0:00 hey guys how's it going and welcome to this tutorial on the scintillation mechanism and the sodium iodide detector so the most commonly used detector is your sodium iodide detector and as you can see there is a TL in the brackets which stands for thallium and usually what uh what is done is the sodium iodide detector is or the sodium iodide crystal is activated using thallium and the word activated basically means that it is doped or an impurity is added to the sodium iodide Crystal and that impurity is thallium and this sodium iodide crystal is an
0:48 inorganic Crystal so it's scintillation mechanism differs considerably from the organic crystals so in this video I'll be telling you guys or rather explaining this insulation mechanism so let's without further Ado let's begin okay so what happens is you have these kinds of balance bands and conduction bands in your sodium iodide Crystal due to the delocalized electron bonding and also one more thing um there's impurity that is Italian is what it does is it replaces one in Thousand atoms of sodium iodide so so it is a very small impurity so
1:33 what happens is your sodium iodide Crystal without thallium has these two valence and these two bands that is a valence band and the conduction band due to the delocalized bonding and when you add this impurity that is the thallium atoms then what happens is you get these kinds of electron orbitals between these two bands so the higher orbitals are called activation Center excited States and the lower orbitals are called actuation Center ground States and also the the electron orbitals produced between these two bands are basically called The actuation Centers so when your sodium iodide crystal is kept without any radiation incident on it then what happens is all the electrons stay in the valence band and
2:19 there is no electron in the conduction band however when you place your crystal inside or in the presence of some radiation then what could happen is um your gamma ray Photon maybe if you have some gamma ray radiation incident or a crystal then the gamma ray Photon would impart its energy to one of the electrons of the crystal now this electron would gain a lot of energy and will move out the crystal causing ionizations and excitations as it moves about now most of the time what would happen is the excitations that this electron would cause they would also like this electron with the impart its energy to more electrons and what will happen is those electrons will lose their energy by in the form of heat
3:06 however some of the electrons would gain enough energy to be raised into the conduction band so they will jump into the conduction band from the valence band and once they are in there they would want to come back to the valence band because it is of lower energy however this process is quantum mechanically forbidden so and so this cannot take place and when this electron goes from the valence band to the conduction band it leaves a hole inside the valence band so what could happen is one of the electrons from the activation centers could come to the hole inside the valence band because these two have almost similar energies and this electron the excited
3:51 electron in the conduction band could come to the actuation center excited States because they this process is quantum mechanically possible and they have almost similar energy however this one is lower so it would want to still come into an energy level that is lower and then the conduction band so it can come to the activation Center excited state so when once this electron comes from the conduction band to the actuation center excited state and one electron goes from the activation Center ground state to the valence band so there's a hole here and there's an electron in so this transition from the activation Center excited state to the activation Center ground state is possible and when this electron comes
4:36 back to the ground state then it would release a photon of the energy corresponding to the band cap and that is how you would get a photon of approximately three electro volts energy from the sodium iodide thallium activated detector so let's say if your gamma ray Photon was off um your gamma ray Photon was of like 662 KV for CCM Source then let's say it imparted like 500 KV to one of the electrons in the Crystal and then that electron would cause a lot of excitations and ionization so it will make a lot of electrons go to the
5:23 conduction band and when all of those electrons come back to the ground state by the process that I just explained then that would lead to a lot of photons of energy three electron volt that corresponds to the ultraviolet or blue range of the electromagnetic spectrum so a lot of photons in within that range would be um emitted and that is how you see the flashes of light or the scintillations in the sodium iodide and here is another diagram I should have rather used this diagram to explain the process in a better way so you can see here that the first process that is the a process is that one of the electron goes to the conduction band then this cannot come back directly and
6:10 when it went there it caused a whole to be here and so one of the electron from the ground state of this band would come here and this electron would go to the activation Center excited state and then finally it would come here and this C process will lead to the emission of its scintillation Photon so well that's it I hope you guys understood that it was pretty easy to follow and if you liked the video don't forget to hit the like button and subscribe to my channel for more videos like this I have two or three more videos on scintillation counters so you better check them out thanks for watching and have a great day
6:59 thank you
