 Hello everyone. Welcome to this lecture on radius and detectors. In the last lecture, I discussed the basic principles of radius and detectors and certain properties of detectors like energy resolution, detection efficiency, take time, etc. Today, we will discuss the basic principles of gas field detectors and what are their applications. The gas field detectors, the basic principle is schematically explained using this diagram here where we have a rectangular geometry of the gas field counter. It could be a tube, it could be a rectangular box which contains two electrodes namely cathode and anode. And you have to have let us say some window through which the radiation can enter the chamber. Once the radiation enters the chamber, already you know that whether it is alpha or beta or gamma which will cause ionization and excitation. And the detector functioning depends upon collecting these ion pairs. It could be positive ion and electron in some cases electron holds and so on. So, the idea lies in collecting the electrons at anode. So, you apply a bias at the anode. Here you apply a potential through this battery, electrons go towards the anode. And then when the charge is collected at the capacitance, this capacitor will discharge through the resistance and you can put a voltmeter to see the voltage signal at this particular place. So, essentially it is the mobility of ions, particularly the electrons in that electric field gradient that dictates the performance of the detector. So, when the electrons are moving across the electric field gradient, that gradient is given by V0 upon D. So, it is like a parallel plate chamber. So, between two plates there is a linear gradient in the electric field. So, if V0 is the potential applied at the anode, D is the distance between the cathode and anode, then the electric field gradient will be given by V0 upon D. And then the how fast the electrons will drift towards the anode depends upon the drift velocity, which is given by mu epsilon upon P, where mu is the mobility of the electron, P is the gas pressure and the epsilon is the electric field gradient. So, the typical electron will take about microseconds to reach the anode, maximum you can reach much faster, but later on you will see there is an avalanche of electrons produced in different types of detectors. So, that may make the process little sluggish, whereas the positive ions being bulkier take hundreds of microseconds to even milliseconds to reach the cathode. So, the principle of the detector system for a gas field counter is explained by this figure here, where we have different zones of applied voltage V0 at this place. And then that the voltage signal at the voltmeter, the relationship between these two quantities, the voltage signal at the voltmeter in the applied voltage dictates the functioning of different types of detectors. So, to begin with, when you have very low applied voltage, let us say we are in the region 1, then this the electric field gradient is not sufficiently high that the electrons will be able to reach the anode and positive ions will reach the cathode. Before that they can recombine. And that is why as you increase the applied voltage more and more electrons and positive ions are reaching the electrodes, but even then all the primary ion pairs, electrons and holes are not collected. And so, this is called the recombination region means where the electrons and positive ions combine to form neutral atoms. So, it is not useful for the detector. Then come to the second region ionization chamber, IC is ionization chamber. And in fact, these two graphs are for two radiations of different energy. So, higher the energy of radiation, higher will be the pulse height that you get in the detector. So, in the second region ionization chamber, the primary ion pairs that are produced as a result of interaction of radiation with the gas material, all the primary ion pairs are collected. And so, that is why you see the pulse height remains constant over this region of applied voltage. This is called the ionization chamber whereby you collect the ion pairs across the electrodes and it is a constant voltage, because whatever the content pulse height, whatever the ion pairs are collected, generally all they are collected. Then comes the second third region the PC, the proportional counter where there is now a secondary multiplication. That means whatever primary ion pairs are produced in the gas medium, they are further multiplied because the electron will acquire energy because of the higher electric field you are applying higher potential. And so, this primary electrons can further cause ionization leading to secondary ionization and there is a multiplication in the number of ion pairs, but that multiplication factor is constant. Therefore, the number of charge carriers is increasing proportionately to the initial energy of the radiation. That is why it is called the proportional count. When you further increase the applied potential to this region, then now there is a sort of saturation type of occurring and this is called the region of limited proportionality. And again, this region is not useful for detected application. But finally, a situation comes where irrespective of the energy of radiation like 1, 2 and here, any other type of energy radiation, you will find that you get the same pulse height in the detector. That is called the GM region, Geiger-Muller region. We will explain the details of the detectors subsequently. So, Geiger-Muller gives you the same pulse height irrespective of the type of radiation or its energy. Now, let us come to the first detector type that is the ionization chamber. I already explained briefly about this that the ionization chamber depends upon the collection of primary ion pairs. That means, let us say the alpha going to A and the gas let us say argon, give rise to argon plus iron plus electron. So, this is the primary ion pair. And collection of this primary ion pairs across the anode and cathode respectively will give rise to a voltage signal at the voltmeter. So, let us see how many ion pairs can be produced by a radiation of particular energy. So, for that will depend upon the W value. W value is the energy required to produce an ion pair, which should be in fact, in principle should be equal to the ionization energy. But now we know that all the interactions do not reach to ionization, some of them may lead to even excitation. And therefore, the W value is higher than the ionization energy, the particular gas medium. So, the number of ion pairs that are produced is energy of radiation upon the W value. The typical W values are given for different field gases like helium, argon, methane. You can see here the ionization energies are in the range of tens of electron volt and the W values are higher than the ionization energy. Because the some of the interactions may not lead to ionization, they may lead to excitation. Let us have an idea about what is the type of pulse height that you get in the detector in an ionization jumper. So, the pulse height in ionization jumper can be calculated from that circuit diagram which we show we have a capacitor and its capacitor will discharge through a resistance. So, let us see you have a 5 MeV alpha particle. So, what is the W value? W value let us take 30 electron volt. So, W is 30 EV and there are 5 10 power 6 electron volt energy of radiation. So, 5 this the energy upon W is the 1.6 10 power 5 ion pairs. Now, just take typical capacitors of the detector system as 100 picofarads. So, the voltage signal at the voltmeter will be Q by C where Q is the charge and C is the capacitance. So, you can put the charge due to this many electrons multiplied by the charge of 1 electron that is 1.602 to the power minus 19 coulombs and then they are divided by the capacitance 100 picofarads. So, that becomes 2.56 minus 4 volts or of the order of 0.2 millivolt. You can see here that the signal is quite weak for a just 1 5 MeV alpha. Because of that these detectors are not really suitable in pulse mode. Instead of that if you have a constant source of radiation you can use it in current mode that is what happens in a survey meter. In a survey meter you just keep on detecting the radiation level in a current mode that tells you the level of radiation. So, ionism chambers are actually used in survey meter which are more qualitative idea other than the quantification of the radiation levels. So, the applications of ionism chambers lie in fact there are some cases like fission fragments. Fission fragments have very high energy of the 100 MeV and because of that even the primary ion pairs you will get. So, compared to 5 MeV it is not 20 times more. So, 20 into 0.2 you can see here 22 into 0.2. So, almost 4 millivolt. So, 4 millivolt is a good signal. So, particularly fission fragments can be counted in ionism chamber without any background. And in fact in reactors 35 millilion line chambers are used for neutron monitoring in reactors. In addition to that they are also used in delta e detectors in particle identifiers in nuclear reactions where you have the heavy ions you want to detect them by using a delta e e telescope. But mostly the survey meters in current mode ionism chambers are used. And in some applications like ion chambers where you have a big source of few curies or few millicuries then you can determine the absolute activity using a 4 pi counter where all the radiation that is emitted is deposited in the chamber and you measure the current. So, ion chambers are also used for measurement of activity of intense sources. These are the applications of ionism chambers, but when it comes to the routine counting on pulse mode we do not use them we use the other chambers other called proportional counters. So, in proportional counters if you recall to recall the curve B0 versus the voltage signal then this is the one. So, we had a so you have this ionization chamber region IC and then we have a proportional region where you there is a secondary multiplication. So, at higher voltage than the ionization chamber this primary ion pairs can further undergo ionization in the gas medium. And so, each primary electron that is produced it will cause an avalanche of radiation ionization. And then so, there will be a large multiplication. So, this multiplication of ion pairs is the in fact the property of proportional counters and the important point is that the proportional the multiplication factor is constant. Suppose you have got n ion pairs produced as a result of primary ionization then the total chart that produced is m into n where m is the multiplication factor. So, the main point of these counters is that m is constant and that puts a strict requirement on the parameters of this proportional count. And the typical multiplication factor is temple power 3 temple power 5. So, you can see the pulse amplitude will increase by this much magnitude. But then this factor that fact that the m has to be constant put the requirement on the purity of the gas. So, there should not be any gas medium which will have a high electron attachment coefficient means any electronegative gas like oxygen or moisture they will they can pick up electrons from the ionization. And so, they can change the multiplication factor. So, typically argon gas 90 percent argon and 10 percent methane that is called p10 gas without any traces of the gas having high electron affinity like oxygen or moisture. So, this p10 gas is commercially available and they are filled in the proportional counter tubes. So, the question arises why this methane is used in this proportional counters along with argon. Regionalize that when you have argon if you take pure argon then the charge particle can cause ionization and sometimes it can also cause excitation. So, this excited argon atoms can emit photons ultraviolet or visible photons. And these photons can cause photo electrons from the cathode. So, once this photon is reaching it is escaping out from detector's volume upon reaching the cathode by photoelectric effect it can generate a photo electron and that photo electron is actually not wanted. So, this causes a spurious pulses. So, to take care of the spurious pulses that they do not arise we use a polyatomic gas with polyatomic gas which will collide with the excited argon atoms and the argon comes to neutral ground state whereas the methane gets ionized. So, we have from an excited argon atom we have got x the ionized methane molecule. Therefore, this methane subs has a squinted spurious pulses. So, proportional counters in fact come in different modes like the sealed proportional counters. So, this you have a cylindrical tube where you can fill the gas and the anode wire has to be very thin micron size you know very very thin you can see here typical diameters of let us say the 0.008. So, 0.08 millimeter or 0.8 0.8 0.8 0.08 millimeter. So, almost 80 microns. So, this thin anode wires are the hallmark of proportional counters because they have to be uniform in thickness. So, uniformity is same that means the electric gradient is constant all along the length of the anode wire. In proportional counters if you use them in the cylindrical format then the proportion that you can have very high electric gradients which is given by at any distance r from the anode wire we applied potential upon r ln b by a where b is the radius of the tube proportional counter tube and a is the radius of the anode wire and so you can see here that by applying a small voltage you can have very high electric field gradient. So, r ln b by a just to give you a field if you apply 2000 volts as the potential this battery then and b the radius of the tube is 1 centimeter the anode diameter is 0.008 centimeter then you can see by this formula you have at the anode wire high million volts per meter is the electric field gradient which is much much higher than that you get in the analysis channel. So, because of that you can achieve the secondary multiplication and then you can have the high pulse site in the. In fact, these the shield proportional counters have a problem of you have to have very high purity of the gas there should not be any moisture or oxygen and because of that there are a variety of variant of proportional counters called flow proportional counter. The flow proportional counter what happens it is not a shielded tube, but you have a assembly where you can allow the gas to flow from one side and it is coming out from the other side you have a you keep the source in the center of the base plate the base plate can be lowered down so that the source can be inside the cavity or if you want to when you want to raise when you are going to count you raise it up in the platform and this in fact these can remove it from this site by using a plate system. So, you can lower it and then took it out then you have the anode in the form of a loop on which you would apply the positive potential. So, this the flow of the gas has got the advantage that the purity is need not be so strictly such high you can purify the gas you can have in the in the circuit circuit you can have a loop circulation loop for purification. The advantages are many one as there is now the there is no window. So, just like alpha particle there is no loss of energy of alpha in the window there is no window source is exposed to the base medium second is that the geometry is 2 pi. So, you have almost 50 percent efficiency for counting so very high efficiency resolution is 0.5 percent we can see for a 5 MeV alpha and 30 electron volt is a W value you get 1.6 to the power 5 ampere resolution will be 0.6 percent. So, you have high resolution high detection efficiency and the purity of the gas is not that strict now because you can purify the gas or it is a purity. So, no window. So, these are the advantages on now many places the laboratory neurological laboratories have the pro-proposal counters are available in their counting systems. Proposal counters have a lot of applications like if you are using the wavelength dispersive x-ray diffraction system then for x-ray counting you can have proportional counters filled with high jet gases like xenon and ketone with high jet gases will have high photo fraction. For neutron counting you can have helium 3 to bf3 gas filled with proportional counters and other than that you have the gas proportional counters for alpha counting beta counting even you can use the patient fragment counting in this proportional count. So, there are a lot of applications of proportional counters. So, you have you are usually gross counting of alpha beta or low energy x-rays you go for the electron count. Lastly, I come to the Geiger-Muller counter. Geiger-Muller GM counter we commonly called in fact they are the one of the oldest detectors they were developed by Geiger and Muller in 1920s and despite that you know after almost now it is going to be a century they are still being utilized widely in laboratory experiments because of the simplicity, low cost, ease of operation very easy to operate. I will show you a photograph of the detector system. So, the gas the GM counters still are in the experimental laboratory when you are doing experiments for the MSC students or you know any PhD students initial training in fact some of the applications where simple count is required GM counters can be used. So, if you recall again the graph B0 applied potential versus the signal and we showed that like for two detectors. So, this was the recombination region this was the allusion chamber proportional region and here is the limited proportionality and now here at still higher applied voltage you will find both the radiations have given the same pulse. So, in fact not only these two radiations you take any photon even one electron one photon anything will give the same pulse. So, that is the the specialty of the GM counter but this is also had the drawback that it does not distinguish between different types of radius. So, I will try to explain this GM region in more details here what I have shown here in this graph is called the the cascade of avalanches in proportional counter what happened every primary ion pair every electron that is produced in primary ionization each electron produces one avalanche. So, what I have shown here is the avalanche avalanche means each electron is multiplied several times when it is undergoing ionization in the medium. So, a electron in proportional counter gave 1000 to 10000 electrons. So, the avalanche sizes and the power 3 or 5 8 to 5 whereas in the case of Geigermuller counter each electron that is producing an avalanche and every avalanche can produce other multiple avalanches. So, it is a continuous process of avalanches generated by not only the primary electrons but also by the secondary electrons and because of that this GM counters have very very high multiplication factors of the other row and power 6 to 10 power it multiple times they let the signal is multiplied. So, what is happening is that when the electron is generated at any point let us say it is produced here. So, this is a tube you can see here this is a tube this is an anode wire and now wherever the electron is produced it will try to go to the anode and along the path of the this going towards anode it will be generating ionization that ionization and during that ionization process it can even cause that there will be excited molecules which will emit light. So, UV radiation can be emitted these UV photons can further cause ionization and generate another avalanche again from this you can have another UV photon generate avalanches and it can be from to the other side also it is like an anode wire is you know it is in the tube. So, you have all along the across along the surrounding the anode wire you have the gas. So, on all sides of the anode wire there will be sequence of avalanches and these avalanches can be generated by electrons or the UV radiations that are produced in the the excitation of the excited molecules. So, this avalanches generate a large number of electrons and peers. So, ultimately what is happening that the so many electrons are collected within a few microseconds, but at the same time there will be a sheet of positive ions. These positive ions are going towards the anode but their velocities are much low. So, what happens because of the slow movement of the positive ions along the anode wire there will be a sheet or the cloud of positive ions that is a space charge due to positive ions around the anode. In fact, that space charge becomes so high because that start dictating the performance of the detector. So, what happens that when the space charge become very high the effective potential across the anode becomes low and then the detector stops function because you require a certain applied voltage but because of the positive space charge generated due to several avalanches effective electric gradient becomes low and this happens only when a particular amount of space charge I can create. So, the the process of avalanches continues till the space charge becomes so dominant that this detector stops functioning. So, every time the space charge becomes a certain value then the detector stops functioning because of that the Geiger-Muller counter will stop functioning only when so much of space charge will be created. So, this in fact essentially explains why the Geiger-Muller counter cannot distinguish between different types of radiation because till that time the space charge has been created you are going on creating the avalanches. So, that explains the simple functioning of the Geiger-Muller counter. So, irrespective of the type of energy of radiation and type of radiation the Geiger-Muller counter gives the same pulsate. Further it cannot distinguish between different types of regions whether it is a single electron or this PTZ X-ray or alpha or beta or even friend present fragment all of them will give you the same pulsate. Pulsate but the pulsate is very high these few volts you can imagine 0.2 millibolt was the pulsate in analysis in Jumbo and now you have a very high multiplication almost a million times. So, you will have a few volts signal. So, you do not need even an amplifier and that is why it becomes very rugged. Of course, it has there is a restriction on the fill gas helium or argon pure highly pure helium or argon gases are needed this will be free of any traces of gases which can form negative ions. So, moisture and oxygen in are prohibited from the fill gases. Why this strict requirement of this purity of the gases because again the what happens that there are in fact there is some process called multiple pulse. So, what happens that we just not discussed that the positive ions are very sluggish to move towards the cathode. So, there is a cloud of positive ions along the anode wire but eventually even if it is few hundreds of microseconds they will reach the cathode and once they reach the cathode the ions positive ions will they get neutralized during the process that energy is released energy equivalent to the hydrogen energy of the positive ions. If that energy released is the energy of the gas minus the work function of the cathode that much energy is released. So, if this energy released happens to be more than the work function of cathode then what happens it can generate a electron from the cathode. So, if the energy released during the neutralization of the positive ion more than the work function of the cathode then another electron will be generated at the cathode and then that electron which is not wanted. So, that electron can generate another geiger discharge and that is called multiple pulsing. So, we do not want that the positive ion when it is reached the cathode generates another electron. So, that is called the multiple pulsing which is not wanted in the geiger number count. So, the geiger the multiple pulsing has to be quenched and that is done by different methods one is the external quenching electronically that means when you can reduce the high voltage temporarily for a time for a few 100 microseconds. So, reduce the high voltage for a fixed time after every pulse you can have an electronic system. But you know it affects the count rate you cannot handle very high count rate. So, normally it is not a very preferred mode other methods are internal quenching where you use a 5 to 10 percent of a second polyatomic gas of low hydrogen energy and having a complex structure. So, like for example, ethanol vapors. So, what happens now when this you have the positive ions the positive ions can collide this polyatomic gas and this polyatomic gas one could get dissociated. So, instead of that positive ion reaching the cathode and generating another electron or multiple pulsing they can collide with these molecules and lead to annihilation or it can even get dissociated. But then this eventually what happens that this polyatomic gases will get exhausted because they will get dissociated and because of that the lifetime of these geocubes are shot. Another method is halogen quenching where you have a chlorine or bromine gas which which also get dissociated. So, when the positive ion collides with this chlorine molecule Cl2 you have the chlorine atoms produced. And so, but this chlorine atoms can again subsequently combine to form the chlorine gas because of that they had a long light. So, 11 quenched GM tubes are very commonly available in the market. Lastly, the applications of GM counter as I mentioned you cannot distinguish between different radiations and different energy also. So, they are used for simple counting like if you have a simple beta count beta active sample you can count the activity or in a laboratory you want to do experiments on statistics for that time determination of a GM counter. So, this is a simple this is a GM tube and you can put the samples in different geometry depending upon that there are these are the slots where you can put a sample and you can see there is no amplifier and straight away you take the signal to that this counting unit you can apply the voltage. And this is a very low cost instrumentation maybe about 50,000 rupees you can even now get a diagram on the count. So, these instruments are very commonly used in the laboratory or demonstrating experiments on statistics that time and if it is a pure beta source you can even count activity. So, I will stop here I will take up the other detectors in such a minute. Thank you.