 Well, hello everyone. I hope you've had a good day so far and that everything is working all right with the lectures. I'm going to now talk to you a little bit about the natural instrumentation for small angle scattering so we've talked in the various lectures today about some of the theory of scattering. Some details of how we calculate what the scattering would look like, form instructor factors and so on, some things about how we make use of chemical variation to highlight and hide different parts of our sample. And so now I'm going to talk about, as I say, the details of what small angle scattering instruments are actually made up of and how we actually embody them in a way that allows us to make these measurements. So, just a reminder of what of summary of what I said this morning. You know, what do we measure in a sands experiment. I spoke in a general sense this morning but in the sands experiment in particular. We are interested only in the correlations in space so we're interested in the structure of our sample on the nanometer to micrometer length scale. And so what we do is we have our instant beam and we measure the number of neutrons that are scattered as a function of Q and you'll see when we talk about how we design the instruments, the different ways that we go about enabling us to do that. And so you'll have heard from Vojtek. So then basically we can then take the information we have where we have intensity as a function of Q, and we can use that to determine the shape and size of particles in solution, or particles in a matrix, or determine information about the structure of our sample in general. And just remind you that that Q ends up as being an inverse space, it's the Fourier transform so we end up with inverse space, which means that smaller Q values mean measuring larger structures in real space. So the bigger things we want to measure the smaller Q values we need to measure out. I'm just saying that we're only interested in the variation as a function of Q, but this morning I said that, you know, we could also measure the variations function of omega which is the energy transfer. So, so what's happened to that in a small angle scattering experiment. In sort of conceptualizing it and when I gave the explanation of scattering from a single nucleus, I worried only about elastic scattering and completely ignored any energy transfer. Of course in the real world, the atoms and molecules and neutrons that we fire at them, don't ignore inelastic scattering or quasi elastic scattering. And so that still happens. But fundamentally what we're doing in a small angle scattering experiment is we're essentially integrating over all energy transfers. So, rather than doing of actually doing an elastic measurement, what we're actually doing is we are integrating over all of the inelasticity so if you imagine if we have something like this quasi elastic system here. I mean, when we say we have some amount of momentum transfer with zero energy transfer what actually we're doing is we're integrating about omega equals zero to get that to get that signal. And this is an important difference in the fact that if it was purely elastic then we wouldn't have to worry ever about non elastic scattering. However, in the real world we do, and this leads to some effects and actually is a big contributor to the background signal so the incoherent inelastic scattering is actually quite a large component of the background from hydrogenous materials. But just that's really just a conceptual thing to bear in mind while we're we're talking about the instrumentation for all intents and purposes the interesting bit of the data the coherent part can be treated as being purely elastic. Alright, so what does the sense instrument look like. It looks something like this. We have a source we have some means of transporting the radiation from the source. We have some means of choosing what wavelength we're going to use. We have some means of collimating the beam. We have our sample and we have a detector. And these are all placed controlled distances apart. And this is the same whether we're measuring with the neutrons or x-rays. The details of the components may be different but the principles are fundamentally the same in terms of what those components do. Today I'm just going to talk about small angle neutron instruments. So we can begin with the very simplest sense instrument we could think about building indeed these were in fact some of this first sense instruments to build. You would place the columnation directly outside the reactor shielding and then have a sample and detector. So we have collimation because we want to measure at small angles. We had no collimation then of course the neutrons would be spreading out in all directions and we wouldn't be able to measure any divergence of the beam. So the only way we're able to measure divergence of the beam from straight is by making the beam straight to start with and that's what the collimation does. From the previous things you noted that the wave vector q is given by 4 pi on lambda times the sine of the scattering angle or half the scattering angle depending on how you choose to define it. And what this means is if we remember that lower q means larger structures is that there are two different ways we can achieve measuring larger and larger structures. And that is either we can increase this distance L2 so we increase the distance between our sample and detector and this allows us to discern smaller and smaller angles. With respect to the direct beam just through simple geometry, or we could choose to measure with longer wavelengths, which again would reduce the q vector we were measuring out and thus allows to measure larger structures. In general, the technique is called small angle scattering because we are always measuring at small angles and by small here we generally say that the scattering angle is less than 45 degrees and 45 degrees is pretty wide. Whereas if you were looking at a pure diffraction instrument to look at the atomic structures you'd be talking about having detector banks at 90 degrees and greater angles. So that's all very well and good. I mentioned that we can make L2 longer in order to get to lower q. But I also said we could get a longer wavelength to go to lower q so we obviously need some form of managing the wavelength and so we put in our wavelength selection device, which is purple in this picture. And then we put in various pieces of neutron guide and optics and you might be wondering why we need this. Well, simple as is if we just have our source, then the intensity the neutrons are given off in all directions so the intensity of radiation would diminish with r squared. So in order to improve the count rate and be able to put the instrument further away from the source of neutrons and thus fit more instruments around the source of neutrons. We make use of various types of neutral optics in particular neutron guides to transport the neutrons from the source to where we want them. So that actually moves the source out from where we actually produce the neutrons to where we want to start the instrument. And if we want to measure as much q space as possible, then we might want to measure with multiple detectors and this is fairly common now on sans instruments. As people are building new ones and upgrading old ones is to add a second bank of detectors so that we can measure at more angles at the same time. I said that you know one of the differential cross section is given by the number of scattered neutrons in a given direction divided by the total number of incident neutrons per second. And so we need a means of determining how many neutrons per second they're actually hitting our sample, and then how what fraction of those are scattered. And so we have here a series of series of beam monitors that allow us to measure those. And then we also potentially have what we call a beam stop in front of the rear detector, so that our detector isn't overloaded by measuring all of the unscattered neutrons. And then around it all, we put some form of radiation shielding designed suitably, depending upon the shape of the instrument and geometry and the type of source that we have. So that's the pictorial view of what a stands instrument looks like. And now I want to talk a little bit about how we actually go about using the sources that we have. So we talked this morning about different types of neutron source we talked about reactors, primarily about reactors and spallation sources. So reactors essentially are on all the time. Once the chain reaction has started, and it's up and running, it's a continuous source of neutrons until it's chosen to be shut off. In the case of a spallation source, they are intrinsically pulsed so you make pulses of protons that hit the target, and then that produces pulses of neutrons. The ISIS spallation source runs at 50 hertz SNS runs at 60 hertz. Those are traditionally because they were traditionally locked to the mains frequency in the country. ESS will actually operate at 14 hertz. And the second target station for instance at ISIS takes one pulse in five so actually operates at 10 hertz. So we get pulses spaced out in time. So what does this mean in terms of how we can use those for making small angle scattering measurements. So in the case of a reactor, we have this situation. We have all of the neutrons coming out of the source. They travel to our instrument we do some wavelength selection which cuts down the intensity because these are basically all wavelengths, and then we choose some wavelengths. And then those neutrons are scattered and arrive at the detector. So we're using some of the neutrons, essentially all of the time. So what we do in the spallation source case is we have an intrinsically pulsed source. So neutrons are produced for a short period of time they come in bursts. And we have all of the neutrons different wavelengths produced in this period. What happens then is that the longer wavelength neutrons are slower, they have lower energy. So they take longer to get to the end of the instrument. So they arrive later in time than the neutrons that have shorter wavelengths and go faster. And so what that means is that by knowing when the neutrons were produced and knowing when they arrive at the detector. We know how far apart these two things are. And we know the parameters of the neutrons so we can work out what the neutrons velocity was by the time it took to get from here to here. And that allows us to know its energy and thus its wavelength. And so what this means is that we can now make use of all of the neutrons, but we're only using them for some of the time. So there are times when we're not producing neutrons compared to that on a reactor. In both cases, we still measure scattered intensity as a function of Q with in the case of the monochromatic or continuous source version. We have to measure different angles to get to these access these different Q values for a given wavelength. We can choose to measure a different wavelength than that's a separate measurement. But in one measurement, we measure one fixed wavelength and make use of varying angle. Whereas with the time of flight instrument, we simultaneously have multiple wavelengths and we can measure at multiple angles. So in general, the time of flight allows us to measure a much wider range of Q values at the same time. But in general, we will have fewer total neutrons at any given wavelength than we will for that wavelength in the monochromatic case. We can also turn a continuous source into a pulsed source by making use of what we call a neutron chopper. It's basically a spinning disk with a hole in it. Depending on how fast we spin this disk, we can choose what spacing we want of these pulses. And that's often done in order to deliver this enhanced simultaneous Q range at continuous source. Excuse me, can I ask a question? So then using chopper, we don't necessarily choose the neutrons with the higher velocity, right? No, right. So I'll talk a little bit more in a minute about how the different technologies work. But essentially, with a chopper, you can adjust it on a continuous source. You can set it to whatever speed you like to produce pulses at that frequency. And there you will be getting whatever range of you're producing or you're still getting all of the neutrons that way. The first chopper will not do any wavelength discrimination. Okay, yes. The reason that I asked the question was that we said we are interested in larger wavelengths, for sense, right? So then we want the neutrons with the lower velocity. That's right. Exactly, yes. So then what we do, and I'll talk about this in a minute about how we do the wavelength selection, is we have different choices. So this is about generating the source, right? And so in all these cases, we have all of the neutron wavelengths that come off on moderator, more or less. We then in the instrument have to make a choice about which wavelengths we want. And there are different ways of doing that, which I'll talk about in a moment. Okay, thank you. All right, so I mentioned before neutron guides. This is how we actually take the neutrons from the source, be it the reactor or the splatium source and get them out where we can make use of them. So these make use of total reflection of neutrons from thin layers of nickel and other materials, sometimes nickel titanium, multi layers, or various other materials depending on the properties you want, on a glass or metal substrate. And those of you who are working in reflectometry groups will be well familiar with this concept of total reflection for neutrons in that just like with light, where you can get total internal reflection in something like an optic fiber by having different refractive indices, we can do exactly the same with neutrons. In the fact that if we put neutrons onto the surface below the critical angle for that given wavelength, then we get reflection. And the reflection is related to the scattering density difference of the interface with air and substrate. And so here one can construct complicated multi layers to improve the reflectivity, or even, and we can also make different shapes of guides to be selective about which neutrons will actually meet the critical angle condition in order to be transmitted. The next thing we have to do is actually choose the neutron wavelength and there are a number of different ways we can do that. If you, the sort of classical way is to use a monochromating crystal. So here we take something that has ordered atomic structure and make use of brag diffraction to select only the wavelengths or wavelength that meets the brag condition at a given angle. So we can make use of different materials with different d spacings and then align them in the right way with our neutron beam to give us different wavelengths. And we use various different materials we use graphite, we use copper. We use various other materials such as sapphire and diamond have been used. But for neutrons with the long wave links we generally want something like graphite that has a large spacing is preferred. Or we can use something like silicon and this is used in some of the instruments I'll talk about when I talk about usands later in the week. So here's an example where if we take the one on one plane of silicon that has a d spacing of 3.136 angstroms. And if we choose to take off angle of 90 degrees so that's 2 theta is 90 degrees. Then we will get from the first order peak a wavelength of 4.4 angstroms transmitted. Now this is very good but it's very specific and the more perfect the crystal is the narrower that wavelength range is going to be. And so what we end up with is we end up with a very, very specific carefully defined wavelength, but we've thrown away an awful lot of neutrons. The other thing we can do is actually filter out the neutrons we don't want. So the previous example was only letting through the neutrons we do want so only the neutrons we do want end up being diffracted into the instrument. In this case, we can use things called filters to get rid of unwanted wavelengths. Usually for sands this means we cut out the fast or thermal neutrons, the ones we don't want, those higher energy ones that were mentioned before and allow the longer wavelength slower ones to pass through. So the common thing to use on a sands instrument is actually a nitrogen called beryllium filter. This has a cutoff wavelength below 4 angstroms so it passes 4 angstroms and above. Another popular choice for sands instruments is to make a new use of a neutron guide with a particular shape. And this is then controlling which wavelengths of neutrons will actually meet the total reflection criterion by shaping the guide. And so that then provides a cutoff in wavelength. In particular curve guides or multi-channel benders and optical filters are all examples of these devices. And fundamentally they all work in the same way you're manipulating the shape of the optics such that the angles are controlled as to as to which angles are possible for a neutron to reflect at. And then so here's another example of a filter this is sapphire and you can see this is the attenuation curve so you can see that whilst we do a good very strong attenuation at very short wavelengths. There is still also some attenuation at long wavelengths and so something like sapphire is not terribly optimal if you want to use say something like eight or 10 angstrom neutrons, but can be quite good as if you're actually interested in three or four angstrom neutrons. The most common way of actually choosing the neutral wavelength for an experiment on a continuous beam source like a reactor sands instrument is what's known as a velocity selector. And this is a rotating device that's either made up of blades around a drum or a series of discs stacked together. And it has it's made of absorbing materials with gaps in. And the gaps are arranged helically around the cylinder. And what this means is that depending on the speed you rotate that drama as the neutrons come through. They will, they might they'll see they'll see a gap. And then if they have the right velocity as this rotates they will still see a gap still see a gap still see a gap still see a gap and make it all the way through. If they have the wrong velocity, they will either hit they'll hit the absorbing edge on one side or the other. And so by varying the speed of rotation of a velocity selector, we can actually change what wavelength we pick. So most sands instruments will allow you to choose somewhere between four and 20 angstroms. And that's all done by varying the rotation speed of the velocity selector. And you can see here, this is the equation that determines what wavelength gets transmitted. And it's dependent upon exactly how long the velocity selector is the size of the gaps and the speed at which we rotate it. The last mechanism we have is what I mentioned before and this is a chopper. This is sort of like a dumb velocity selector in the fact that it's a single disk with a usually with one possibly more gaps in depending on the design of the instrument, but usually just one gap. And so basically it rotates and on a pulsed source. Well, you can use it to either generate a pulse pulses and then you serve the choppers to refine that or you have a pulse source. What you have to do is you set it up such that it's synchronized with the pulse pulses so that only neutrons that have have the right velocity by the time they get to the chopper see an open window. Everything else will hit the absorbing part of the chopper. And so by varying the phase of the chopper in relation to the source, you can vary which what wavelengths get passed through by having different sized openings on the chopper. Window, you can vary the range of wavelengths to the past through. And if we look in more detail at how a chopper works. We use what we call time distance diagrams or distance time diagrams depending upon your preference to visualize the chopper operation. So along the this is as the example of a potential chopper setup for an instrumented SS. So here we have a neutron pulse 14 times a second so every 71.4 milliseconds. And the pulse is here. What we have is then neutrons are spreading out in time all of the neutrons. But we have our chopper setup with this particular opening in it and rotating such that it's open at this time and closes at this time. So the front edge of the hole opens and then passes across the guide and then the the trailing edge passes across the guiding and then only absorbing is materials in front of the guide. And this is rotating at the same frequency as the source in this case so it's rotating at 14 hertz. So here we can see the fastest neutrons come from the source here. They go up here and they pass through and then the slowest neutrons also come up and pass through here. And you might think well we only need one chopper to do this. But in practice if we only had one then you can imagine that we've some very long way like neutrons from here could go sneak through over here. Right. So what we do is we then have a second chopper which we usually call a frame overlap chopper. We call this first one a bandwidth chopper and the second one an overlap chopper. What happens here is that the neutrons can appear and then they're also can pass through the second chopper. Any neutrons that are coming at a longer wavelength. So here, even if they get through this one, they're going to be stopped by this one. And the key thing about a time distance diagram is the nice thing is that and the slope of the lines is the neutron velocity and so is directly proportional to the wavelength. And we can do various calculations of what the time of flight is for a given neutron and work out then what the open opening time for a chopper needs to be to pass a certain wave range of wavelengths through. And in practice these choppers can be made up of two discs next to each other, and the relative phasing of those discs can be varied to change the opening size so we can actually change the bandwidth, as well as choosing where exactly it is in respect to the neutron pulse. So we have a lot of control over both what wavelength we choose and what range of wavelengths we choose for our time of flight measurement. And we can do some really complicated things where we can take a pulse and we can make lots of small pulses from it even at a pulse source. But I won't go into that in any detail but just to say that that there are a lot of complicated and clever things we can do with choppers to make the most of the instrument geometry. All right, now we move on here really to the heart of a sans instrument and this is actually the bit of sans instrumentation that as an experimentalist. You might become most familiar with because this is the primary tool you have to vary the configuration of the instrument and determine the balance you get between flux resolution and minimum q. So, we have a set of pinhole apertures, these are either actual sort of holes in discs absorbing material or adjustable slits that are made of absorbing material that we can make apertures from. And we use this to determine the minimum accessible q value that's the primary tool we have. In an ideal world, we would have our source aperture. We would then have an actual pinhole at the sample, and we would image that source aperture on our detector. This would give us the smallest possible scaphing angle so what we the smallest angle we could determine is then the difference between this half of this angle and a little bit further over. Unfortunately, if we do this, we would have an infinitely small pinhole and infinitely few neutrons. So in the real world, we have to increase the size of this sample aperture on. In fact, we have finite sized samples. And as we increase this you'll see we now have this number, as we call it, of neutrons that are passing through different path lengths. And we see that the shape of the beam becomes more trapezoidal. And then what we get to is what we say is the optimum configuration for a sands instrument. And this is where the source aperture is twice the diameter of the sample aperture. And this we call this the optimum because this is the condition under which we get maximum number of neutrons for the same width of the full width half maximum of the beam. And the full width half maximum of the beam is important because that determines our ability to know how well we can measure the scattering angle. If you've ever done microscopy, you'll be familiar with the point spread function, which gives you the uncertainty and thus the limit of resolution of a optical microscope. This is essentially a similar thing for us in the fact that we can't know our angle any better than the full width half maximum of our beam. That's the best we can determine the angle. And so when you set up an instrument, you have a choice between improving resolution that's making the full width half maximum smaller. And you can do this by changing the distances or by changing the aperture sizes. Or you can increase that and relax the resolution and go for as many neutrons as possible. The thing to note is that as you relax the resolution, you also change the minimum Q value that you can measure out. So when you go for very long collimation distances with very small pinholes in order to get to very small Q values. So that's making sine theta as small as possible. Then you end up also reducing the flux. Thankfully, as you'll have seen today, the scattering intensity goes up significantly as you go to lower Q values in most cases. And so you don't lose as much intensity as it first appears. And there's an equation here that you can use to figure out what the damage to the beam will be given all the other parameters. All right. How do we actually go about detecting neutrons? One of the advantages of neutrons and we sort of touched on it earlier, but not in great detail is the fact that they're very weakly interacting with matter. This is why we see they have small cross sections for many elements and can pass through materials like metals and allow us to see inside. This also means that with this weak interaction, they have a very simple form of interaction potential with the nucleus. And this allows us to do those calculations of form factors and so on and scattering and do modeling of neutron scattering in a relatively straightforward manner. However, this is a significant problem if we actually want to detect them. I mentioned this morning that we have very few neutrons, we have a low intensity of sources. And so we really want to make sure that we're counting as many of them as possible. Thankfully, there are a number of elements that undergo nuclear reactions with the neutron and produce detectable products. The ones we use in detectors are boron 10, lithium 6, helium 3, and gadolinium 157. And then you can see the reactions and reaction products in the top right. And in general, so what we do is we use these materials to generate something that can be detected. So generate something that strongly interacting usually a charged part of it. And the absorbing material, the boron, lithium, helium or gadolinium can be gaseous or solid or solid or even liquid in the case of scintillator detectors. And the most common detectors used on sands are actually proportional counters containing helium 3. And what this happens here, how this happens is the neutron interacts with the helium produces charged particles. These ionize the gas mixture and produce a cascade of charge, which can then be detected on a series of wires, or a single wire in the case of a tube detector. So you can work out where the charge was collected on those wires, and that tells you where the neutron was detected. And so by then knowing where your detector is in space, and knowing where on the detector the neutron was detected. We can calculate the angle of scattering from the sample and determine Q. And then we just count the number of neutrons as a function of position on the detector, then that allows us to get number of neutrons scattered as a function of Q. In the case of solid detectors, like many of the boron 10 detectors that we're deploying at ESS, the principle is similar. We have a very thin boron layer, the neutron interacts with it, and the layer of boron is thin enough such that the reaction products can escape. They then ionize a gas, which produces a charge cascade, which we can detect in various ways. We're actually used in scintillator detectors. And here what happens is that the lithium is mixed in a glass with something like zinc sulfide. And the zinc sulfide will scintillate, namely it will produce light when the reaction products from the neutron lithium six reaction interact with it. And measure the light intensity and use that to localize where the neutron was detected. And then the gadolinium detectors work in a similar way. They generally produce a gamma ray, which can be used to ionize gas and measure where the neutron was detected. In general, lithium and gadolinium are used in their solid state. As I said, helium is obviously gaseous in this case. Boron tends an interesting one. We use it in the solid form these days, but historically boron trifluoride was used widely as a detection gas in neutron detectors. It's fallen out of favor as any of you who have done any fluoride chemistry might suddenly be aware in that when it interacts with any moisture. So if there's a leak and it interacts with moisture in the air, it produces hydrogen fluoride, which is generally not a very nice thing to have floating around in the air. And so it's sort of become less popular as a detection gas, as a detection fluid, although it is a detection gas, sorry. It is rather a very good detection gas, though the chemical problems stop us from using it. So I said that we count the number of neutrons. There are two different ways of doing this. The traditional way is to build up histograms. So basically you store a histogram in equipment memory of where in space the neutron was detected and the time of flight if that's relevant. And then we process these histograms to produce the reduced data set. The advantage of this was that it uses fixed storage, so older detector electronics were able to do this in solid state memory. But you choose up front what your histogram looks like. More recently we've moved with enhancements in both networking and speed of electronics and data storage to what we call event recording. And here essentially every neutron detection event is stored individually. And then these collection of neutron events can be processed into histograms in Q space to be turned into our final data set. And the advantage is that you can choose after the experiment how you want to cut up your data. So if you need higher Q resolution, you can choose to do that with worse statistics in each Cuban. If there was if there's some one particular area of the data where you want to have higher resolution, particularly at a time of flight instrument you can choose to only use some neutrons and not others. And you have much more flexibility in how you can slice up the data. This also makes time resolve measurements much easier because you can simply just start counting and then afterwards decide how you want to slice up your data in in time. Once you see what it looks like. The last topic I'm going to cover now is that of shielding. So, why do we need radiological shielding. And that's a question to you all. So I want to see some hands up. Can anybody give me some reasons why we need radiological shielding. Anybody think of any. Is it for safety. Yes, absolutely. And what, what are we trying to keep safe humans. Yeah, we're trying to keep people safe right so we know that radiation can cause damage to the human body and apparently my slides on stage so I've given you the other answers now. So it can cause damage to equipment. And also it can cause problems with our data we get background from our data. So, if we look at the effect that it has on the human body. And this is the one thing that most people think about when they think about about radiation and the risks associated with it. In Europe, we use the steved as the measure of dose of radiation received by the human body. And this has the units of jewels per kilogram, however it's not a pure energy deposition. There are biological damage factors, which are published by the International Commission on radiological protection based upon images and evidence from people have been exposed to radiation as to what affects different types of radiation different energies of radiation have on different parts of the body. And so there is it is the, you take the the energy deposition and you modify it by a this dose factor. And then you do a feel for the size of the units. The sea that is a very large unit of radiation. And so the natural background radiation. In general is around one millisecond a year is the dose you would receive from just natural background radiation. It's maybe slightly higher in Sweden if you have a basement and spend a lot of time in it. Or if you live in one of the rockier areas because of radon. But fundamentally, it's on that order. Less than five milliseconds per year is defined as minimum control necessary. And this is the range we operate for users of neutron facilities. In fact, our goal is to, in fact, our design goal is a maximum exposure of one millisecond per year so equivalent to background. And we take strict measures to ensure that users of the facilities are not exposed to high levels of radiation than that. In the five to 15 millisecond range is what we call professional exposure and this is where people who work at the facilities generally fall into this category, since we're working there all the time, whereas visitors come there less frequently. And this means that we have regular checkups and and the like just to test our health. And then we go into the range of 15 to 15 milliseconds where strict goes and dose control is necessary and this is often where if we have to do work on instrument components that have been irradiated. We have to plan that work very carefully. We have to do calculations of what those somebody might receive by doing that hands on work. And then we have to strictly limit their time as well as providing them with the appropriate personal protective equipment and training in order to do that work safely. 15 milliseconds per year is the absolute upper range of occupational dose. So no workers are allowed to be routinely exposed to more than 15 minutes per year as part of their job. If you're exposed to more than 50 milliseconds per year you require immediate medical checks. This is we're still in the middle range. These are thousands of CVS. It's only once we jump up to a whole CVS that we are starting to talk about the serious to lethal range of dose. And once you're up to about three and a half CVS or your chances of survival, that dose become very low. And so as you can see, we keep the exposure that that users of facilities have and even the staff down to extremely low levels by design and an important part of designing a neutron instrument is doing the calculations to figure this out. On the other hand, we also worry about our equipment. So a lot of the equipment is things that we will actually have to put very close to or actually in the neutron beam in order to operate the instrument. Things like the beam monitors and the detectors and so on. And so the electronics for that will be very close to the beam and potentially in the scattered be. Here we make use of the gray as the unit and this really is just energy deposited in joules per kilogram. And you can see in this table that different types of materials have different radiation tolerances so material like captain, which is a polyamide film has a very high resistance to radiation. And as such we often use this in neutron detectors as a support for circuits and things like that and detection elements. Whereas your general electronic components things like microchips and stuff are very intolerant and these single event failures are actually something that is tested at some neutron facilities so when when electronics are you know people say you know military spec or military grade or aviation grade or whatever. These are devices that have been tested and designed to be tolerant to single event upsets. So where a neutron usually a cosmic ray in the real in the outside world. When it gets it, it will cause something in the chip to go wrong as it changes, maybe a bit in memory, or changes the way one of the logic gates works. And so, if you're putting things in aeroplanes or safety critical devices, or sending them up into space, then their tolerance to radiation is very important. Generally, we make sure that we just put shielding boxes around the electronic components, or keep them as far away from the neutron beams possible. And then the final one is our experimental data. The problem problem that we have is that I mentioned before that you know neutrons are difficult to detect and we come up with. Careful design to our detectors to detect them. And so the problem is that if we have neutrons that haven't gone through our sample and aren't the ones we ought to measure will still detect them the neutron doesn't know that the detector doesn't know where they've come from. It just knows it's a neutron. And so this actually turns out to be usually the most stringent requirement on shielding, because our detectors are so sensitive for detecting neutrons. This is usually about 100 times more stringent a requirement on the shielding than the safety component in terms of neutrons anyway. And sometimes in terms of the gamma radiation, although we can mostly discriminate that on the test. So as an instrument builder, we spend a lot of time. Well, the first thing we have to do is satisfy people that we're not going to injure anyone. We then need to make sure that our equipment is going to last. And then we spend most of our time go those relatively straightforward to fix. And then when we're operating the instrument, we spend an awful lot of time trying to work out where neutrons are coming from that we weren't expecting. So what do we do to actually shield ourselves against neutrons. And again we use materials that will absorb or scatter neutrons. And ideally we use mixtures of materials that do both. We use materials such as concrete, which contains hydrogen amongst other things. And will attenuate the beam. We use steel. We use lead to attenuate at any way gamma rays, and we use boron in many forms we use boron carbide in various different forms powders. Mixed in with epoxy resin to shape it formed into a sintered ceramic tiles is before sea. Even these days, mixed into polymers and 3d printed. And we use these cross sections that you see down here to allow us to attenuate the neutron beams. What this means is that around a neutron instrument, you will usually see quite a lot of thick shielding. And it's usually less at reactors than at sphalation sources. The reason for this is that sphalation sources often have a lot of much higher energy neutrons coming down the beam beam line then you get at a reactor and so you end up with larger shielding. And also because because of the sphalation source you end up with a larger background of high energy neutrons that you need to stop as well. To wrap up I thought I'd just talk a little bit about sands instruments around the world and where you could actually go and get your hands on one of these things to do an experiment. And there are neutron sources all over the world and if there's a neutron source you can almost guarantee that it has a sands instrument sands instruments are very versatile. Given that they measure material structure on nanometre to micrometre length scale, which is a key length scale for materials properties. And it's not surprising that they're very popular, both in terms of their application, but also in terms of the choice to build them. And you can see that in Europe we have quite a few in Europe we have quite a few. Unfortunately with the closure of LLB and HZB. We have lost quite a few sands instruments, although with LLB they are planning to move a sands instrument to ILL and one to PSI. So those will be rehoused and we'll get a new life. But you can see that even some of the smallest reactor sources will have a sands instrument. And so there's plenty of choice of where you can go to do your experiments. So just to summarize, we've gone through today all of the key components of sands instrument from the source through the optics and guides that are used to transport the neutrons, different ways of selecting the wavelengths that we want or blocking the wavelengths we don't want, ways of detecting neutrons, be it so we can make measurements of the beam flux in a beam monitor or measure the scattering as a function of Q in an area detector. How we do collimation and the effects that has on the flux we get on our sample, the minimum Q we can measure and the resolution of the measurement, how well we can measure the angle. And also then we've talked about shielding and why we need it and the different types of materials we can use. And I just want to finish by reminding you that fundamentally this is the equation that governs how we set up and design a sands instrument. The length scale we can measure is determined by the Q value we can measure, and that Q value is determined by the wavelength we can transport to the sample and the angles at which we can measure scattering. So with that, I'm happy to take a few questions for the last few minutes. Hello. Have you ever come across any computer program that can be used to simulate the neutron absorption cross section or range of if so, can you please recommend one. So, it is actually quite tricky. So we can, I mean, all of the neutron absorption cross section tabulated. And so, in theory one could can do a fairly straightforward Monte Carlo approach to firing neutron raising and determining what their interaction is in fact, the package McStas, I believe can be used to make such calculations. But actually in terms of just for simple geometries one can fairly straightforwardly writes some Monte Carlo routine to do that if you know the materials properties. If you just if you're interested just in what the transmission of a given component will be given its composition, then you don't even need to do a simulation you can do a track and attenuation coefficient calculation. I would point you to how to do that. It's it's a fairly straightforward equation you can plug numbers into. I've got a spreadsheet you can borrow if you like. Yeah. I have a question about this wavelengths or this number of values. Let's say if you want to study a matter of material that contains a different phases, they have a different D spacing values. So how should we consider that may choose a one or choose a number. Right so so in general the D spacing doesn't affect the signal you'll see on sans right I mean we we're looking at length scales longer than that and we first take the average scattering length density of each of the phases. So what we'll get from the sans measurement is generally things like grain size distribution of the phases phase quantity so what fraction is in one phase or another. These type of measurements. You won't be directly getting information about the crystal structure of those. The measurements you can make. If you're if you can do sort of brag edge type measurements where you move over an edge where you then enhance the brag scattering and so reduce transmission. You can look at transmission dips in your spectrum and that can tell you about the phases of course in general you'll be you'll be looking at a powder so rather than a single crystal but you can get information about the phases that way as well that's a slightly more advanced That by that I mean that's a brag edge measurement right. Another question. And I'm just curious about the sensitivity you know when we detect some small amount of stuff in a complicated materials. So it's better to have a larger number of neutrons. So it's always better to have more neutrons. You can never have too many by March. The the the in terms of sensitivity. It's actually once you're over a certain threshold for for backup. Then for for instrumental background, then it's really driven by the properties of sample. So depending upon what the major phases that will be the major contributor to your background signal. And so really it then comes down to how big the scattering contrast is between your minor phase and your major phase as to whether you would see it and they're more neutrons generally won't help you it'll just help you realize you're not getting a signal faster. Thank you. I have a small question about the spellation source like the pulses are have a much higher flux than like continuous. So I was just wondering if, even if the average flux is the same. Is your background also that much lower. So your signals noise during reports should be exactly that. That's a good observation. And in fact, actually that's one of the major advantages of spellation sources is the fact that you're generally measuring while the source is off. So so so as long as as long as there is nothing so as long as your source is well shielded close to it. So your signal is shielding is good. So you're not getting say neutrons that come out, reflect off the ceiling of the of the hall, get moderated in your shielding and then in the detector. Then which is can be a problem. Then then it should be very quiet from the source background term while you're measuring. And so the dominant should be with a well designed source and instrument dominant term should be just coming from instrumental presence and the sample. And so that could be a lot quieter than a than a reactor source. Thanks. Any other questions. Ah, here we are. That was the question. Oh yeah those are the two questions that just been asked they were in text chat already I didn't see them. Okay. Any other questions about the things that we spoke about today. Then I'll wrap up and see you tomorrow.