 Welcome again to the second day of this small angle neutron scattering course and today we will start talking a bit more of the practical details of performing a science experiment. So we are going to talk about things that happen and you have to take into consideration when you are planning and performing a small angle neutron scattering experiment. So let's see if I can actually get this to be a laser point, yes. So the first thing that I'm going to do today is start with a refreshment of some of the important concepts we saw yesterday. So as we explained yesterday, a small angle scattering arises from in homogeneities in the scattering and length density profile in the mesoscopic scale. So basically we have this amplitude of the form factor that has this scattering and length density distribution. And that's actually what we want to characterize because here's where the structural information is. So basically what happens is that we go to our experiment and we get some scattering intensity that is the macroscopic scattering cross section and then from there we have to reconstruct this scattering length density profile. But what actually happens when we do one of these experiments is that we get some scattering intensities that come from your sample plus some other things. So what we have to do is to apply some corrections to determine this macroscopic cross section which will ultimately be the data that we have to analyze. And today we're going to talk a bit more about how we get from this scattering intensity to this macroscopic scattering cross section. So with a small angle scattering we can measure, we can prove structures between one a few nanometers to hundreds of nanometers. And it's very important that we are in the right Q range because this will let's remind the length of scale that we are effectively proving using this experiment. And then there are some important aspects about contrast and composition that will tell us what we can measure with sands and if we can measure that sample with sands. So for example, if there is any, let's say, natural contrast in the sample, for example, if we are going to measure something that has some contrast by itself, or if we need to do some specific iteration or isotopic level in the schemes. And then if we can use some of these contrast variation approaches that I explained yesterday to enhance the information gain that we can get from this type of system. And then, and finally, some like other considerations. For example, if the use of isotopic substitution will affect the behavior of our system. So that's something that we have to keep in mind when we use one of these contrast variation methods. And the last important thing that I wanted to remind you was the scattering vector. So this is one of the most important concepts when it comes to a scattering in general, but it's really important when it comes to a small angular scattering. If you have come across some crystallography data, especially x-ray, many times you can see that they plot their intensities against the scattering angle. So this is something not common in sands because we use the Q vector. What happens here is that we have some point of scatter, something common, wave vector, and a scattering wave at a given scattering angle. Okay, so by playing with the Bodil relationship, we can get that the scattering vector is a factor that contains information about the scattering angle and the wavelength of the radiation we are using. This means that the Q vector standardizes, I mean, this means that regardless of the instrument or configuration that we are going to use, we are going to get the same answer. So for example, let's say that we have a feature at 0.1 at Q equals 0.1 inverse sands. It doesn't really matter if we are going to measure it in one instrument, that it's time of flight, or in another instrument that has, I don't know, six sands from wavelength or four sands from wavelength, so it doesn't really matter because they are going to always appear at 0.1 inverse sands from. And that's because the Q vector kind of, it's used to standardize the region of interest of these experiments. And if we go back to Bragg's law and we replace this wavelength and this scattering angle by Bragg's law, we get that the Q vector is inversely proportional to the real space distance. So this means that if we're going to gain information about small objects, we have to go to high Q. If we want to gain information about small features in the system, we have to go to low Q. And that's something that we have to keep in mind also when we do a small-angle scattering experiment. So today we're going to build up a bit more some of those concepts and we are going to talk about what we have to keep in mind when we are planning a small-angle scattering experiment and what we have to look for when we are one of these experiments. Then we're going to talk a bit more about how we do the data collection and how we do the data treatment to get to that microscopic scattering cross-section that we want to use for the, which is data we are going to use for the analysis. And then I will give you just a brief introduction on how the different access schemes to these type of instruments, the different routes for accessing these instruments. So I think that Andrew explained you a bit more about the instrumentation of a small-angle scattering. So I'm just going to go to the important point. So this is the classical schematic representation of a small-angle scattering instrument. So basically we have incident bin. In our case it's going to be neutrons because it's the sound scores. We have our wave vector for the incoming bin. We have our sample and then there's some scattered radiation at a given scattering angle. And then we measure everything and we measure this scattered radiation in this 2D detector. Okay? So the first important thing that you have to keep in mind is the Q-range that you're going to measure. The Q-range of, we can call this like the figures of merit of a sound instrument. So what happens here is that the Q-range is a very important parameter because it will determine which length scale we are proving. So we have to know that whatever it's outside that Q-range we are not going to say it. So we have to keep in mind that for planning the experiment. Then the Flassox sample on sample means the amount of neutrons that we are getting in our sample. So the merrier, so it's important to keep in mind that when we are going to do an experiment this might be critical to determine if we are measuring five or 50 samples. Instrument resolution. This is something that I'm going to introduce a bit more in detail today. And basically when we are measuring a Q-value what happens is that we have contributions from Q-points around that Q-value. So it's not perfectly defined. And this is important because these will kind of tell us how sharp will be the features in our scattering curve. Then the instrument background is basically the noise and we want to keep that at the very minimum because the lower it is the more defined will be our small-angle scattering data. Then we have the sample environments and Judith will explain more about this in the next lecture. And basically we are going to have a series of different sample environments that can perform different different roles in experiments from just keeping the sample into position to controlling the sample temperature for example or to or some very advanced small-angle scattering sample environments where you can do complementary methods for example spectroscopy. And then another important aspect about the instrument is that if they are available and this is something that for example next year is going to become crucial because ILO and NICES are going into a shutdown. So means that their instruments are not going to be available. So we have to keep this in mind when we are considering which instrument we are going to use. Okay. So if we want to consider an instrument for an experiment one of the first things that we have to do to consider is the queue range. So how big and small are the things that we can measure in that instrument. And we have here for example some of the specifications of some of the of some instruments around the world. So we have LOQ and Sans2D which are time-out flight instruments. And we have D11 which is a dialo so it's a continuous source. And we can see here that they have different specifications. So for example the wavelengths that they use for the experiment and the momentum transfer that they can they can reach with with with these configurations. Okay. So when it comes to time with when it comes to queue range we have to the first thing that we have to ask ourselves is that are we going to use a time-out flight or a continuous source. Normally what happens is that with a time-out flight we use a set of different wavelengths and that means that with one detector position we can reach a relatively wide queue range because we are using different wavelengths. So if you think about the queue vector we have the scattering angle on top and then we have the wavelength at the bottom. So it means that at a given scattering angle we are getting different queue values because we are using different wavelengths. And normally in one single shot without moving the detector this is the configuration that will give us the widest queue range. Whereas continuous source normally uses one continuous source normally uses one specific wavelength. So for example here at D11 you have to pick a wavelength between I don't know but it's probably something between 4 and 10 or 12 angstroms. And what happens here is that if you want to achieve a wide queue range you're going to probably have to pick one wavelength and then take different detector positions. Or normally you can also play with the wavelength but you measure different detector positions or different wavelengths to achieve the desired queue range. So another important aspect about the queue range is how big is the detector. So the bigger the bigger is the detector the wider is going to be the solid angle that we can reach with that single detector position. So it means that we are going to get more queue values from that specific shot. And then sample to the detector distance. This is something that we can play for example in substitute you can also vary the sample to the detector distance but it's not something I mean you can do that at the beginning of the experiment to set up the queue range that you want to measure but it's not normally something that you change during your experiment at a time of flight instrument. But basically what that's something that you have to do on continuous sources for example on D11 or D22 at ILL in which you basically take maybe two or three detector positions sample to the detector distances to collect a wide queue range. And some other some other aspects that can affect the queue range is the beam collimation. And then if you want to get to a specific queue regimes normally what you do is you use some advanced sans geometries for example USAPs if you want to go to really really low angle. And I think that Andrew will explain more about this type of instrument configurations that are not the common the standard sans setup. When it comes to flags basically what happens here is that the main contribution to the flags will come from the source. Normally reactors provide a higher flux on sample but then we also have to think that we are going to select one wavelength so we are chopping away lots of neutrons so we have to keep in mind which wavelength that we're going to use so that wavelength is the second factor that we have to take into consideration and then instrument geometry. So these three factors will determine how many neutrons we are going to get in our sample and this is important because as I said before the more neutrons the faster the measurements will be. Okay so when it comes to the resolution basically what happens is that the intensity that we measure at given Q value it has contributions from nearby Q vectors. Okay so there are two different types of contributions so this is the resolution of the Q vectors and we have one geometric contribution and one wavelength contribution. So what happens here is that these two different contributions come from different parts so the geometric contribution comes because our detector elements so we have normally these healing-free tubes and they have a finite size. Okay so it's kind of like five millimeter or eight millimeter it depends on the instrument that you are using but it means that you're going to have an array of different tubes and the detector can detect which tube is being hit by a neutron. Okay but basically that limits your resolution it's not that perfect resolution and that's not Q-dependent you can see that the geometric contribution is uh oh yeah sorry what I was saying yeah so the the the angle contribution the geometric contribution is not Q-dependent or it's wavelength dependent but not Q-dependent and then the other the other contribution comes from the wavelength that you are picking and the the the configuration of the instrument so what happens here is that we have different different types of let's say velocity selectors which basically will decide which wavelength we are going to use for our experiment and those will give a defined a defined wavelength for the neutrons and I will show you in the next slide how that works more or less so basically this will give us some wavelength resolution and as you can see here this is Q-dependent so we have at the end that this Q-dependent resolution if we add these two contributions we have a resolution that depends on the Q so normal on the Q-vector so normally when you go to do an experiment they calculate the resolution for you that then you can use to smear your theoretical models okay so just keep in mind that depending on which type of instrument configuration you use you're going to have two different types of contributions to your resolution one is the wavelength contribution that comes from how you select your wavelength for the experiment and the other one is your detector okay so for example helium free tubes have different spatial resolutions but a CCD camera will have a much better resolution but it has also some other disadvantages so so basically what happens is that when you have this like resolution effect in your data what is going to happen is that it's going to smear some of the features in your data and I will give you some examples later about it and I thought I had a picture here but apparently I don't maybe it comes later so basically what happens here is that we it's going to smear some of the features in your scattering signal and it's actually very difficult to let's say the convulate the resolution from your data so a common approach is actually to smear the theoretical models that we use for the analysis of the data I'm going to talk about this tomorrow more in detail how we smear these theoretical models but what happens here is that then we are going to have a function that accounts for this resolution the resolution that we get from the instrument configuration because as I said these values will be calculated for each q vector and then will be will appear in our data and we will use that to smear our theoretical models and then it's important to know that there is always a compromise between flux and resolution so the higher is the resolution normally the lower is the flux and sometimes to improve like our resolution by maybe I mean if we want to go from a 10 percent resolution to a 5 percent resolution maybe we have 12 orders of magnitude of our flux is too far 12 orders of magnitude lower so we have to actually really decide if resolution is that important for us okay another aspect about science experiments is the the background that we have in our in our measurements so it's kind of like the mean the the bare minimum signal that we can measure in our system so we have a scattering that is weaker than our background basically we're not going to be able to see it because it's going to be completely dominated by the background so we have to keep in mind that the lower is the background the better so the background normally comes from from different contributions so one is the stray radiation so neutrons coming from others from other instruments and from space and things like that and electronic noise from the detector and things like that so our things that actually we cannot control we can try to minimize it by for example placing some shielding and by playing with instrument geometry for example using like curve guides in in time of light instruments and improving the detector electronics but the thing is that we cannot just easily change the background in in in an instrument it's not that we can go there and just play some more shielding and I mean during an experiment and that will improve our background so actually what normally happens is that when they have internal time they just basically put all of the shoulders down and they just measure the signal that comes from the environment and that's the background and then they're gonna we're gonna subtract that to our to our to our to the to the experimental data we get okay so this is something that we cannot easily play with that is something that we can subtract and then there is also some background scattering that comes from the sample it's the incoherent scattering that we mentioned yesterday for example the presence of hydrogen will of protein will increase the the incoherent background and this is something that we also need to account for and subtract but this is a different type of background okay so just to give you a couple of minutes of of of fresh air I'm just gonna show you some pictures of different instruments so this is sound study at ices and this is the detector chamber and this is d22 at ilf and this is the detector chamber and the first thing that you can probably see is that the detector chambers of sound study is much shorter than that of d22 and that's because one is a time of flight instrument and as I explained before we can measure a wide-q range just by using one the texture position whereas at d22 we have to move the detector to reach like a wide-q range so you can go I think down to 20 meter and uh and then sample to the detector distance and I think that in d11 you can go down to 40 meter to get to the lowest q value okay but just keep in mind that the different time of flight and continuous source sounds instruments will will will be different in terms of of building and this is just the sample area of d22 there using some kind of like dialysis unit here and this is rob playing with the with the sample changer and larmor and and as you can see like this instrument it's a it's a bit more complex because they can also do like stuff with magnets and things that I can barely understand okay so what do you have to do when you do your experiment is you have to pick the q vector that you're going to measure that's very important and this will that's remind the wavelength that you're going to use and the detector type and position that you're going to use for this experiment and then there are some more complex configurations basically using the the the the collimation and the size of the apertures at the sample position and so on and uh also the sample environment will that's remind which configuration you can use but it's just something that Judith will mention later I guess but basically here what happens is that by playing with the different uh different uh characteristics of the instrument like wavelength detector position aperture and collimation you will get uh different q ranges so basically you will have to pick what you want to what you want to what you want to get in terms of q vector so another aspect that will be influenced by that will be the resolution of your measurement so this is the classic pinhole collimation and then we start to open the aperture and basically what happens is that we change the profile of our beam in terms of wavelength okay so this is something that will affect our resolution will increase the flags but it will affect the resolution as I mentioned before and and and basically by playing with this uh with these different aspects of the instrument this will determine the q range the flags and the instrument resolution and I said before that I was missing a picture and this is the one that I was missing just to show you how uh for example a velocity selector selects a given wavelength with a given resolution and how that will influence the flags okay so this is the neutron flags at n c n r and what happens here is that the total number of neutrons per unit area and time we are getting is the integration beneath this curve okay so if we want to select an area of the neutral I mean if we want to select uh a neutral wavelength we can let's say open or close uh I mean the let's say that the the velocity selector will select a wider or narrower uh waveband so what happens here is that if we for example want to have a better resolution this velocity selector will will select a narrower uh a narrower wavelength waveband but this will also mean that the the the integral beneath the curve will be smaller which means that we're going to have a much lower flux whereas if we peak if we are less picky with the resolution and we have like a wider uh waveband we are going to have more neutrons okay so this is basically how we can just like in a simple way understand how the the the selection of the resolution will uh of the wavelength resolution in this case will determine how many neutrons we are getting okay because of the integral minus this uh this curves okay so how this affects the data that we are actually collecting so I have here some really simple and probably not the best examples but some really simple uh plots where I'm showing the scattering from the spheres and now I'm gonna uh just like get you to think about uh q range for spheres okay so basically let's say that we have these uh 20 ounce from the spheres we measure this entire q range for the green curve and basically we see uh the different parts of the curve so we see this guinea region around here and then we see the oscillations at high q and the pull-up region and this means that we are collecting the entire data set so we are collecting the q range that will require to resolve the structure of the system but then when if instead of picking the instrument configuration that gives us that q range we pick another one that doesn't go to high q what happens here is that we get information on the size of the particles because we get the q range but the interfacial scattering that comes from the high q expansion of the data has disappeared because we haven't actually proved it using that instrument configuration so we don't have this region here and there will be things like polydispersity or an isotropy and things like that that will that will be difficult to prove using this configuration so we have to be aware of what we are actually proving and then in the in the in the next example so we have some slightly bigger spheres so this is 80 ounce room I think I could have picked a better example that is more even more exaggerated but here what happens is that we are not uh the the q mean that we are proving with this instrument configuration is getting close to this guinea region so we are actually not getting this plot or that the data reaches a low q and what happens is that uh if we if we get this to the extreme there is going to be a point in which we are just going to see maybe just this part of the data because we are not going to a q values that are low enough to prove the entire size of the scattering so we are going to be missing that bit or information and this is a problem when we have something that is really big like probably hundreds of instruments and so on or even more um and I think that we'll have a few examples tomorrow about this uh another important aspect is the resolution as I said and I want to show you how the resolution affects the data so what happens here is that I have exactly the same uh model being simulated and what is happening here is that I'm applying I'm applying different resolution uh resolution values to this data so M1 has a perfect resolution so we have like one single wavelength uh we have like an infinitely good geometric resolution so basically we are doing like sax with a sound instrument because in sax we have super good resolution so as we can see at low q we have basically no effect in this configuration for the different resolutions but we can see how these sharp features are getting worse and worse when we are moving up in resolution so zero percent is the pink one and then we go to five percent so these these are less defined and then we go to 10 percent and we start to see that the peaks here high q start to disappear and when we are at 20 percent which is not that uncommon basically all of the features are gone okay so all of these features are gone so this is not if for example your system is it's polyvase burst it's not really important so it's not the end of the world but if you are looking at something that is crystalline or we are looking at something that you really want to have uh good information on for example is a small deviations from from isotropy then you really want to have like a good information here good resolution at high q and and and to achieve that you have to select wisely uh the instrument configuration okay i would say that a normal uh sounds configuration uses uh about a 10 percent resolution uh depending on the q value that you are using of course but uh normally the resolution at low q is low and at high q we have better resolution uh with the configurations that we normally use so we can get to to pretty good uh resolution at high q so basically what happens is that by playing with the different uh configurations that i mentioned before we will determine the cure rates the flux on the sample and the instrument resolution but i know that there are many things to keep in mind and it's not always easy to just they are not always straightforward so my recommendation uh when you are going to pick some instrument configuration and i think that this is more or less what people use is some things about it is that first uh you should simulate the data or try to kind of like just pick a sound software a small-angle scattering data analysis software and just say okay i think i'm going to have some spheres of this size because this is something that i've seen in dls or with nmr so do some characterization first and then basically just like try to get some information on the q-range that you need to prove that structure then basically you have to also think about the resolution that you will need for example do you have any peaks in the system is your system very one of these pairs is very isotropic uh because if it is probably you will need a good resolution to to look at those specific features so then that will basically if it is important resolution is very important for you you will have to tell the v-line scientists like yeah i need this resolution five percent resolution 10 percent resolution and he will basically tell you what is the best instrument configuration that will give us will give you the highest flux which is what you want but there will always be a compromise so if you want a higher resolution it's going to mean it means that you are going to have a lower flux if you want to go to really low q it means that your uh your your uh spot your time will be longer okay so this is something that you have to keep in mind so moving to the next part i'm going to talk a bit now about uh the different uh calibrations that perform uh in the small-angle nature and scattering instruments so this is something that the v-line scientists will normally do for you but it's something that you can keep in mind so you know what is actually happening so in a perfectly small-angle neutron scattering you have a constant flux that you know you have a normal neutron spectrum and it's perfectly defined and you have no background but actually this doesn't exist so this means that to to to get the data that you want to get you will have to apply some corrections to the raw data that you obtain in a small-angle neutron scattering experiment and then you have to do some calibration measurements for this so you have to measure the wavelength and the wavelength spectrum the incident flux the efficiency of your detector and the dead time so i'm just going to go very briefly through this because as i said this is something that is performed by the by the by the v-line scientists and it's not that unique to know all of this for your experiment but it's good to actually be aware of what is actually happening so one of the first things that perform is that you have to calibrate your wavelength so you have to determine the neutron wavelength that is getting to your instrument so to do so you have two options one is to use an standard that has normally a peak and it has a so it has like a known scattering so we know how this is going to scatter so this is for example silver v and a and it has a peak around it has a d spacing of 58.38 angstrom so if we use bragg law we can see that there is going to be a peak appearing at 0.01 inverse angstrom so basically we measure the scattering from that and we know that this peak will have to be at that given q value okay and if there is deviations from that it means that our wavelength is not correct okay so we have to correct the the wavelength that we are so this will be used to kind of like know the wavelength that we are using for that experiment and then another way to do this is to use the time of flight time of flight configuration let's say so what happens here is that we have two small detectors that are called pencil detectors before the sample position so what happens is that the instrument hits this pencil detector so we know so there is like a chopper stopping neutrons so the chopper lets some neutrons in they hit this first detector and then the neutrons travel and they hit the sans detector so basically there is a time that the neutrons take to go from here to here and we can actually use that to calculate the velocity of the neutrons and by using the boggle relationship we can calculate the wavelength of the neutrons so those are two typical ways to calculate to calibrate the wavelength in some instruments but this is something that is normally performed before you go to do the experiment okay so the incident flux is measured in I only mentioned one but there is another way to measure it so the incident flux is measured using the direct beam so basically you have a direct beam hitting your detector normally is attenuated so you don't burn your detector but you have your beam hitting your detector and it's going to tell you how many neutrons that detector is to is how many neutrons are reaching the detector and basically that's going to give you your flux another way of doing this is by using a standard sample so for example a polymer and then we know that that should show an eye of zero so so the intensity at angle zero should be I don't know a given value it's an standard and then we can use that value to calibrate the the incident flux and that's normally that is and that's something that is normally performed at a time of flight instruments instead of this direct beam so the other calibration that you had that it's performed is the detector efficiency so you have to think that these detectors have an array of tubes and not all of them are perfectly equal so there's going to be uh uh let's say there's going to be an efficiency associated to each of these pixels so what you have to do to measure these uh to correct for these is that you have to pick something that is an coherent miscatering because as Andrew said yesterday in coherent miscatering it's a four has a four pi dependence which means that it's just going to go everywhere it's not it doesn't have any uh angle dependence so what happens here is that there is going to be some uniform signal hitting the detector uh if we use for example h2 or plexiglass and then we use that to calculate the the efficiency of each of the pixels and correct for that so uh the time is basically uh the the time required for the detection of new of a neutron uh uh to to detect a uh a neutron hitting the the detector and this uh relates to when the detector is saturated so if we are sending too many neutrons and too fast to our detector we just might get to the situation where we saturate the detector and you know it's not capable of measuring them uh there is not capable of measuring them so basically what happens here is that they have to do some uh some some calculations to measure what is this uh depth time which means that if we go above this depth time the detector will not be able to cope with the neutrons and we're going to have to use a lower flux or a weaker scatter in this case and basically yeah this is something that it's also performed at uh before you go to the to the facility so uh once you've done some of these measurements what you have to do is that you will have to correct the data that you get because basically the scattering intensity you have will have different contributions so we'll have some contribution from the instrument configuration because it has optics has different things on the beam path and that's going to affect your scattering as you can see in this schematic representation so it gets to this aperture and suddenly it just hits this corner of the aperture so there's going to be some scattering coming from that so there is some contribution from the from the instrument there is going to be some contribution from the sample which is what we actually want to measure these orange lines here then we have some contribution from the cell because basically the cell is also in the beam path and even if we normally use sample environments that have very very very little interaction with the neutrons there is always some interaction so what happens here is that when it hits the cell it just like gets scattered and goes somewhere and then we have some background that it's basically coming from somewhere else outside the instrument some electronic noise and straight neutrons from other instruments or outer space so what happens here is that we have to correct for these different things I just like put here some of the you know what each of these symbols and letters means so we I'm not going to go into detail but so you know for example we have neutron flags detector efficiency the soil angle of the of the detector and so on so there are some things that as I said before we have to determine before getting what we want to get which is the microscopic cross section of the sample okay so we have here our sample we have this microscopic cross section which is the data that we actually want to analyze but this is what we are actually measuring so we have to correct for all of this okay so there is some like let's say standardized protocols to do this and and as I said the beam light scientist is aware of this it's not that you know you need to know all of this by heart but I think it's good that you have an idea of what is actually happening there so basically to get this scattering the microscopic cross section of your sample what you have to do is that you have to measure the scattering from the sample the scattering so the sample in yourself and you put it there in your sample environment then you measure the scattering from the from an empty sample holder you measure the block position so basically you you cut all of the incoming neutrons and this means that you can for example better mine the background signal you measure the direct beam intensity to get the total flux you measure the transition the transmission which is the transmission is the neutrons that pass through the sample without being scattered you measure the transmission you measure the an empty the transmission of the of the empty beam and then you measure the detector response and efficiency and finally you can also measure the solvent scattering to subtract that so basically with this first seven you're going to perform the corrections that will allow you to go from the intensity that you actually measure to the microscopic cross section of your sample okay and then there is one step more which is a step number eight which is the the scattering signal that comes from the solvent or from your matrix so what happens here is that once you apply all of those corrections that I mentioned before you get what it's called the reduced file so you go from your raw data to your reduced file so your reduced file is basically going to have the scattering from the sample plus the solvent okay so basically it's sorry is that it's the scattering from your particles plus the solvent it's like here represented as the as this black curve so what happens is that to actually get the microscopic cross section from your particles you have to subtract the solvent contribution or this matrix contribution so what you have to do is that you perform a measurement where you measure these signal from the solvent and then you subtract that contribution to get your scattering signal which in this case is the blue okay so this is actually quite important to do it right because normally this is actually log of the data of intensity you overestruct these values what will happen is that you will have some negative incentive negative intensity values which means that if you are if you present them in a log longer scale they are just going to disappear because you cannot calculate logarithm of a negative value so what happens here is that you have to be aware that you are not over subtracting or under subtracting the this solvent signal and the way to do that is to you go to the high q expansion of your data you see what is the intensity of your sample and the intensity of your solvent and because of this high q data mainly coming from the contribution of the solvent because it's what it's most important there basically you can just like scale yourself and up and down until you get the desired scattering signal so I think that this is my last bit and I'm going to talk about how you can access this type of instruments and I think and I think probably some of you are already aware of this but I think it's important to know that at different scattering facilities they have different access routes so this is for example from ISIS and they have this nice diagram that will tell you what is the best way for you to access this to access the instrument to get some beam time okay so for example if you need some quick measurement and if it is something relatively straightforward that doesn't require complicated sample environments then you can apply for a spread access access basically it's just some mail-in system that you send your samples a few of them and they will measure them for you to actually just maybe know if Sans is a good technique for the characterization of your sample or something else or like complete a data set or something like that if you want to do use a more complex if you want to have a more complex experiment for example having an advanced sample environment or you want to measure loss of sample you will have to apply for a full proposal and there are two different ways of doing this one is the normal access so we have the diet access which basically it's let's say the standard access for these facilities and then we have that I'm going to explain a minute and then we have rapid access so rapid access is when you have something that is very urgent so it's something that is either a very hot topic and you want to get it out as soon as possible or for example you are a PhD student that you are about to finish and you need to do some measurements okay so so you can try to bribe the the beam line scientists and by saying like oh yeah I need to do this and so I need some beam line some beam some access to the beam line really really fast and and this is a some of the routes so basically these are very fast they will allow you to perform either a few measurements or an entire experiment and this normally takes a bit longer okay so basically what happens here is that for the standard access route which is what I what I see is called the direct route basically you get a full experiment and your proposal will be reviewed by by some external scientific panel there is normally two calls per year and you have to wait for a bit before you go to do your experiment so you have to plan this ahead so discretionary access is the direct access so what happens here is that you get a full experiment and it has to be either a hot topic or that you're you're a PhD student you're about to finish and so on and you want to convince them that it's very important for you to get your experiment as soon as possible so this is a rolling proposal mode that basically means that you can submit this proposal at any time and you can typically get your beam time in one month because they always save some experimental time for this type of proposals but it's also very very scarce so it's difficult to actually get this type of access but if you are in a in an urgent need you can always do that so express time or test access or easy it has different names at different facilities what happens here is that you want to just just check some samples for example I'm sending some samples today to ILO for for some easy experiment it's called like that the type of access so what happens here is that they are going to measure some of the samples for me to get some data and to submit like a full proposal for the next round so you can use it for collecting some preliminary data it's also a rolling proposal that you can submit at any time and normally in a few weeks you can you get your experiment on if it is something relatively straightforward and then something that is not included in the in the previous in the previous slide but it's something that and it's not that common but basically you can also pay for having access to these instruments you can get like full experiments and it's very expensive I can tell you that and it's normally used by industry because they want to either keep all of the preparatory knowledge for them or because they won't just have the BIM time available for when they want to do the experiment and basically yeah they just pay for the BIM time and they just get access to those instruments so yeah so what happens here is that depending on what the type of proposal that you're going to submit is going to go through a different process so the first thing that you have to consider is how to write your proposal okay so I'm going here to one proposal that I submitted to NCNR a few years ago and basically it had more or less this structure so basically I have a standardized way of writing proposals and that works for me not I guess that not everyone does it in the same way but basically what I do is I use some like scientific background to explain why why why the science that I'm performing is relevant it's important to put some references because it looks like you know what you're doing then it's also very important to include some preliminary data because you have to show them that SANS or any other technique but now we are talking about SANS will be useful for the characterization that you want to perform on that sample so you have to put some structural characterization normally or some data that relates to a structural feature so you can present some NMR or the spentroscopy too but normally some structural characterization it's always good DLS or SANS for example so you present that data and then the important thing and this is very crucial for almost any SANS characterization is that you have to explain them why SANS is needed because otherwise they will tell you oh your sex rates they are available in labs you can access easily you can access them easily at a synchrotron facility so they will tell you do sex but you have to explain them why you need to do a small-angle neutron scattering for resolving the questions that you are trying to to to answer so basically it's important to tell them why SANS and also why you selected that instrument in particular then basically you have to present your experiment plan so what you're going to measure how many samples how many contrasts the configuration that you want the sample environment and for how long are you going to be measuring so for example here I think that we requested three days and then you can also put like a brief statement on what are you going to do with the results so are you going to how you're going to analyze the data if there is going to be if you're going to involve some other techniques for example SANS or NMR if this is planned to be like published in some journals or whatever if it is part of a PhD project so this is something important to actually because they want the community to keep growing and the way to do that is by by educating and forming new scientists on the technique so it's important that they know if this is part of a PhD project or or a master project or something like that so what happens when you submit your proposal is that you have your research problem so you have to ask your question can a small-angle nutrient scattering help me with this so you write your proposal if you think that the answer is yes you have to say what is the relevance of the science that you are performing what you want to achieve with the experiment and what is your plan so you have to submit it if it is the standard proposal round you have to submit it before the deadline because otherwise you will have to wait half a year for the next call and then they're going to gather some scientific panels that will evaluate the quality of your proposal and then they will accept it or deny it we have to keep in mind that this is a competitive scheme you're going to be competing they have a limited amount of resources that they can allocate so you're going to be competing against other scientists so for example next year that these two facilities are going into a shutdown iolite analysis there is going to be uh there is not going to be much uh being time available for small-angle neutrons scattering in europe which means that it's going to the competition will be brutal and and the likelihood of getting your proposal accepted will be lower which doesn't mean that your experiment is not interesting or the science that you are performing is not interesting it's just that it was not the time okay so you have to keep that in mind and and and they will in case that it gets rejected they will give you some feedback on how to improve it and what how they think that the the proposal could be a bit better for for the next round and so on so it's always good to to to also listen to that feedback and if you get your proposal accepted what is going to happen is that a few months later the beam life scientist is going to send you emails saying okay i have to put your experiment we have to schedule your experiment at some point uh what do you think about this when you're available so you give them some dates for this experiment and they are going to tell you okay come come come here whenever you i mean for these dates and we will do the experiment so you will have to plan the experiment for those dates and you will have to keep things in mind like for example if you have to travel because now in covid times is a bit more complicated and many experiments are being performed remotely uh you're gonna have to get the chemicals for the experiment especially if they are deteriorated this is something that you have to keep in mind and you might need to plan ahead for example do i need to contact a declaration facility to to to get all of these chemicals or do i need to buy them but just what i said yesterday just think that not everyone has deteriorated cotton candy in the lab uh then you have to come out with an experiment plan and what you're going to do at the experiment and you have to have this very clear because you're going to have to work a lot you're going to have to do night shifts so it's very to have a plan and not just go cowboy style and hope to get there and decide everything last minute and then just like sort out the logistics for example if you have to travel you have to book your flights and so on so these are some important aspects that we have to keep in mind okay so in summary uh what it's important when it comes to performing an experiment is that you have to come out with a plan and you have to really decide if you need neutrons for uh for answering the question and you need to actually figure out how you're going to access those neutrons uh to perform that experiment so basically then you will have to pick an instrument and you will have to decide what configuration of the instrument uh you're going to use and how that will enable you to access the information that you want to get then you will have to to go to the facility or if it is a remote experiment you will have to send your samples to perform the experiment you will have to reduce the data and then that basically means that you have your macroscopic scattering cross section which is the signal that you will have to analyze and then probably spend at the very least a few weeks analyzing the data and having fun with that and tomorrow we are going to talk a bit more about data analysis which is the next part of my series of lectures and this is all I had to say about experimental small-angle neutrons scattering so if you have any questions you can either ask them now or we can leave them for the future I think I put my question yes is there any protocol concerning the handling of measured samples because many of these materials can potentially become radioactive on cities it has been exposed to neutron beams yes uh so it depends of what materials you are working and when you submit your experimental risk assessment before the experiment you basically have to state the compounds that you are going to measure so for example if you have like you have some neutron absorbers and things like that they will be they will get hot and they will be radioactive so what you have to do is that after the experiment they're gonna so they're gonna give you a protocol on how to deal with those samples so the standard protocol is that all of the samples that have been in the beam will be tested for different types of radiation but then if you have something that is marked with like yeah with like especially radioactive or activated material then they will have also established protocol and sometimes you have to leave your samples there for like weeks or months until they get deactivated or if they don't get deactivated they will dispose them for you as radioactive material but that means that you cannot take your sample back so it's sample dependent and that's something that they have like an established protocol for that when they receive their the experimental risk assessment thank you you're welcome I'm sure there is another question here in the chat