Radiation Detectors | Part-1 | GM-Counter

In this Introductory part we will discuss about the basic principle of radiation detectors and detection of ionizing radiation in details.

 

    We can not see radiation  with our eyes but when neutral particles interact with radiation then charged particles are liberated, the detection of which gives an idea about radiation.

The principle of detection of nuclear radiation may broadly be classified as:

1. Method based on the detection of free charge carriers, by the ionization produced by them e.g. Ionization chambers, Proportional counters, Geiger-Muller counters and Semiconductor Detectors.
2. Method based on visualization of the tracks of radiation e.g. Wilson's cloud chambers, Bubbles chambers, nuclear emulsion plates, Spark chambers etc.
3. Method based on light sensing e.g. scintillation counters, Cerenkov counters etc. 

Now, we will discuss some important counters of the first type.

DETECTION OF IONIZING RADIATION | GM-Counter

How the ionization takes place? 

    When some external nuclear particle like an alpha particle comes into a material medium it collides with the molecules of the material medium and transfers energy to the molecules of the material of the medium. Now the electrons in the outermost shells of these molecules i.e. atoms absorb some of that energy and if the energy is sufficient enough, these electrons will become free and it leads to the creation of a positive ion and a free electron.

Ionization Chambers:

This type of radiation detectors depends on the interaction of gas inside the chamber with the incoming radiation. These detectors are usually called gas-filled detectors.

• Construction :

An ionization chamber consists of a closed vessel having a suitable gas (e.g. argon) in which the ions have long life and two parallel plate or cylindrical shaped capacitors where a moderate voltage (form few hundred to few thousand) is applied. The gas inside the chamber is subjected to some presser to ensure that the gas molecules collide with the incoming charged particles or the photons.


The construction of ionization chamber is shown in the schematic figure bellow,

• Working Principle :

The incoming radiation enters in the chamber through the thin mica window and collides with the gas molecules inside and ionize the gas-molecules. The ions are attracted to the electrodes due to the electric field generated by the voltage between the plates. Thus, a voltage pulse is generated between the plates. These Pules are amplified and registered.

The number of ions produced helps to detect the nature of the radiation.

Ionization chambers can detect α, β rays . they also detect γ - ray and x-rays.

Proportional Counters:

This is a type of gas filled detector modified from ionization chamber. Low energy ionizing particles can not be detected by an ionizing chamber because the generated voltage pulses are very small in amplitude. If the amplitude can be amplified then the pulses can be detected. This is done in proportional counter by utilizing gas multiplication.

• Construction:

Proportional counter consists of a metal chamber filled with gas and having a thin wire running axially through the Center. The wire acts as anode and the metal chamber acts as cathode. The are connected as shown in the above figure. A voltage of 250 to 800 V, which causes no discharge is applied between the electrodes.

• Working Principle: 

When an ionizing radiation enters into the gas chamber through the thin mica window then the produced electrons are attracted towards the anode wire. Since the electric field in the very high then the electrons gets high kinetic energy. When the collide with the gas molecules then additional ion-pairs (Secondary ions) are generated this process is called gas multiplication. Thus the pulses are amplified and the low energy radiations are detected.

Geiger-Muller Counter:

One of the very important nuclear detectors is Geiger Mueller counter. In the following section we are going to discuss about it.

What is a GM counter?

 

 GM- Counter is a kind of a nuclear detector which is capable of detecting different kinds of nuclear radiation like alpha particle, beta particle, gamma radiation.

Now the principle on which this kind of a GM counter is based upon is very interesting but before we go through that let's first talk about the construction of this kind of a counter.

Here, the construction of a GM counter is shown in the figure.

 This mainly consists of the Geiger-Muller tube which is nothing but a metallic cylinder which is hollow from inside and it contains some kind of a gaseous medium and the gaseous medium is usually consisting of some kind of a noble gas like argon and a mixture of other alcohol compounds. Now this metallic cylinder is connected to the negative terminal of a very high tension battery so this is acting as a cathode and through the center of the cylinder you have a metallic electrode which is made out of tungsten that is basically connected to a load resistance and which is again connected to the positive terminal of the same power supply. Now across the load resistance it contains some kind of electronic setup which is basically capable of determining any kind of a potential drop that happens across the load resistance. It can also measure the amount of current pulse associated with that. It is also associated with some kind of a counter that can count these events over a period of time.

 

Working mechanism of GM-Counter:

• Basic Working Mechanism:

The basic working mechanism of a Geiger-Muller counter or the principle of a Geiger-Mueller counter is based upon the Townsend avalanche effect. The Townsend avalanche effect is quite simple - When an external nuclear particle enters into the Geiger-Muller tube it creates ionized particles - free electrons and positive ions. They experience a huge amount of electric field inside the Geiger-Muller tube then they are accelerated and cause further ionization. This process leads to a chain effect that can cause an avalanche along the entire Geiger-Muller tube itself. That is the general working mechanism of a Geiger-Muller counter or GM counter.

• Detailed Working Process:

The working mechanism of GM-counter is very simple. When some kind of an external particle like an alpha particle or a beta particle or a gamma-ray radiation enters into the gas filled tube then it basically leads to ionization of the medium. It basically ends up by getting a positive ion and a negative electron due to the ionization caused by some kind of an external nuclear particle.

Now what happens to this positive ion and the negative electron when they are created by some external nuclear particle?

                They are subjected to an external electric field because the central electrode is connected to the positive terminal of the battery and the metallic surface is connected to the negative terminal of the battery (so the central electrode is acting as an anode and a metallic surface is acting as a cathode) and the electric field is created which is directed in a particular direction. So, when the positive ion and electron is created they will be subjected to an extremely high electric field.

 Now, GM counters are distinct from other nuclear detectors like proportional counters or ionization chambers in the sense that - Here we have a battery which is capable of creating a potential difference of around thousand to three thousand volt so this is a huge amount of potential difference under this potential difference the electron will experience acceleration towards the anode and the positive ion will experience acceleration towards the cathode direction.

 Now what happens is that once the electron is accelerated towards the positive anode it also is capable of causing further ionization. So, when an external nuclear particle induces ionization and leads to the creation of a free electron and a positive ion, the positive ion moves towards the metallic surface and the negative electron moves towards the central electrode. So, if the potential is extremely high enough, the electron will be accelerated to very high velocities and this electron will also collide with the gaseous molecules that lie in its spot and this electron is also capable of further inducing secondary ionization. Thus, an electron created during the primary ionization process, collides with the nearest neighboring molecule and ionizes that molecule then that molecule also lead to the creation of a positive ion and electron.  

Now, you have one electron which causes secondary ionization and again another electron from the above process so you end up by getting the previous electron and the secondary electron. Now these two electrons will also get accelerated towards the central electrode and they will also cause further tertiary ionizations along its path. So, this process will lead to some kind of chain reaction or an avalanche effect which will cause a whole lot of ionizations along the path before reaching the central electrode.

This kind of an avalanche effect leads to a complete discharge of the entire Geiger Muller tube. Now, when the electron is ionizing the gaseous molecules, the electron can interact with the potential field of a nucleus and experience deceleration. So, when an electron interacts with a potential field of a nucleus and experiences deceleration then the electron can lose its energy and lead to the creation of a photon. This kind of an effect is known as a bream strolling effect. Due to bream stalling effect during this process, when the electron is getting decelerated due to collisions with nearby molecules, ultraviolet radiation can also emitted. This kind of an ultraviolet radiation can travel to other corners of the Geiger Muller counter and create their own Avalanche effect in those places.

Even presence of a single electron will lead to an avalanche effect along the entire Geiger-Muller tube. Now once all of these electrons reach the central electrode they will get absorbed by the anode and then they will complete the circuit and lead to a potential drop across the load resistance. This potential drop is measured by a pulse detecting counter. 

Again the metallic surface then basically has the ions accumulated on the surface and they recombine with electrons from the circuit to create neutral molecules. Thus, it brings the entire setup back to its original state. This entire process, which was induced by the existence of one particle, will create one count in the counter of the Geiger Muller tube.

Dead time of GM- Counter: 

The entire process takes a little bit of a time when the electron leads to an avalanche effect and the electrons on the anode complete the circuit and the positive ions recombine with the electrons. During this time period the detector remains dead or it is inefficient. It cannot detect any further nuclear particles as long as this process does not get completed and the Geiger-Muller tube comes back to its original state. This time period is known as dead time. It is usually of the order of 200 to 400 microseconds.

 

Quenching in  GM- Counter: 

 Another topic associated with Geiger-Muller counter is known as quenching. When the electrons recombine with the positive ions in the Geiger-Muller tube then basically the electrons are free electron and it recombines with a positive ion.

If a free electron combines with the ion to create a neutral atom this process might lead to the emission of a photon. Now this photon is also capable of inducing another Avalanche effect but we don't want that because the count has already been made and we only want those counts that are associated with external nuclear particles. So, when this kind of an electron is recombining with the positive ion we do not want these kinds of spurious events or these kind of emitted photons that might lead to further avalanche effect. To prevent this from happening certain measures are taken and those measures are known as quenching.

The most popular are chemical quenching. In chemical quenching what we have is the gaseous medium is mixed with certain kind of other organic compounds like alcohol. In this case we have 90% argon and 10% alcohol. The purpose of this alcohol molecule is to absorb the excess amount of energy. Whenever the electron recombines with ion, instead of the excess energy being emitted in the form of a photon the excess energy is usually given off to a nearby alcohol molecule which absorbs that energy in the form of either vibrational energy or rotational energy.

There is another quenching method which is known as external quenching that is whenever the electron completes this circuit the Geiger-Muller counter basically shuts down for a very small period of time so the voltage almost becomes near about zero so that any kind of a photon emitted might not lead to any kind of an avalanche effect.

These methods which are used to prevent or reduce any kind of delay in the dead time during the particular recombination of ions with electrons in Geiger-Muller counter is known as quenching.

Advantages of Geiger-Muller Counter: 

 Now there are certain advantages to this kind of a GM counter. GM counters are usually quite simple and easily available in the market.

Disadvantages of Geiger-Muller Counter: 

They also have certain disadvantages. The first disadvantage is that it has a very high dead time that means it has a dead time of around 200 to 400 microseconds that means it cannot be used for extremely high counting rates. It can only be used for low counting rates.  If there is large number of nuclear particles which are coming in it might not be able to count every single one of them.

 The second disadvantage is that it cannot distinguish between the energy of two different nuclear particles. Let’s suppose you have two different nuclear particles which are entering this kind of a chamber. So, whether it is a high energetic particle or a low energetic particle? Both of them will lead to a complete discharge of the GM counter. Therefore, current pulse for the less energetic as well as a high energetic nuclear particle is exactly the same and it cannot distinguish between different energies of the particles.