 Just a comment before starting with the following talk is why we want to calculate thermal scattering. That is because the question you made. Why we want? Because we... Well, we have two options at least at this moment. I noted two options. For example, for MCMP calculations, we need the cross-section library. We need the thermal cross-section libraries. This is what Ariel did in the poster outside. Then you can see that. But this is what you need for that kind of calculation. But also here in this expression, what you have, as I said before, you have the information of the sample. You can use neutrons as proof particles. You can use neutrons to analyze the structures and the dynamics of a sample. The theory is exactly the same. The result is different and the use is different. But you can use neutrons to study materials. There are many experimental facilities where you use neutron sources to collide into a sample and to get information about the dynamic and about the structure. The good thing of neutrons is that all the information of the neutron is only in this thing and all the rest depends only on the sample. That is the good thing about neutrons. Also, this is for physicists. Physicists use neutrons as particle proof. Engineers need to know how neutrons interact with matter and they need this kind of cross-section library. These are two applications of thermal scattering. To continue with this, now I will talk about the measurements of thermal neutron data using a low-intensity pulse neutron source. This is the neutron source we have in Central Atomic Biology. As I said before, our group that is the neutron physics group grew towards this neutron source. It was created in 1969, so our neutron source is really old but still working. This is a picture of our neutron source. This is an electron linac-driven neutron source that is 25 MeV. It is in our department, the Neutron Physics Department. It belongs to the National Commission of Atomic Energy of Argentina. It is a pulse accelerator which uses a microwave of 2856 MHz to accelerate electrons up to 25 MeV. The electron pulses can be extended up to two microseconds with a repetition frequency of 150 pps. The maximum neutron production is at operating at 100 pps with a mean current of 25 micromperes. This is all the technical information. So how neutrons are produced? We can accelerate here. This is a zoom of this. We can accelerate the electrons, collide into a lead target when these electrons are stopping. They produce premstralum radiation. Gamma radiation is produced. And through a gamma-neutron reaction, again inside the lead target, neutrons are produced. That is the way we can get neutrons from this. These neutrons that are produced in the lead target are born as fast neutrons, so they have to be moderated. To moderate these neutrons, 2 cm of foliatilane is put there and used as a moderator. So when neutrons, this is the moderator can be put in the two lines. We have two lines. One is for transmission line and one is for scattering line. This draw is for scattering, but you can put this green foliatilane wherever and you can moderate the neutrons. You can get thermal neutrons for your experiments. So to get the thermal spectrum, you can put here a thermal moderator. You can put instead of a thermal moderator, you can put a cold moderator. As we said before, if we have a moderator with a lower temperature rather than the room temperature, we can get a Maxwellian spectrum, but a little cold. So wherever. This accelerator is used for education and training activities. The main activities that these are two students, they are changing the distribution of the shielding in the target. Some of the experiments that are done is the neutron airway time as a function of the moderator dimensions, neutron flux distribution, total cross-section measurement that is one of the biggest activity, resonances, neutron spectrum using time of flight, and the multiplication factor of the fuel assembly. Those are the main activities for students. Also, and not for students, we have researchers that are working around this neutron source. One of the main activities, as I said before, is the total cross-section measurement for subthermal and two epithermal energies. Neutron spectra measurements for multiplicative and for non-multiplicative systems. We can also do neutron diffraction, deep in elastic neutron scattering, determination of hydrogen content, the study of cryogenic materials, complementary techniques for cargo scanning, and textured studies. Right here we have the application that I mentioned before. Here we have these kind of things, the textured studies are done by physicists. They need to study the samples, they put it in the sample holder, and they throw neutrons through it, and then they get information about the sample. These cryogenic material studies that was done for getting information about how neutrons behave when they go through a material that then can be used as a coal moderator. So you have many applications of this. As I said before, when you have a sample and you have an incident being most of the neutrons are transmitted and some of them are scattered. The transmission experiment consists in, you know that the intensity of the amount of neutrons after passing the sample will be lower than the intensity in the incident part of the sample. And the way to calculate that is this that depends on the deep, on the sample, on how of the flying path they say of the neutron inside the sample, and on the macroscopic total cross-section. That is something that is only dependent on the sample. So the experiment, the transmission experiment, consists in measuring the spectrum with the sample in the beam of neutron and out of the beam. If you compare the two spectra with the sample in and with the sample out, you get information about the total cross-section of the sample that was in the middle of the beam. The good thing of this, this is called the transmission. The transmission is the division of the intensity. You get outside the sample and the intensity at the beginning with sample and with no sample. So the idea is that you have neutrons, you moderate them, you make them collide through this. This is a sample changer. So you can put the sample here and you can measure one more than one sample at a time. And then with this detection bank you can detect the neutrons that are right to that bank and you can calculate the transmission. So after having the spectrum, we measure the spectrum and we have to make some corrections to that information. The most important thing of a transmission experiment is that you don't need to take into account the efficiency of the detector because you are doing a division. You are dividing the spectrum with the sample and the spectrum with no sample. So the efficiency is there but it's cancelled because it's in the numerator and denominator. So this is a very good fact of transmission experiments. So one of the things, one of the corrections we have to do to the calculated spectrum, this is not a correction, sorry, one of the things, the techniques that we apply is what is called sample in and sample out. Sample in and sample out means that, as I said before, the normal measurement is with sample and with no sample but one option could be I put the sample, I measure three hours and then I take out the sample and I measure other three hours, right? The thing is if I measure all the spectrum together, the microwave that accelerates the electrons can fluctuate. So I will keep that fluctuations inside the measurements. So a good thing to go through that fluctuation, to go inside that fluctuation is to put the sample and to put it in and to put it out in short measurements. With that short measurement we do a lot of short measurements and then we make an average. If some of these measurements are not correct, we can throw it away, right? So this is what is called sample in and sample out. If the signal is steady, you don't need to do this. But just in case you can do these short measurements in and out to avoid these fluctuations, right? And not to lose data. So what we did here is we put in and out the sample and the measurements took 20 minutes approximately. Then another thing we did, all we did, we normally do, is the measured spectra are normalized using the integral count for a monitor of helium free that is placed close to the sample and that is only used for this thing. So this is the expression I will be showing. So the transmission is the spectrum, oh, it's in Spanish, sorry, the spectrum with the sample over the spectrum without, this is open bin with sample and with no sample, right? And each of these spectrum have to be normalized by the spectrum of the monitor, right? Another correction is that we have to subtract the background, the background that also has to be normalized. Normalize with what? With the spectra that is measured by the same monitor but when we are measuring background, right? So the transmission expression is the sample, the normalized sample spectrum over the normalized open bin spectrum both with the background subtracted, okay? Another correction, no, another technique that we apply to get to know the energy of the neutron is the time of flight. We normally place the detectors at the distance of 8 meters approximately so we have that flying path. We can know the energy associated to that flying path depending on the time neutrons arrive to the detector, right? And also as the moderator has 2 centimeters we can also correct with the expression that is for the mean emission time of that moderator that tells you corrects that not all the neutrons are born at the same time in the same place of the moderator. It's a very small correction. Sometimes it's not necessary but sometimes it is. So we only have to take into account. Another thing that we have to take into account, here it is, is the dead time effects that is related to the electronics, to the detectors and all kind of that. So with that neutron source, this technique has been applied since we began with this neutron source in 1969. With these things, long time ago the first paper was published in 1972. It was the measurement of Mylar at low energies. This is exactly the same technique that we use today, right? The first publication in an X4 database was in 1974. You are going to have a talk tomorrow about X4. X4 is a database where all the measurements or the experimental values are kept, right? So this is a place where you can share your measurements and where you can put your measurements and where you can get measurements from other people. For example, I showed you before the calculation of the silicon cross-section we made. We were not able to measure the silicon sample because we were not able to get it and we were able to get the information for this X4 database, right? So the first publication in X4 was in 1974 and nowadays we have a student that finished his PhD that is in NASA, that is the third involved in our group. His PhD was on light and heavy water and the best measurements we could find for his PhD were still the ones that we made in 1974, not me, but the other people, right? So these measurements are really good, are still available and are still used. From that time to these times we made some changes in the neutron source. We were able to acquire a cryostat where the samples are placed inside and are cooled down to the desired temperature. It works with nitrogen, with lithium nitrogen. It's a closed circuit, so the sample is there. This is a cryostat. This is the way here. We have the sample here. We have the sample. So we can have very low temperatures. The lowest temperature we could reach was 32 Kelvin. That is really good. And in the same way we could measure cold samples in exactly the same way I mentioned before with room temperature samples, right? The temperature with this system is controlled with two thermal resistances that are placed at both sides of the sample holder. This cryostat was bought when we started working on cold moderated materials that are these hydrocarbons that are mesitalin, toluene, are mixes of them that I will show you some measurements here. We measured these kind of things. We measured the mesitalin at room temperature at 90 Kelvin and other temperatures we wanted to analyze. And we got this information that is really good. The error we get with this technique is lower than the 3%. So I think it's really good. Mostly as we study pairs of data with the sample in and with the sample out what we do is an average. So the main source of the error of this measurement is due to these statistical matches. And as we were able to measure these things we also wanted to compare to a calculation, right? Honestly, it was the other way. We have to do the calculation and then we were able to get the sample so we could measure it. And the calculations were done for this material. This material, the mesitalin, is a benzene ring with three carbon-hydrogen-3 groups. They are called methyl groups. This is a very good moderator material because it has a very high protonic density. It has a lot of hydrogen, so it's very good moderator material. Also, when you freeze it, it behaves properly to radiation, right? For example, for moderator materials, the best material is the methane. Methane is excellent, but it has a very, very low resistance to radiation. This material is not as good as methane, but resist happily, let's say, the radiation. It gives a spectrum that is really similar to the methane. That was the reason because we wanted to study. It's also very easy to handle because it's a liquid at room temperature so you can handle it in the bottle and you can put it in the neutron source and you freeze it in the moment you need to work with it. This is good for small sources. If you have a very big source, you need a liquid moderator material because you need to circulate, but for this kind of sources as our Linux, this kind of material is excellent. We decided to do the calculation with this. We have to study the frequency spectrum for this material. We got experimental information from people from Dugna. They sent us the continuous part of the frequency spectrum and then we started studying this material as an integrant of the family of the benzene. If we study the benzene and we study the tolua and data materials that are well studied, we can construct in a certain way the frequency spectrum for this material. What we did is we used the continuous part where the collective movements are included. We took it from people from Dugna. Then we add to the ENSHOI three vibrations. One that is related to the ring breathing. The ring breathing is this movement associated with this kind of breathing. It is exactly the same energy of the benzene. Then we included two stretching. The stretching in the material group is the stretching here and the stretching in the ring. What we did is we made a mix of these things. We put the continuous part and three discreet energies. We put that in ENSHOI and we got a cross-section. That cross-section was extremely similar to the experimental data. The only thing we have to optimize is the weight of the different parts of the spectrum. One of the most important applications of our neutron source is the measurement of the total cross-section of any material to validate our model for the cross-section. This material is also important because it was the first material that we were able to measure at low temperatures. This material was first studied as a sample, but then it was used as the coal moderator for the same source. After knowing about the existence of these kinds of materials that are easy to handle, that can be freezed in an easy way and that are not expensive, we decided to construct a coal neutron source. Not very cold because for cooling the material we need nitrogen. Nitrogen is extremely expensive within a closed circuit and it was almost impossible. We decided to start with the almost cold source, almost cold because we were using nitrogen. Nitrogen is the temperature of nitrogen in 77 Kelvin, so the temperature of the moderator was close to 90 Kelvin, depending on the moment. This is another PhD. It was designed for a PhD thesis. This is only a container of nitrogen. This is the moderator. Here we have the values, but they cannot be seen. This is placed instead of the polyethylene moderator. It can last 8 hours, so for a working date it is more than OK. When the day is finished, we put nitrogen again. Not what the day is. When we start the day, we put nitrogen and it lasts the whole day. We can have the cold neutron source. When we use this cold neutron source, we got that. Using mesetylene at room temperature, we got this spectrum, the red one, that is a Maxwellian spectrum for room temperature. And with mesetylene at 77 Kelvin, we got a colder spectrum. This was measured directly with our neutron source. The background is the black one here. It is subtracted from this information. This is exactly what we were expecting. So now we have this cold neutron source that can be used to get neutron for lower energies. What is the advantage of having colder neutrons? Why we want to have neutrons of lower energies? Because sometimes we want to study as samples materials that have branch edges. The branch edges appear for low energies, so we need to have cold neutrons to impact with that kind of samples. That is part of the textures analysis that are going to be carried out in the NINAC. This is the gain of using a cold moderator over a room moderator. This is clear. This is the important part here, close to the energy associated with that. This is the experiment. We took the spectrum from one of them over the other one. The experimental and the calculation, and we got exactly the same value by exactly similar values. So the idea of our NINAC is to get information about cross sections of any material at almost any temperature. The cryostat can be used for cold samples and to heat them. We can get, I don't know up to what temperature, but we can have cold and hot samples, so we can measure them. Mostly the idea of these measurements is to publish them in a normal journal and then are shared in X4. Again, we share this information in X4. Many years ago we were able to publish directly in X4. We sent the information to X4 and the information was published. Now you have to write the paper and I think that hours after you send the paper you get the email from X4 asking for the measurement. So it's really good to put the things here because they can be shared. X4 is not very popular, but it should be more. So throughout these 45 years the linear accelerator in Centro Atomico Bariloche was enabled to do the development of many activities. Research and development in basic science and in nuclear engineering. Remember that it's also used for students. It's used for research. And it also is used, for example, from people from MIMBAP. MIMBAP is a company that is in Bariloche. That is the one that is designing and constructing reactors, now satellites and kind of technology. They normally need, for example, to study the resistance to radiation of certain electronic materials. They go to the Linux and they put them there in a neutron field or in a gamma field and they study the behavior. So we can give that services to companies. Also, in the academic field, technological innovation in the field of nuclear energy, training of human resources. In the area of neutronics, the good thing of the Linux is that the watch is, how we said, the regulatory body. What it's taking care of is of the target. You can change anything you want in the source. But the target. We have two autoresized targets. One is of aluminium and one is of lead. We had in the past an uranium target, but it was a little dangerous. So now we are not using anymore. But if you change the target, you have to call the authority, the regulatory body. But if you don't change, you can change whatever. You can go inside the source and you can change what you want. You can take out the lead target and you can use the electrons directed to irradiate your sample. With this neutron source, we also were able to produce 39 master thesis, 20 PADs. We have contributed with 46 databases to X4, not only in our Linux, also in the nuclear reactor. And we are able to use this information for the evaluation of thermal scattering libraries through the transmission of total cross-section measurements and also with the neutron spectrum measurements. Both things are useful for validation of thermal scattering. I think I speed up a lot. With the Linux, we contribute to the mission of the National Commission of Atomic Energy of transmitting to the society the benefits of the peaceful use of nuclear energy. But Iloce Linnak is the only operating post-neutron source in the South's air missile, in my sphere. It's very small in neutron flux, but you can put hands on it. That is the good thing. And it's very useful for testing ideas. And in particular, it's extremely useful for total cross-section measurements. That nowadays, I think that there are few places in the world where you can go and measure total cross-section. That is not the only thing you need for validation, but it's the first you need. You need more, but this is the first thing you need for validation. Again, this is our group. And if you want to download our cross-section libraries, you are kindly invited to our website where we normally put the cross-section libraries when we have time. So it's a little not updated now, but you can download whatever or you can write to us directly an email and we can send you the cross-section library to share opinions. And of course, we need the feedback of that, so you are kindly invited to do that. I don't know if you have any questions. The flux is extremely low because you have 25 MeV electrons, so you are having a very, very low flux. But the resolution, I honestly don't remember. But the experiments are not quick, let's say. For example, you need three days for experiments that maybe in a big facility, you can do it in a couple of hours. The good thing of this is at home. And especially for us that we are really far away from the real world. That is really good because I can do like this and I can put a sample when I am doing something else. And also remember, this is a very old source. Forty-five years ago it was something extremely good. Now it's old and it's almost fading, but it's what we have. And the thing is that the people that have been working there, have been working there for a long time and they know everything on the source. They know each detail that we have no idea because now you buy a closed box and you put it there and use it. That was built with the hands of the people. We have helium-3. We have 7.7. No, 7.3 helium-3 detector is a bank. Hope they can survive a little because we are thinking of changing them because of all the problems with helium. But now we have helium-3. No more questions? On the size? Well, normally, that is a very good question. Normally samples are big, really big. For example, samples are not that big. Maybe you can have these samples. Depending on if you have, for example, hydrogenated or deuterated materials, deuterated materials are really big. You have room enough to put the samples, but you have to... Samples are placed here. This is the flying tube and this is the out, the face of the place where neutrons are born. You have a big place for putting the sample, but you have to be careful about if you are doing a transmission, for example. If you put a very big sample, it is going to take out for the measurement all the neutrons. You need to calculate more or less the size of the sample taking into account that you need 50% of the transmission, approximately. Not to have a very low transmission in order to measure something or not to have a very big transmission the other way. You have to be in the middle, but normally are very big sizes. That is a limitation of also this neutron source because you need... For example, we wanted to measure magnetic material, but the people that work in magnetism use samples very, very small. When we asked them, I think that we needed three grams of the magnetic material and it was almost impossible to get it. We have the facility, but they didn't have the big sample. This is how an Exfor file looks. Tomorrow you are going to have a talk about Exfor. We send the information. This is an example. This is one of our... This is mesitile. This is what you get in the database. I think it's all. If you don't have any more questions...