 So, now we come to the fact that what can we see with neutrons in condensed matter? That's the question. The first is structure and dynamics is the broad answer. If you talk about x-rays, you cannot see dynamics, you can of course see structures. Now, when I say structure, that means the measurements are intensity versus angle. We don't care about the outgoing energy. We just impinge neutrons on the system and we measure the intensity of the scattered neutrons as a function of angle and that gives me the q vector and that q vector, that means I get iqt and what I mean is that I get iq as I showed you and its Fourier transform over q is related to my gf. So, in general, this is a most general statement for diffraction. So far, possibly some of you have been introduced to x-ray diffraction from crystallographic material, but in general, diffraction is structure at various length scales, hills depending q, wave vector transfer because wave vector transfer and space, they're Fourier transform of each other. And our experiments are at this, so this is the most general definition of diffraction. Like in case of optical diffraction, we know if you have rulings on a board, then we can from the diffraction pattern, these gratings, we can find out the spacing between gratings, but that's a much larger length scale maybe at micron level because optical rays have wavelength in the range of 4000 to 7000 angstrom. So basically a diffraction gives me structure at various length scales and that's the purpose of our doing neutron diffraction. So as I said, so when I say measure intensity versus angle, so what range of structures can we see? So we can say crystal structure that's angstrom. We can see in homogeneities at typically 100 angstrom, so that means we can see things like micelles precipitates. So sometimes when you are mixing two materials depending on their miscibility, one might start precipitating in the other metallurgists often face this problem. Micelles are basically there are something called surfactants and when surfactants have a head group which might like or repel water and the tail group which is just the opposite, which might accept water. So if I have a hydrophobic tail and the hydrophilic head, then this surfactants they form conglomerations in a solution known as micelles and you can study them using diffraction, neutron diffraction basically or even if I go to even smaller q values or smaller momentum transfers, we can see pores at 10000 angstrom in micron size pores, rocks are often made of pores and we can find out such pore distribution in diffraction. So this whole lot of experiments on the left of this slide tells me about diffraction experiments at various length scales, but diffraction experiments are easier because we are not measuring energy. For dynamics, I need to measure intensity of neutrons as a function of energy and angle. So when you measure energy difference, then of course, we are trying to probe dynamics in the systems and depending on how much energy transfer we are seeing in an experiment, it will sort of look at one specific kind of dynamics in the system and at various q values. Angle means at various q values. So what I get is actually in a dynamics experiment, it is s of q and omega from where we try to get a dynamics at various time ranges. So I have just listed a few. We can measure ponons, the time scales are typically 10 to the minus 15 seconds or femtoseconds or we can study rotational diffusion at which I have got a time scale of typical rotational diffusion means, let me give you an example of rotational diffusion, by rotational diffusion I mean, suppose I have got an NH4Cl, NH4Cl, NH4Cl crystal, then this crystal lattice has got this tetrahedral of NH4, we know N, so I have got this HN and this is on a sphere because NH bond lengths are same, but now interestingly with time this NH4 does not remain static, it might just suddenly it might undergo rotation, this H goes here, that is goes there and or it can tumble, this H comes here, this H goes up, so it is continuously undergoing this kind of rotational diffusion on a sphere and this can be measured, energies and the time scales of the diffusion are measured using neutron inelastic scattering. Also we can look at dynamics using very, very small energy transfer and when you talk about very small energy transfer, we are talking about longer or slower processes, here I have taken example of relaxation of polymer backbones at nanosecond, 10 to the power of 9 as 9 second, so time scale is increasing here and the demand on doing experiments at lower and lower energies is increasing, so when you want to do experiments at lower energies, then you also need neutrons who themselves have very low energies because you are supposed to measure the energy difference in an experiment, if the neutron has very high energy, it will be difficult to measure a small energy transfer as a percentage change, that is why you will also need neutrons with lower energies or what are known as old neutrons in the parlance of neutron scatters, what they are I will come to later and on top of everything, the structure at various length scales and the dynamics at various length scales, we have the magnetic moment of minus 1.91 nuclear magneton for a neutron, so it is a tiny magnet for us, so I can do magnetic measurement at all these length scales and all these time scales using polarized neutrons or using a sample polarized and the neutron is unpolarized, both of these kind of experiments gives us magnetic information, so like I talked about crystal structure, now neutrons are possibly the only tool which can tell you about magnetic structure, crystallographic structures have always been done using x-rays, we know that actually many of us are familiar with the powder diffraction data table where we try to identify our sample, we have prepared fresh from their black piece and say that phase has formed or not, but here in addition we can also find out the magnetic structure, the magnetic structure means you have for example for 3D transition elements like cobalt iron and nickel, you have unfilled shells in D orbit and they have their own magnetic moment, so the atom has a magnetic moment and the neutron has a magnetic moment and it is an interaction between these two magnetic moments, similarly in homogeneities even they can be magnetic in homogeneities and I can use polarized neutrons to study them, similarly when I come to dynamics I have talked to about phonons, but we can also talk about magnons which is the counter part of phonon where the magnetic moments they persist around their mean position and these are known as the wavelength of this precision is known as a magnon and magnons are also measured using magnetic scattering, this is possibly a unique tool which can give you idea about crystal structure in a magnetic crystal structure and also magnetic dynamics among all the probes, so now we will talk about a Q omega space, so omega is basically energy, so energy and momentum space, so on this there is a representation you can see that the Q space is covered by various kinds of spectrometers like spin, nickel, no sorry backscattering, time of light, etc. and also the energy space is also covered by many of them, so these are typical slices of this Q omega space that means the Q omega values in the Q omega space that we can measure using various instruments like triple axis, backscattering, spin, nickel, these are dynamics, an instrument which can measure dynamics, triple axis is an instrument which also measures phonons, I will discuss them and time of light machines they can measure structure and dynamics both and the only thing I want to point out to you is that neutrons cover a very large range of Q omega space which gives us a good handle to use them to understand dynamics and structure for various kinds of material and length scales, also I would like to notice because Q and length scales are inverse of each other as Q goes up, your length scale goes down, so for lower Q experiments your length scales are about 100 angstrom, whereas when you go I said I gave you an example of 10 angstrom inverse, you talk about length scales which are 1, similarly when the time scale time goes up that means things become slower, we talk about energy transfers which are smaller and when the time becomes faster or smaller we talk about energy transfers which are larger, they are inverse of each other, this is just a schematic representation of how we cover the Q omega space in various spectrometers possible, so with this now to cover this we need various kinds of arrangements and those are actually I just wanted to show you some examples, schematics basically, this is one very famous reactor at NISD USA known as NCNR, NISD Center for Neutron Research, I must tell you that you can see that the core is here on the left bottom and there are various instruments and you can see long lines going out, these are actually neutron guides, I will discuss them also in my talk in my course and there are various instruments accommodated on them and actually in reality it looks like this, what I showed you here has long lines into the guides, these are actually they look like this, so you can see the brick shieldings around the guide tooth because in any experiment with a reactor or escalation neutron source using neutrons, we have to take care of the radiation that accompanies the neutron often when it comes out from the source, so there are shieldings and these guides are running through them and the reactor is top left corner from there the beam is coming and goes like this, similarly if I talk about the best reactor in the world, Institute L'Ave-Langevoir in Grenoble, this is the typical look for the reactor, the reactor hall and the guides, so there are 2 to the power 5 that means 64 instruments in this place and that is what they mean and this is how they are accommodated on the guides and again the look of the guide hall, I am sorry I haven't got a photograph here, so it will be similar to what you saw in the last video and then we come to our own neutron scattering facility at Dhruva, Dhruva is a 100 megawatt thermal reactor because the moderator is at close to room temperature, the thermal flux of 1.8 10 to the power 14 neutrons per centimeter square per second, I must tell you that these are very expensive because this flux is when you compare it to even to a tablet of extra source, this is much smaller and so our design of instruments have to be judicious and proper so that we don't waste neutrons and this is a schematic look like, so this is the reactor, the small circle at the center is the reactor core and then you can see the beam lines, there are many beam lines, some of them are radial, some of them are tangential, I will discuss why and then at the end of the beam lines, at the end of the beam hole we say, beam mouth we say, we put our instruments and they are used for various experimental purposes, you can also do guides like you found the guides there in ILL and in NIST, we have also two guides over here which are running into a guide hall next to the laboratory and this is how the photograph looks like, again it's a large hall, this large cylindrical drum that you see at the beam hole mouth, they are called monochromatic drums, I have used this word earlier, but their utility I will be discussing briefly later, so now you see when we talk about neutron diffraction, I gave you the formalism, the theoretical formalism and the reasons for doing several experiments at several angles from the point of view of Q range structures we are trying to look so now there is several points we shall bring to you the core configuration, the optimal use of neutron starts right from the core configuration, then as I told you for very slow dynamics we need slow neutrons, but then we should have neutrons which are slow, so we have something called cold neutron source, similarly also hot neutron source, I will describe them to you, then we have beam lines that transport neutrons, beam lines transfer from the core to the outside of the reactor and then the guides take them far away from the reactor, to define the energy and direction of the neutrons, we have in pipe collimeters, filters, solar collimeters, I will describe all of them, monochromatars to choose the specific angle and one most important thing, neutron detectors because and monitors, they are a very important part of neutron experiments and I will try to discuss them in reasonable details during my talk, I will also tell you how neutron guides work and how they design, so with this I will stop here and come back with the next.