is the time during which the counter fails to record any other ionizing particle entering the counter.
As the positive Ions move away from the anode (i.e. central wire) toward the cathode, the value of the electric field increases and another electric discharge takes place due to the production of electrons when positive ions strike the wall of the cathode. As a result of this, the continuous electric discharge takes place and the counter is not ready to receive another incident particle.
In order to record the chain of incident particles (i.e. ionizing particles), it is essential that the electric discharge caused by the first ionizing particle is completely quenched before the arrival of a new particle.
Though quenching depends upon a number of parameter like voltage, shape, and size of the counter it is the mixture of the gas which is of vital importance. Alcohol has a low ionization potential of 11.3 eV and Argon has an ionization potential of 15.7eV. Argon iron gets neutralized by acquiring an electron from the alcohol molecules.
This means that the ions reaching the cathode are particularly that of alcohol, where they get neutralized. The energy which these ions carry is absorbed in dissociating the alcohol molecule. The alcohol vapor also absorbs the ultraviolet photons emitted during the avalanche stage and prevent them from ejecting photoelectrons from the cathode.
This means that ethyl alcohol has two specific functions. First, it prevents the photoelectric effect of the quanta at the cathode. Second, it prevents the creation of secondary electrons, when the positive ions impinge on the cathode.
GM counter Working
The operation of a Geiger Muller counter is explained as follows: A radioactive source whose half-life period is very long compared with the duration of the experiment is taken and placed at a suitable distance from the counter. The applied voltage is gradually increased in steps keeping the source counter assembly undisturbed.
When the applied potential is low, the multiplication factor (m) shall below show the output pulse will not be strong enough even to operate the mechanical counter. The counting rate shall therefore be zero.
As the counter potential is raised, the multiplication factor increases, and more and more of the ionizing particles, starting with the ones which produce maximum ionization in the counter, start getting counted.
When the high voltage is reached, secondary ionization takes place and we find that there is a region where the number of impulses increases almost linearly with the applied voltage. This is known as the multiplication region where the total number of Ions produced is proportional to the number of original ions. This region is most suitable for proportional counters.
The counting rate rises and soon the Geiger region is reached when pulse due to all the ionizing particles become of the same size and are recorded, This operating potential is called Geiger threshold.
When the voltage is increased beyond the threshold, the counting rate remains the same for a large variation in counter potential though the pulse height rises. The pulse rate remains the same as all the particles are being counted.
This straight portion of the curve is called a Plateau. This plateau a continuous, till the excess potential over the threshold, called the overvoltage become so high that the counter breaks down into continuous discharge.
For a good Geiger Muller counter, the plateau region extends over 200 V. The operating point is so chosen that the value of overvoltage is not critical and small fluctuations in it do not affect the counting rate.
Recovery Time and Resolving time
The counter remains dead or insensitive till the positive ions have moved sufficiently away from the avalanche sites to put the wire back at the Geiger threshold potential. The counter is insensitive to further ionization pulses as the ion sheath moves towards the small.
The time for which the counter is dead is called dead time or resolving time. The recovery time is the time after which the original pulse levels are restored. The usual resolving time of the order of 100 limits the counting rates of G.M. tubes to about 5000 pulses per second.
Electronic Circuit for External Quenching
Neher and Harper used the following circuit for external quenching where self-quenching is not possible. The wire W is connected to the plate of the pentode (T). The outer cylinder is connected to the control grid.
Rp is the plate load which is of the order of 2 Megohms, Rg ranges from 1 to 15 megohms. Ep is the plate voltage, whereas Eg is the grid bias. The grid bias is adjusted in such a way that no plate current flows before the arrival of the ionizing agent.
On the arrival of the ionizing particle, the gas gets ionized and the current flow in the plate circuit because the grid becomes less negative.
If the change in the grid voltage is sufficient, the voltage drop in Rp will result in the effective counter voltage dropping below the threshold value. Thus the counter discharge is extinguished and the circuit recovers to its original condition.
Efficiency of Geiger Muller Counter
The efficiency of the Geiger Muller counter is defined as the ratio of the number of particles of radiation detected to the number of particles of radiation emitted.
That is, η = Number of particles of radiation detected / Number of
particles of radiation emitted
if α-particles do not have very high energy, they are absorbed by the gas and enclosed in the cylinder. β-particles cause ionization of gas molecules in the Geiger Muller tube and hence Geiger Muller counter is most efficient for detecting β-particles.
γ-rays are detected when they scatter an electron in the Geiger Muller tube enclosing the gas. Thus, Geiger Muller counter detects all types of radiation:α, β, and γ but it is not most efficient for β-particles.
Advantages of Geiger Muller counter
Geiger Muller counter has the following advantages:
- It is durable and easily portable.
- It is a low-cost device.
- It can detect all types of radiation. i.e α, β, and γ radiations.
Disadvantages of Geiger Muller counter
- Its efficiency is low.
- The output pulse from a Geiger Muller counter is always of the same magnitude irrespective of the energy of the incident radiation. Therefore, the Geiger Muller counter can detect all types of radiation but it cannot distinguish which type of radiation is detected.
- It cannot determine the energy of the detected radiation.
- During the dead time, it cannot detect any incident radiation. Therefore the measured count in the Geiger Muller counter is less than the actual count.