 So, this brings us to a new kind of detector which I will describe to you known as position sensitive detectors or PSTs. What do you mean by position sensitive? You can see here I have shown you that this is an incident beam and these are the scattered beams. Now, this position sensitive detector not only detects the neutron, it can tell us at which position is detected. So, that means I can get the information about the momentum transfer from the position information on this detector. What I mean to say, so I have a detector here and there is a central wire, if I can find out where the neutron has entered with some uncertainty certainly we cannot find out exactly but with some resolution if I can find this out then I know this is the direct beam if I know this distance if I know this distance then from this I can calculate out the angle and from there the Q value which is 4 pi by lambda sin theta where lambda is the incident neutron energy because if you remember what was my target was to find out intensity as a function of Q in our experiment and this is the experiment where we can find out structure and I know that IQ is related to the G of R the pair correlation function which is structured at in very different way you can say structure G of R the average picture so I can calculate if I can calculate theta because I know at what position the neutron has struck so from the theta I can find out Q so I can get information on a very large range of Q when I come to actual experiments I will show you how we get so basically a large range of Q can be covered in a PhD so it will save us time so let me just show you once again I showed it to earlier so the neutron scattered I am showing on a circle but this circle basically part of a sphere actually it is going in all four pi directions and for every direction I can calculate a Q out and then I am moving earlier this we used to move the detector the way I am showing you from of course I showed you negative side but generally from zero to positive side I can move the detector I can collect information but now if I have let us say I have shown some angular range if I have two PhDs I can cover this entire range using two PhD at this at the same time so it is something like parallel processing my signal so not only I am counting neutrons I am also counting neutrons at certain positions I will show you the actual counts how they look like and with that given the incident direction and energy I am considering an elastic experiment I can find out the outgoing direction and a direction and then the Q vector and the intensity as a function of Q so how we do it so typically this is how position sensitive detector looks like this is again a gas detector and you can see there is a wire the central anode wire which is a resistive anode typically around 10 to 25 micron diameter and this shows schematic just which I showed that the there are the sample scatters at various angles and the PhD not only counts the neutron also gives me the position which I convert to angles knowing the sample to detector distance so how do you find out the position is something interesting there are several ways but I will tell you the way it is done in our group in BIRC so this is a called a charge division position encoding so we have the neutron charge cloud so so what I mean to say that there is a resistive wire here there is a central anode wire which is micron diameter now the neutron got converted into charged particles and with some uncertainty the shower is arrested here this this is position X if the total length is L total length is L for the anode wire typically these are these are around 1 meter long our detectors are around 1 meter long but this length can vary but typically 1 meter long when we use a 1 meter long detector then that the whatever charges are produced they move in this direction and travel in this direction and they give rise to voltage pulse at the end we are proportional to this length so if the total length is L and from left if this is X then it travels a distance of X and L minus X when it goes in two directions and then the voltage is proportional to X and proportionate L minus X at the two edges so now you see now at the both ends we have preamplifiers then spectroscopic amplifiers then a circuit which does this job this algebra so let us talk about the voltage in this end and voltage in this end and what is the Z Z is the input impedance of the preamplifier so voltage is proportional to resistance per unit length of this wire central area of the position sense with detector into L minus X into plus the input impedance of the preamplifier that's what we get here and the other end we get which is proportional to F this is V B and this is VA the way we have written it here because this distance is this distance is L minus X and this distance is X at a VA is proportional to rho into L minus X and added to it the input impedance of the preamplifier these two and on the other side it is V B is proportional to rho X rho is the resistance per unit length so it's a resistive wire into X the distance it travels plus Z the input impedance of the preamplifier now you can see that VA plus VB is rho L plus 2 Z and if I divide VB upon VA plus VB we get X upon L for input impedance being much much less than this value so that means the voltage at the two ends are proportional to the distance it traveled and once I can do something called a ratio I take which is VA upon VA plus VB upon VB upon VA plus VB from these things this circuit does the job of finding out the position and then it can send to a multi-challenge analyzer so ratio output can be obtained from analog or digital ratio circuit we use a digital ratio circuit here so rho is the specific resistance of the anode wire per unit length and L is the total length of the ratio so that is how we figure out where the neutron has struck so there are some uncertainties because neutron has not struck the anode wire neutron has a heat a helium 3 atom that has produced tritium and proton this tritium and proton has been collected through a charge multiplication process on the central anode wire but those lengths in which they are absorbed and the charge cloud is produced are much smaller compared to the total length and the length resolution we get is of the order 3 to 4 millimeter that happens in within micron lengths so they are small and zero and we can consider neutron as a point being detected on the central anode wire once I get this information I can start recording in my multi-challenge analyzer position versus intensity so this position versus intensity is nothing but it is collection of IQ data at all cues at the same time so I show you data taken by our detector group head Dr. Sratat Desai at BRC so this is the PhD what we do actually we prepare a neutron beam which is very narrow to going through a slit and we either we put the detector across the beam or move the direct neutron beam generally move the detector across the beam in a shielding box you can see as the position is changing you can see the source beam counts a changing position as a function of channel you can see and almost same everywhere so we move by 10 centimeters and check and this is how a position sensitive detector is sort of clarity is calibrated and also tested so for one minute all this is connected collected and peak width with plus minus one channel it looks like this so this position sensitive detector can now go for use in a spectrometer so this is how the test facility will test the neutron detector for its position sensitivity so now I show you some specific example I will discuss this spectrometer later this is a liquid and amorphous refractometer photograph I just wanted to show you the photograph along with the schematic to give you a idea of the scale of the thing there they there is a human size so you can see it's a huge shielding but inside the shielding box you can see there is a sample the neutron beam is coming from a monochromator here goes to the sample and then sample can scatter the neutron there are 1, 2, 3, 4, 5 so there is the five position sensitive detectors covering a range of angles which is 2 degree to 140 degree and these are helium 3 base detector so we have got 10 atmospheric pressure of helium 3 and 2 bars of krypton krypton is used as a quenching gas that I will not get into right now but basically helium 3 detects the neutrons in this large inside the large shielding material and this is the monochromator drum the at the center of which the monochromator is there and then the monochromator brings sense it to this detector sample position and the sample scatters into the large angle because this is a liquid and amorphous spectrometer I will come to it later when I actually discuss the principle of these spectrometers and because it is a liquid and amorphous spectrometer we need to cover a large Q range to get the pair correlation function in this case also I let me show you another example of reducing time by using multiple collection at the same time this is a small angle scattering instrument this also I will discuss it a multiple psd base the earlier days I come to this earlier days that there was a sample which was reflecting we had one detector one detector so let me so now when there is one detector this scattering is like a debaucher or a cone so from here from a sample a cone goes out a cone goes out but this detector just cuts one segment of the cone rest of the cone is not detected so you do get in information about angles as you go from the direct beam to larger angles but you don't cut the entire debaucher or cone because you are using a single psd now it has been improved to the extent now that you can see here we have used one two three four five six seven sorry so we have got we have made one two three four five six seven eight eight detectors over here and so we can cut we can now if it is a debaucher or cone then I can have detector I have had I have taken detectors over here now instead of one detector if I have multiple detectors then with respect to the sample position I have got this circle which is showing and there are detectors like this detectors like this so this debaucher or cone which is there cut for the same angle theta if I consider this angle as theta I have got now number of detectors counting neutrons so I have got a multiplicity of eight multiplication of eight in intensity for the same time of counting so I just show you here and this is a data collected this is a central beam and these are the scattered beams that you can measure on these detectors and similarly the psd position and it's shown with respect to the sounds data of a CTAP micellar system all these things I will discuss later now I am just showing you the advantage of using position sensitive detectors detectors that use one after another that means not all the angles at the same time earlier you were using detectors rotating from one position to another now we don't move the detectors the detector collects data over the entire queue range in this case and the whole data is converted into counts versus channel number and later counts versus Q which is important for structural determination so now I will come back to so this I will further add to it that now so far I have been talking about cylindrical detector but there are also in the world nowadays two dimensional detectors so there actually you have a square chamber in which you have wires like this wires going like this like this coming out this can be the anode wire and the cathode wire another one can be going like this like this and that's signals are taken out the electrical signals go out from these points so when I have this then the charge produced in this lattice somewhere can be immediately converted into X and Y components and then can be converted from the distance from the sample if I consider it kept at a distance from the sample like this from the sample if the charge is detected here immediately I can convert it into Q information but now not one not multiple but you are covering almost entire device error circle or the diffraction circle using a single detector these are two dimensional detector and this is also used in major neutron sources like PSI and other places we have we are working on it right now and we have made the prototype two dimensional detector in our Dhruva also so where you collect the entire Q space value in one shot so it's a very big advantage in time with respect to time because neutrons are low intense sources are ping again and again and that's why we need to improve the data collection efficiency and two dimensional detectors one step ahead of one dimensional detector where we can collect the entire scattering cone at one go now let me get back to what do I mean by monitor detector this most of us who have done X ray scattering might not have experienced this reason being neutron experiments takes longer hour experiments take longer time hours to even days because neutron intensity is low and we need longer time to count and the reactor is a very big setup so the reactor power may fluctuate so if the power fluctuates then for a particular setting as I showed you that the react suddenly something happens reactor power might go down after sometime they might restore back to the higher power it might again go down due to some problem because reactor operation is a is a technically involved job and they need to keep the reactor operating safely so everywhere these powers it is not a planned fluctuation but it might be stable for some time then it might go down then might come up so then if I do a serial counting or if serial counting means one setting of detector then another setting after that one setting how do I do it because the time if I fix there is a 10 seconds I will count for 10 seconds in my first 10 seconds the reactor power may be high in my next 10 seconds the reactor power may be low so I am not counting for let us say for a peak two positions in the peak for the same time it will cause errors so here instead of time we normalize with respect to a monitor count so what is a monitor counter so I have just showing you a schematic now I just show you that it is again from the side of the side so you can see before the sample there is something called a monitor counter and then of course there can be point detector there can be one DPS there can also be two DPS depending on what you have caught with you so but there's a monitor counter now the monitor counter what does it do so a monitor counter is a low efficiency counter it is placed in the incident beam before the beam is incident on your sample so here I show a solar collimator and I put the monitor counter in front of the solar collimator now what I do here I don't count for time but I count for a fixed monitor count and not with respect to time how it helps you can see I have just shown you in a simple diagram if I am counting neutrons for a fixed monitor count suppose a reactor power goes down when the reactor power goes down less number of neutrons are coming from the source from my reactor so it will take longer time so monitor count will the same count let's say I have fixed it for 30,000 counts so 30,000 counts will take longer time again if the reactor power goes up I have shown it goes up then the same 30,000 counts will take shorter time so the monitor adjusts with respect to the reactor power and at every point I'm counting for 30,000 monitor count so I have written the fact that the reactor power can fluctuate I am doing it for a fixed monitor count so slight fluctuations in reactor power are automatically nullified by monitor count by taking it longer and lesser time and because I'm counting for fixed monitor count my two points in my data they remain equivalent but this monitor count has to be low efficiency because it is put in the incident beam so we make low efficiency monitor counters and then put it for the serial counting system mostly in elastic neutrons spectrums we have to put them in the beam path and my counting setup is such that my counting setup works I have to set the number of monitor counts for which each point will be counted and then the counting goes on so this is a photograph I am thankful to Dr. Sardar Desai for giving me this photograph these are the various neutron detectors that we have developed at the Solicited Fields Division of BRC and they are these are the one meter long position sensitive detectors there are monitor counters and we also make extra counters so one DPSDs are there and also curvilinear PhDs have also been made so these are photograph of various kinds of detectors that we make similarly there are monitor counters and now let me come to the part of scintillation detectors I haven't discussed it so far because these are mostly used in spallation neutron sources and here I show you a scintillation process some of you may be familiar with it so there is an incident particle inside the beam medium what the medium is it's basically and which causes scintillation that means it absorbs the incident particle produces photons so and these photons they are through a electron with a window gets inside a photo multiplier this is basically they multiply photon they are multiple first converted into electrons and they multiplied out these are called dynodes these electrodes are called dynodes they're all negative with one this one is negative with respect to this so the photon comes and maybe gives you few electrons this electron they move to this dynode then this dynode further multiplies it sends them toward this dynode and this process goes on and finally you get an electron shower on my electrode so and then we can count the particle so here again the neutron enters the neutron has to enter the medium and produce photons so how does it do so neutrons to charge particles to photons the process is like this so I'm just copying it from ICS detector room so you see they are discussing about a prototype detector for Polaris instrument is a scintillation detector scintillation that we use that zinc sulfide scintillator zinc sulfide scintillator the zinc sulfide is mixed with lithium fluoride so as I told you earlier that lithium-6 it is enriched with lithium-6 with neutron it gives me proton and triton these are charged particles they are absorbed by zinc sulfide zinc sulfide produces photon so there are very few in the actual two so here it is lithium in this link layer in alpha particle and triton particles sorry this is generated they go through the zinc sulfide lattice causing ionization so now these charged particles cause ionization and when the ionization they come back and combine back they give up a light flash and so each neutron is converted to light flash and then the light flash is taken into a as I showed you to this through these dynodes and then finally they're counted so there's a photomultiplier tube which creates so let me just so this is neutrons to charge to ionization in the medium and when they recombine recombine the ions in the solid medium mine they give rise to photons these photons travel through the zinc sulfide matrix and enter this photomultiplier tube and their counter this is an example of a scintillation detector I've given the reference if you want some of you are interested for more knowledge you can get it from there so this is as opposed to gas detector this is a scintillation detector and this is used for final detection and not as a monitor so I just wanted to show you a photograph this is a huge detector bank in which this ZNS impregnated with lithium fluoride are placed in this polarized diffractometer so how do I find out Q values for this these are medium it's a it's a diffractometer so it is done because now this huge detector bank huge detector bank they are having a matrix of this I'm just showing it in a simplistic manner matrix of these matrix of these detectors and it's a long travel path so you can say for one particular element here the angles are fixed so once they're detected then these light signal is taken by a photomultiplier out I mean optical fiber they're taken out and converted into electron power electronic signals voltage pulses and that's how you know the position from the detector one detector in the matrix one matrix detectors in the matrix they provide the information on the Q value so you collect on a very large Q value that you can see because the you can see this is the people standing so you can see the scale of things but this is a detector for polarized diffractometer and this detector bank it's in the there's a spallation neutron source and these simulating glasses glass detectors they collect the final signal and give you the Q information so I think I have more or less completed of what I wanted to say regarding the neutron detectors in this talk we'll next go over to a kind of spectroscopies that you can do with neutron and then we'll really enter the actual field of neutron diffraction as well as neutron elastic sketch thank you