 Hello, everyone. In the previous lecture, I discussed the gas filled counters like ionization chambers, proportional counters and Geiger-Muller counters. Now, these gas filled counters you will notice that the density of gases is low and particularly for gamma rays, they are not very suitable. So, now I will discuss the detectors which are solids in shape, solid state type detectors, they can be emitting light or they can be semiconductor type. So, I have to the first lecture, I will take the Sintlation detectors. As the name itself applies, when the radiation falls on this material, certain materials are you know luminescent materials or they fluoresce. So, they are basically the fluorescing materials and the light falls on the radiation falls on them, they produce light and then that light can be used to in a protomultiplier tube to generate the electrons. That is the type of detectors we will be discussing in this lecture. So, Sintlation detectors are those detectors, which when radiation falls on them produce light. Now, there are certain requirements for a good Sintlative material. The first thing is that this should have a high Sintlation efficiency. Like if you have a 1 MeV of the gamma ray, then how much of that how much of the energy of the gamma ray is utilized in producing light. So, that is an important point. Higher the Sintlation efficiency, higher is the detector efficiency, the resolution becomes higher. Then once this light is produced in the crystal, it should be taken out of the crystal and so that the PRT can generate an electronic signal from here. So, this should be transparent to the wavelength of its own emission, the second important property. Then that decay, the fluorescence decay should be fast so that all the light is generated in a very short span of time. Otherwise, if it is a phosphor material, phosphorescence, then the light will be produced very slowly and we may not be able to integrate the charge that is being collected. So, short fluorescence decay time, typically few nanoseconds to few microseconds the order of time for fluorescence decay. Thirdly, fourthly, to make it compatible with the PMT, we require that the refractive index of the crystal should be compatible same order of the same order as glass because the PMT has a window made of glass. Sometimes you have the UV radiation being emitted so you require the UV compatible, UV transparent PMT windows. So, in general for normal PMT the glass window is used and so refractive index of the crystal should be of the order of 1.5. And lastly, it should be possible to manufacture these crystals in large sizes. So, all of them are important requirements for a material to be used as a scintillator. So, the block diagram of the detector scintillator is you have a crystal. The crystal could be 1 inch by 1 inch, 2 inch by 2 inch. There are some crystals which are available in 3 inch by 3 or even 5 inch by 5. So, it depends what type of application you have in mind. Then you have an optically couple the crystal with the PMT. We have a 1 inch by 1 inch crystal and you have 2 inch by 2 inch PMT. You have to have some optical coupling. So, there are optical coupling materials. Then you apply the pre-amplifier. PMT will have its filter circuit. Apply the high voltage to the anode of the PMT. And then the pre-amplifier signal is amplified using a scintillator amplifier. And the amplifier output analog signal you can take to multiple analyzer or it can go even through a timer scale. So, this is a block diagram of a scintillator detector. The heart of the detector lies in the crystal which has this above properties. Now that the scintillators can be of two types. One could be the organic scintillator or it could be inorganic scintillators. Organic scintillators are having their own advantages and their own disadvantages. Advantage is that they have a molecular structure. So, each molecule is fluorescent unlike inorganic scintillators where the entire crystal lattice generates the light. So, organic molecules with pylatron structure like pure organic crystals, anthracene, styrene and so on. So, each molecule and the light falls when it gets ionized. Then you can excite the molecule and the excited molecule can lead to fluorescence. So, you have this is the typical diagram for a fluorescence fluorescing molecule the singlet state, ground state, excited states. You can also have a triplet state which will undergo phosphorescence and the singlet excited state will undergo fluorescence. So, there are several molecules which will undergo fluorescence when they are ionized or they are excited. But the typical ones which are used in detecting detection of radiations are the liquid organic scintillators like topo, this is the triane-octile ospin oxide or PPO 2-5-9 oxazole. Basically, you have a pylatronic structure in molecules, conjugated double bonds that will give you the fluorescing nature of the molecule. So, you have each organic molecule acting as a fluorescence. Now, this organic molecules you have to dissolve in some solvent. So, you can use toluene, xylene and dioxin. So, for organics solution, you can use toluene, xylene or aqueous toluene, you can use dioxin. And many a times, you know, these fluorescence of these molecules may not be compatible with the PMT portability that tube. So, you use some other reagents called wavelength shifter like PPO 1-4-2-5-9 oxazole, benzene. So, these molecules take the radiation from the fluorescing molecule, get excited and redshift the light that is emitted. So, if you have a redshift, so then that the light that is emitted from these molecules will be compatible with the PMT. So, these organic molecules are very good to detecting the alpha particles or beta particles of low energy, but they have a low photofraction because they are mostly hydrocarbon material, carbon hydrogen oxygen. And so, the photofraction, the photoelectric effect which requires high jet material that is very small. So, there are some applications which they are useful, but for particularly for gamma counting, they are not very useful. So, organic scintillators are mostly used for gross counting of alpha and beta. So, alpha liquid scintillation counting, alpha you have for example, you have radium or you have peronium or plutonium, very small quantity microliters of solution can dissolve in an organic solvent. So, here we have a bile, this 30 ml bile and then you have this yellow with cocktail, cocktail of blood popo and PPO and popo and you dissolve the activity in this organic solvent. So, you have toluene or xylene, in that you have this activity. And then so, now this sample, active sample is a part of the detector material itself. And then you have to put it on the over the PMT and the PMT will take the light signal which you can go to the subsequent circuitry. So, here the advantage is that there is some sample dissolving detector material. Every time you want to count a new sample, you take a new detector attention. So, the efficiency is 100 percent because the radiation or radiations are being stopped in the detector material. For beta counting, efficiency is close to 100 percent, but since the range of beta is more, some part of beta may get lost. But mostly the scintillation counters are used for alpha beta counting. There are a class of materials which we here you can polymerize a producing material. So, it is a solid material, but it is still low Z, they are called plastic scintillators. These plastic scintillators are used to for gamma ray spectroscopy, but for timing, not for energy spectroscopy. So, organic molecules have very slow, very short fluorescence decay time of the arrow picoseconds. And because of that, they find applications wherever you have, you are doing lifetime spectroscopy. So, lifetime spectroscopy where you use like in positron endless spectroscopy, you have, you are detecting the time between the two in the birth and decay of a positron or a positronium atom. So, there you require very fast detectors which will generate a signal within hundreds of picoseconds. For those purposes, these organic scintillator based detectors are very useful, but they do not have a higher atomic number. So, for gamma counting, they are not very useful. For time purposes, yes, they can be utilized. Now, I come to the workahorse of gamma counting is called the inorganic scintillators, particularly the sodium iodide doped with thalium. So, inorganic scintillators are now based on their band structure. So, you have any crystal, a single crystal of sodium iodide, a single crystal. So, you have the valence band and the conduction band. So, it is an insulating material that the band gap, if it, the difference between the conduction band and valence band is of the prime electron bolt which falls in the insulating. Sodium iodide as such is a very poor scintillator, does not emit fluorescence with high yield. And secondly, even if it is emitting fluorescence, that is in the ultraviolet future. So, sodium iodide as such is not used for counting of gamma rays. Instead, if you dope this sodium iodide with a small quantity, 0.1 percent of thalium which is as an impurity, then this thalium impurity generates activator sites in between the valence band and conduction band. And now, so when the radiation is exciting, this valence band electron to conduction band, these electrons can get trapped in the activator exaggerated states. And these activator sites then undergo fluorescence to come to ground state of activator site and thereby emit light which is incompatible to the automotive light tubes. So, because by adding 0.1 percent of thalium to a sodium iodide jet crystal, you can have a, a fluorescing material of high fluorescence yield and compatible wavelengths to the PMG. So, the thalium impurity acts as a wavelength shifted like pop-up in the organic scintillators. Even then, the, the fluorescence intensity is not very high because there are computing processes to fluorescence. For example, the electron when it is generated in the conduction, it is going to conduction band, this may occupy activator site which all these activator sites may not undergo fluorescence emission to ground state. So, the ground state, so those radiations which are like they can be reduced in less transitions. So, those are not useful for detection practices. So, reduced in less transition between excited in ground state and the trapping of, occupying trapping of this electrons in the activator sites those transition programs should be forbidden that will, they will lower the fluorescence intensity of this crystals. So, sodium iodide thalium though it is pretty even this does not have very high simplest efficiency, but it still it is the course of gamma counting. So, advantages are that it is mostly why the scintillator used scintillator in since 1950s is available in large sizes I mean 3 inch by 3 even 5 inch by 3 I mean sizes available. You can machine it in such a way you can make a well type. So, you can have a detector you know. So, you have a well, you can put the detector the counting tube you can put here in this. So, you have a well type sodium iodide and the tube. So, the source is here. So, it is almost giving you 100 percent geometric efficiency. So, counting tubes can be inserted in the well of the sodium iodide thalium. Excellent light yield though it is not very high, but you see certain percent in the efficiency, but still in parallel soft scintillators it is called excellent. Only thing is it has got it is a hygroscopic in nature. So, when you are making a detector you have to house it in a air tight container to hermetically seal it in the aluminum kc and also so you should make sure that it does not moisture does not get inside. Resolution is not very bad very good it is about 6 percent we will discuss more on this. The scintillation efficiency is one of the best that you have among the different scintillators though it is much less than 100 percent still the 13 percent is considered to be pretty good. Now, why this resolution is so poor? We expected about 0.1 percent or 0.2 percent, but it is 6 percent and because of that the reason is that all the photons that produce in the crystal do not read the photocathode of the PMT. So, the light collection efficiency is very, very low. Similarly, the photocathode has its own quantum efficiency it is much less than 100 percent. So, first of all the crystal itself has got very low quantum efficiency yield then all the light that is produced does not reach the photocathode of the PMT and the photocathode does not have a good quantum efficiency. So, because of these three factors this sodium alder thallium or for that matter any scintillator does not have a good energy resolution. So, let us do an exercise here to compute the energy resolution of a sodium alder thallium. Why this resolution is so bad? So, typically let us see for a 1 MeV gamma A. So, out of the 1 MeV gamma A the scintillation efficiency is 13 percent means what? 13 percent of this 1 MeV is utilized in generating photons and so you can see so 1 MeV means 1300 1 MeV. So, 130 KV, 130 KV energy of the gamma A utilized in producing scintillation. And if you have 4 electron volt is the energy of each photon then you can calculate how many photons will be produced. So, you will be having 32500 photons of 4 EV, 32500 into 4 will be 1300 electron volt. So, that is how 130 KV you can calculate 13 percent of 1000 KV, 130 KV and 130 KV divided by 4 EV will be 32450. Now, this many photons are producing the crystal all of them are now going to photocathode only 20 percent of them may reach because you have a crystal here and you have PMT only one side. So, the light can be produced in all directions. So, you will have very small fraction reaching the reaching the photocathode. So, that is let us say 20 percent. So, out of 32500 only 6500 reach the photocathode and then photocathode has its own quantum it is in that state 20 percent. So, out of 6500 only 20 percent 1300 photo electrons will be produced at the photocathode. So, these are the primary ion pairs like you know charge carriers which will determine the resolution of detector because after this now the subsequent multiplication of electrons in the protein multiplied 2 you will get 10 power 7 or 8 electrons in the n order but they will not increase the resolution. First any multiplication will not improve the resolution is the primary ion pairs of electrons that will determine the resolution. So, the resolution you can see 100 into 2.35 upon root of n that is n is 1300. So, it becomes a 6.5 percent. So, I hope this explains the region for the poor energy resolution of scintillation detectors. Even then you will find that the sodium and the thalium are widely used in counting of gamma ray radioactive samples. Resolution is 4, 6 to 7 percent but the efficiency is very high. So, one of the positive points of the sodium and the thalium detector is the detection efficiency is very high. Why it is so? Because of the atomic number of iodine that is fifth. So, high atomic number means the photoelectric effect is more. So, because of that there the majority of the interaction will be with the density effect though Compton scattering also will take place but because of the high z of the iodine photofaction is very high. So, typically sodium iodine are in fact available also in well type just now I explain what is the well type sodium iodide and if you put the sample in the well then you get almost 100 percent efficiency because geometric efficiency is not 100 percent. So, for gamma ray energy less than 500 kV the efficiencies are of the order of 100 percent. So, for example you have 100 kV, 200 kV, gamma ray emitting source the efficiency is close to 100 percent. So, this is the typical gamma spectrum as a function of E gamma you have the counts and you have this Compton edge. So, you have the Compton edge and this is called the full energy peak. This full energy peak can be due to photoelectric effect or multiple Compton scattering. So, this is a broad peak because of the poor resolution and so it is sitting on a background. So, you can take the area of this peak by what you call as the linear subtraction of the Compton background. So, Compton is like linear. So, you can make a you can make a trapegym and take the gross area subtract the background and you get the total effective peak area of the gamma ray. Then this peak area you can divide by time to get the counter rate counts per second and then subsequently you correct for the efficiency and the abundance to get the absolute activity disintegration per second. So, while sodium iodide thalium is the workhorse for gamma counting when you do not need high resolution, but there are other scintillators like caesium iodide thalium, barium fluoride, barium fluoride used for it is emitting UV radiation. So, you have UV compatible PMT, but it has got fast timings. The sodium iodide thalium has got the producing decay time of 230 microseconds whereas barium fluoride has got in 200 picoseconds. Zinc sulphide silver groped. Zinc sulphide is not a single crystal. It is a polycrystalline material and groping with silver, silver is like no that weblens sifter. So, for alpha counting you have very thin layer of zinc sulphide powder on a perspex plate and coupled with the PMT you can use for alpha count. Bismuth germanium oxide BGO. Bismuth is a good material, high Z of the Bismuth makes it very high efficiency, but resolution is worse than sodium iodide thalium. And recently one new detector has come in the market, lanthanum bromide detector which is doped with cerium. So, lanthanum bromide detectors are another class of detectors which we will be discussing very shortly. So, in commercially you will find a sodium iodide thalium detector well type. So, this is the, you can see the pencil source half of it is going inside the detector well and this is the potter multiplier Q, this is the bleeder circuit and we have the unit timer scalar which will be giving you the counts. It will not give you the gamma spectrum. If you want to give the gamma spectrum then you can you can connect this with their multi-channel analyzer. So, the well type sodium iodide thalium is very widely used in the laboratories. It has got the 100 percent geometric efficiency and so for gamma energy of 500 kV or low it has got the even absolute efficiency as 100 percent. And the lowest lower energies let us say 50 kV and below at that energy because the aluminum casing it has got the aluminum casing that aluminum will reduce the gamma rate in intensity for the low energy gamma rays. So, efficiency will become low for low energy. So, typically you will find that the efficiency no energy of the gamma ray efficiency. So, it should have been going down like this but that at low energy again will fall down because of the aluminum window. So, this is how the efficiency for detection of gamma ray will change with the energy of the gamma ray. So, almost up to 500 kV you will find it is is close to 100 percent. Now, this is the advancement which has taken place in the last one decade or so more than just 10, 15 years. So, there are always research going on to develop new detective materials which are having better performance than the existing detector systems. And in this series a new crystal lanthanum bromide cerium doped synthesis detector has been developed. In fact, in a technical university delt they have developed this crystal. This detector offers the best energy resolution among the scintillators not all not any detector, but among the scintillators these detectors offer. So, like since 1950s, sodium iodide thalium has been rolling for the gamma counting for gross gamma counting and even for pure gamma sources. But this detector was developed and it has got 30 percent higher yield than sodium iodide thalium. So, when we say 30 percent higher fluorescence yield means it was having 13 percent. Sodium iodide thalium was having 13 percent fluorescence yield lanthanum bromide has got 16 percent. So, because of high yield you will find the resolution is better because more photons are produced for a same energy. So, the light yield photons per KeV sodium iodide thalium 38 photons per KeV lanthanum bromide cerium 63 photons per KeV compare this with garium chloride and VGO. So, if the number of photons per KeV are low, resolution is going to be worse. The resolving time that fluorescence decay time how fast the fluorescence is decaying. I was telling that this for sodium iodide thalium is to 230 nanosecond for lanthanum bromide is 26 nanosecond, barium bromide is 0.8 nanosecond, VGO 300 nanosecond. So, it is better than sodium iodide thalium in terms of the life fluorescence decay time as well as the number of photons per KeV. So, the resolution we can see sodium iodide thalium 7 percent lanthanum bromide 2.8 percent. So, you can see here this is a typical resolution of a lanthanum bromide detector 2.8 percent. So, this has in fact almost revolutionized the gamma respectability with scintillators. We will discuss subsequently that the this semiconductor detectors are much better in terms of resolution, but this lanthanum bromide detector not only in terms of resolution, but efficiency wise also both wise is better than thalium. Because of that more and more now searchers are using. So, the cost is higher, the cost of lanthanum bromide is much higher still cost approximation is not that high. And one of one only one drawback is there that 138 lanthanum which is a you know long lived isotope of lanthanum. Lanthanum 139 is the naturally occurring radioisotope and stable isotope, but because of this presence of 138 lanthanum which has got very long half life which is available in natural radium lanthanum and it is emitting a gamma ray high energy gamma ray. Because of that if you have a lanthanum bromide detector you will have a background peak in the gamma spectrum. So, though that background peak will be very very small if you count for a long time you will find a peak will appear somewhere in the high energy region. So, that is the only drawback, but other than that lanthanum bromide is going to score all points with respect to sodium iodide and others glycemic So, I have tried to explain the fundamental principles of silicon and the scintillation detectors for organic as well as inorganic. Organic scintillators good for alpha beta counting in liquid solution where you get 100 percent efficiency, but it is not good in terms of energy because of the hydro carbon nature of the organic molecules whereas the inorganic scintillators they are having rugged high Z materials sodium iodide or BGO lanthanum bromide because of that they have high protopraction and hence the resolution is also good, efficiency is also good, but now subsequent lecture we will discuss the semiconductors germanium silicon which though have low Z because of that efficiency is not very high, but you would have excellent resolutions and they are ideal for gamma rays. So, I will stop here. Thank you very much.