 Let me come down to the instruments. I will primarily focus on the instruments that we have at Dhruva. But I will also use examples, some of the extremely interesting and tough experiments that have been done using sands technique. So we have two instruments, sands one if I may call it, sands one and this is sands two. Both of them are Dhruva guide hall and one is based on, I will explain it to you on velocity selector today. Earlier it was a beryllium oxide filter beam. Most importantly for most of the sands experiments because you are doing experiments at a large angle, at a small angle, large distances are involved because you have to collimate the beam very nicely. So the same thing has been used by us for this. So you can see the flight path, but this is not the largest that you can think. For example, if you go to an advanced neutron source, very advanced neutron source where you have got a cold neutron source, you can afford to lose some neutrons. These flights pass a typically 15 to 30 meter round and then a sample comes. Here the samples is an old photograph, falls on a one-dimensional detector, that is one technique. This is another interesting instrument where we don't take it to a large flight path, but what we do actually here, there are two silicon single crystals, very interesting. Silicon single crystals, they have very small mosaic spray. Actually they have got Darwin width, what is known as Darwin width. For a single crystal, the reflected beam is very narrow, very narrow. Few arc seconds, a bright diffracted beam, angle or Q whatever, it's very narrow. This is few arc seconds. Now also it has some mosaic spray if it all. So now if I use two single crystals in parallel beam geometry for at a certain angle, angle should be same. If I rock this crystal, second crystal, keeping that fixed, then the rocking curve actually gives the width of the beam from the first crystal. So you get a very, very narrow beam in intensity. So it's called rocking curve, if you rock it. It's a very narrow beam, which will be a Darwin width ideally of the first crystal, because this is a single crystal, silicon single crystal, which gives me Darwin width. Now after the first crystal, if I put my sample here, it may sample here, then this sample causes small angle neutron scattering of the beam. And then when I rock this one, I watch or I observe the widened beam. So if there is a widening of the beam, I'm just excellent. So now I have got a beam without sample and beam with sample. And now this can measure because this beam is very narrow. It's a rocking curve or it's a rocking curve which captures the Darwin width of the first crystal. This is being a single crystal. So I can see very small Q intensity. Interesting, because when I do a conventional, conventional, conventional small angle neutron diffraction, I have got a very, very narrow beam and I go to a very far place so that the beam spreads out on the detector and I can measure them. And I can measure the small angles at very small angles. This is one technique, most commonly used technique. But the other technique is that using the rocking curve and we are using this rocking curve for one sense machine. So we have got two of these small angle neutron scattering machines. One using the rocking curve principle. One is a common one in which the beam travels through a long path. Then there's a sample after that. This is the detector shielding in which the beam travels. And there's a one meter position sensitive detector or PSD to capture the beam in this case. So this is a schematic from this reference. So we earlier we had this lambda equal to we have a guide which allows the neutron beam to go. So I don't know whether you remember this. We talked about neutron guides. Neutron guides are actually optical fibers for neutrons where neutrons travel through total external reflection over large parts. We have a guide which is 34 meter long and where after 34 meters we have this beam comes from the guide. And there's a cryostat which has got a beryllium oxide filter and quickly tell you what it is. And then there's a detector shielding followed by a one meter long PSD. So why beryllium oxide? This is an interesting thing we did earlier where actually what you used to do that you see if you put a single crystal, a powder crystal in the neutron beam path there are lots of crystallites oriented in all possible direction. And if I can choose de-spacing judiciously we know 2d sin theta is equal to lambda. Now when lambda is larger than the largest de-spacing that part will be transmitted by the crystal, this poly crystal. I said transmission versus energy on lambda. So long lambda will be transmitted but when lambda is less than the largest d then you have some de-spacing to scatter out the beam and you have a cutoff in this region. So beryllium oxide has that cutoff at 5.6 angstrom. If I remember it correct. You see 5.2 angstrom sorry 5.2 angstrom. So now when I put a beryllium oxide poly crystal in the beam then what happens that I have got an android as lambda. Now this is the beryllium which is coming from the reactor this was a lambda. Now if I put the beryllium oxide filter on that then it will cut off the lower side. And what I get actually this cutoff is not so sharp it looks somewhat like this. It is a broadly monochromatic beam which is transmitted when I put the beam through a beryllium oxide polycrystalline filter. So this is a beryllium oxide filter as monochromatic and we get a lambda mean equal to 5.2 angstrom and we can cover we could cover the q range of 0.015 to 0.3 angstrom inverse but this is how the filter this is the transmission cutoff. So here below this all the lambda have been cut off in the transmission beam and this is the tail of the maxillium. So this is what we could get and this is the variation of resolution with q this one has measured and today this instrument has been modified and what we have now actually which is very commonly used in almost all the neutron sources known as a velocity selector. It is a physical velocity selector I had discussed with you earlier when I had discussed monochromatic but let me remind you once again this is a cylindrical material on whose body you have cut down these helical slits. You have cut these helical slits on the body and you rotate it around the axis. So now this helical slit because basically when a neutron is passing through this a neutron in the laboratory frame is moving in a straight line but in this rotating frame of this velocity selector the straight line it becomes a helix of long and if this slots on the body of the velocity selector matches then you have a wavelength and a band. So you have a lambda plus minus delta lambda that is allowed to pass through the velocity selector. The beauty of this technique is that earlier you had a valium oxide filter beam where we could not play with the lambda. Now depending on the velocity not the velocity the rotational speed of the velocity selector I can choose my lambda because the helix for the neutron changes. Now we have got a Maxwellian and depending on the rotation speed omega of this velocity selector I can choose a certain band because different lambda mean and the band because depending on the rotational velocity this helix changes and then the helix gives me which lambda I am choosing so that is what I have mentioned here in my thing. It can be 4 to 10 angstrom and in case of small angle neutron scattering or sands we can do with a large delta lambda by lambda because this I will come to later because my resolution actually is dictated by the delta theta by theta. So today the valium oxide based instrument which I had earlier this is a recent photograph. It has been changed into a using with a velocity selector and the one dimensional detector has been replaced by an area of detector. So at any instant we can collect more data. So I show you the data typical data here you see this central part is the direct beam and this is the scattered beam. So this is the beam which will be used for obtaining various parameters for a sample. So here it is a 0.1 molar C tab. C tab is a surfactant as I was talking to earlier but this comes on the detector channel but now instead of one detector I have got many. So this scattered beam forms a cone and I can arrange the cone on these detectors on these detectors. So it should be centered on this. This is the direct beam and I can collect the data on large number of detectors. So most of the applications are soft matter and nanomaterials and biology in this. So this is the instrument used which I can say one sands one and this is sands two. As I explained to you just now that you have got two silicon 111 monochromators and there is a collimator between the two silicon. And you can see that the second silicon is rocked around the first one and you capture the rocking curve of this one by rocking the second crystal. And this one has been applied and you can see that here 0.003 nanometer inverse. So angstrom inverse it will be 0.03 2.1.7 angstrom inverse 10 times more. The ceramics, cements and metallurgical alloys and precipitates have been used heavily. We call it m sands. Because this one, the other one, this one it can measure typically say 2 nanometers to 10 nanometers. The range of samples that I can use whereas here you can see that typically it can look at 40 nanometers to 1000 nanometers. So they are complementary of each other. One goes from 2 to 10 or 20 other one goes to 42,000 nanometers. And many times the data from both of them have been merged to get a larger cube. So it's a monochromator is a silicon 111 and the same has to be the analyzer. Here the lambda is 3.12 angstrom and here the delta lambda by lambda is extremely small just 1% 0.01. And I must mention to you that in a reactor like Dhruva the flux is this. We don't get disappointed. This is a very very small number compared to if I consider a beam in a synchrotron source or any other advanced neutron source. But with this kind of flux also you can study many interesting problems. So I have just shown you one more example international example is the NIST detector. So it has the same principle only because you have large number of neutrons falling. You can see the pre-sample flight path is just 16 meters. You have got a detector vessel which is further 15 meters. So almost 30 meter length 30 meter sons. And here also you have a velocity detector that gives you the required or desired range of lambda for your experiments. So with this as top I will continue with this in the next lecture.