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  2. What Neutrons are good at?

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- [Instructor] Okay, so let me talk about what neutrons are good at. The neutron source I used is located in NIST Center for Neutron Research, the so-called NCNR. NCNR is located in Gaithersburg in Maryland in U.S. And this is NCNR pictures and we have a 20 megawatts reactor on site. And in NCNR we have total 28 neutron beam stations. And all these instruments, they have all different purpose and today's topic were, I use the main two instrument. One is Neutron Imaging Stations to study the core structures, the core scale structures of the shale rock, and the Small-Angle Neutron's Scatterings is used to study the nano-to-micrometer structures for the carriagings. Neutron is good because neutron doesn't carry much. Neutron doesn't carry charge and so it can be very penetrating through the materials, especially for metals. And this allows to build up complicate sample environments such as high pressure and the high temperature. And this is very importance if we want to simulate the down hill collisions for the shale rock. But most of the high temperatures and high pressures devised involving metal, so it's hard for the x-ray to penetrate through it. That's why a neutron is very important and also, neutron is very sensitive to hydrogens because hydrogen has a very large incoherence or scattering cross section for neutrons And we know that in the down hill conditions that there's a lot of water, hydrocarbons, and organic matter, such as carrageens instruments and then all these things have or are hydrogen rich. So we need to. Since Neutron is very good because it is very sensitive to hydrogen to detect all these components. And this is a diagram for the total neutron scattering cross section and you can see how hydrogen compare with all other elements. It's very large and if we want to study the structures around nanometer to micrometer size, and coherent scattering cross section is what we need. So for x-ray, the coherence scattering cross section depends on the atomic numbers. So basically, higher atomic number elements, you will get higher interaction with x-rays and, therefore, neutrons, this interaction is not depends much on the atomic number. For example, this is hydrogens and this is the coherent scattering cross sections. Though they have the same atomic number, and the chemical is smaller elements, but they have very different coherent scattering cross sections. So we usually, we often take advantage of this concept and combine the first and second advantage; that Neutron is very penetrative and is very sensitive to hydrogens. We can see these lily flowers being put into the, 2.5, centimeter thickness wall lead box. So because you can imagine that this thick 2.5 centimeter thickness, of lead is hard to be penetrated by other radiation such as x-rays; however, Neutron can easily penetrate through this box and see these lily flower. Especially lily flower is hydrogen rich then you can see these dark areas or hydrogen rich areas, so that is how neutron is very powerful and then, so we neutron people always say that, "Oh, if Superman cannot "see this lily flower because Superman "only have x-ray eyes, he don't have neutron eyes." Okay, so also, neutron can do isotope replacement and this allows to, selectively choose the structure we are interested at in this instance. And just like a human can distinguish different materials by different colors. And neutrons distinguish different materials by their Standard Length Density. So you can treat Standard Length Density as a neutron color, so I plot this as an example the different color I show is different Scale Length Density, elements and so there are a green triangles and a blue circle elements put into a continuous medius. And if we can somehow change the colors, the neutron colors, the Scale Length Density of the continuous medius, we can highlight a certain part of the material. For example, if I choose the continuous medius color to be the same as blue circle's, then I can only see green triangle. And on the other thing, I choose the continuous medius color to match the green triangle, then I can only see this blue circles. So by using this, we can highlight different elements in the materials. This is very easy to be done in neutrons because it's, hydrogens and neutrons have very different Scale Lengths. So, if this continuous medius water as a solvent, and we just basically need to mix D2O and H2O with different ratios and then we can choose the continuous medius colors and then, I will talk about later, that we, I, indeed used this technique, this contrast variation technique inside this, solve this heterogeneity of the materials. And the last advantage of neutron is that neutron can cover by using different techniques. And we can cover a wide range of time scale and length scale. So we can cover 10 to the minus 14 second to 10 to the minus seven seconds. And with different instruments, we can cover sub nanometers to several micrometers. And this is important for us to study the hydrocarbons transfers inside nanopores and the, inside, the shale rocks.