0:00 hi welcome to my channel so since the last couple of videos I have been talking about nuclear detectors so what our nuclear detectors nuclear detectors are special instruments that can detect nuclear particles like alpha particles beta particles gamma radiation neutrons protons etcetera and also determine the energy and other such parameters of those particles now one very common way in which many of the nuclear detectors works is like this when an external particle enters a material medium then through some mechanism the material medium absorbs the energy of the incident particle and converts it into a current pulse and by studying the current pulse by looking at the size of the current pulse we can get an idea about the energy of the incident radiation now the detector that we are going to discuss today also works on a similar
0:46 kind of a mechanism so today's detector is the scintillation detector so I'm going to talk about the construction as well as the principle and working mechanism of a scintillation detector so here I have drawn a very simple diagram to represent the basics of a scintillation detector so as you can see the scintillation detector mainly consists of two main parts the first is a scintillator material and the second is a photomultiplier tube alright so let me talk about each of them so the first what is a scintillator a scintillator is a special kind of a material medium in which if a charged particle enters it basically absorbs that energy of the charged particle and leads to the creation of light so this is known as scintillation so scintillation is the property of a
1:32 material medium that when an external charged particle enters then it absorbs that energy and leads to the emission of light how does that happen so basically when some external nuclear particle enters a scintillator medium then the external nuclear particle let's suppose an alpha particle is entering the medium then it basically collides with the molecules of the material medium and every time a collision happens it transfers energy to the material medium now the electrons in a material medium which are in the valence band absorb that energy and jump to the conduction band and every time the electrons jump back from the conduction band to the valence band it emits a particular photon so every time this kind of Adi excitation happen particular photon is emitted so basically what happens is that the material medium absorbs the energy of
2:17 the incident particle and converts it into low energetic photons now different kinds of scintillation detectors use different kind of scintillated medium for example a scintillator made of cesium iodide is used to detect protons and alpha particle while sodium iodide is used to detect gamma radiation and zinc sulphide is used for the detection of alpha particles so the job of the scintillator is very simple when an external alpha particle or a gamma radiation enters the material medium it basically converts the energy of the external particle into low energetic photons in greater the energy of the incident radiation larger will be the number of photons created now all of these photons is focused into a photo cathode so what is a photo cathode the photo cathode is simply a material that can experience photoelectric effect
3:03 what is photoelectric effect so whenever some kind of a incident photons falls onto a mantle surface then if the energy of the photons is sufficient enough then electrons are ejected from the metal surface this is photoelectric effect so some kind of a photo cathode material is placed on top of a photomultiplier tube so when all of these light photons which were created as an incident particle enters the scintillator now these light photons are focused on to the photo cathode which result in photoelectric effect and the emission of photo electrons or primary photo electrons now these primary photo electrons enter the photomultiplier tube now what is the purpose of a photomultiplier tube the purpose is very simple it simply needs to increase the initial photo electrons
3:49 to a very high value so that it can lead to some sort of a significant current pulse in the electronics connected to this particular system how does it do that it does it in a very simple manner so there is a particular metrical construction inside the photomultiplier tube so as you can see when the light photons came and hit the photo cathode it led to the emission of photo electrons and these photo electrons are now directed towards a particular electrode so this electrode is at plus 100 volt now as you can see here these curved surfaces basically all represent electrodes and they are dynodes so there is a particular reason why every single one of this electrode is shaped like this so initially when the photoelectrons were emitted by the photocathode
4:35 they were directed towards the first die node now when the electrons reach this particular die node and strike the surface of the die node and every time the electron strikes the surface of the Tyler it also leads to emission of secondary photo electrons so let's suppose that one electron strikes the surface of a dye node then that leads to the ejection of a secondary photo electron which is now directed towards another dye node or the second eye node alright so as you can see here the first dye node is at a potential of 100 volt while the second eye node is at a potential of plus 200 volt now when the photo electrons hit the first die node they lead to ejection of secondary photo electrons and these photo electrons are not directed towards a second eye node and since there is a potential
5:21 difference between both these two diodes and there is going to be an electric field so that the electrons are now accelerated towards the second eye note itself so every time these photoelectrons strike a particular dye node they lead to the emission of secondary photo electrons now when the secondary photo electrons reach the second eye node again the same thing happens with every single collision a large number of secondary photo electrons are emitted so here again you will end up getting a large number of photo electrons which are now directed towards the third diode because there is a potential difference of 100 volt between them so if I complete this diagram you end up seeing that at every step the total number of secondary photo electrons keeps on increasing
6:11 so as you can see here the effect of this construction is an exponential increase in the total number of photoelectrons so initially the primary photoelectrons were ejected from the photocathode when light photons hit this particular material and they were directed to as a first electrode now every time the photo electron hits this surface it also leads to the emission of photoelectrons the number of photo electrons now increases they are now directed towards the second diode because there is a potential difference there these electrons are accelerated when they strike the second I node they again lead to the emission of more secondary electrons so there is a cumulative increase in the total number of electrons they again are directed towards a third diode which are again directed towards the fourth the fifth and on and on and with every step the total number of photo electrons are increasing and there is a cumulative
6:59 increase in the total number of photo electrons so finally when the total number of electron reaches this particular anode which is connected to some sort of electronic system then there is a huge amount of current pulse associated with it so as you now realize the construction of this entire setup is just meant to increase a few number of electrons to a huge number of electrons so as to have a significant current pulse now the size of the current pulse which is detected by some sort of an electronic setup is basically dependent on the infinite number of primary photo electrons now the primary photo electrons are created because of light photons every single light photon creates one primary electron so the primary electrons are basically dependent upon the number of light photons and a number of light photons as I already told you are dependent on the
7:45 energy of the incident nuclear particle so in a way the size of the current pulse created here is dependent upon the energy of the incident nuclear particle so by studying the current pulse you can get an idea about the energy of the nuclear particle which entered this particular material medium so a scintillation detector is very effective in telling us about the energy effects and particle as well as in telling us about the intensity of incident particles so I hope you have understood the gist of how these kind of detectors work an incident nuclear particle creates scintillation inside a scintillating material which basically leads to emission of photons these photons goes towards the photo cathode which basically leads to photo electric effect and the emission of photoelectrons these photoelectrons are directed to us a electrode where when
8:30 they hit the surface they lead to ejection of large number of secondary electrons they are again directed towards another electrode which is at a higher potential does increasing kinetic energy of the electron and creating for the secondary electrons when they hit the surface which are again directed to another electrode which are again directed to another electron thereby increasing the total number of electrons so that we can observe some kind of a signal current pulse associated with this nuclear particle so that is the general sort of a working principle and mechanism of a scintillation detector that's it for today thank you very much
