تدريب Shadowing: 2 The Principle of the Electron Microscope - تعلم التحدث بالإنجليزية مع YouTube
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How does such a microscope work?
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How does such a microscope work?
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The optical microscope has been known since the 17th century.
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The modern optical microscope has magnification of about 10,000 times and makes it possible for the human eye to distinguish objects that are two ten-thousandths of a millimeter away from each other.
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It is this resolving power of the microscope, i.e.
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the ability to distinguish two very close objects in an image, which is one of the most important parameters.
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The level of magnification just makes it possible to picture how the image was enlarged.
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However, it does not say whether we can see anything in the picture.
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A human eye, at its best, has only a resolution of 0.2 millimeters, while the optical microscope has two ten-thousandths of a millimeter, and the electron microscope has up to 50 picometers.
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Wait a minute.
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That's too many numbers.
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Well, it's as if you were watching a tennis match from the moon and were still able to follow the small tennis ball.
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Mmm, interesting.
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Let's go back to the optical microscope.
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When attempting to reach a better resolution, scientists encountered the limit of the wavelength of the light used for the illumination of the sample.
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It was not possible to distinguish points closer than several hundreds of nanometers.
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In 1920, it was discovered that accelerated electrons in a vacuum can act as light, while the wavelength of these electrons is about 100,000 times smaller than light.
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It was also discovered that electric and magnetic fields influence electrons similarly to how lenses and mirrors influence the light.
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Hey, who is this handsome fellow?
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That's Dr.
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Ernst Ruska, who assembled the first electron microscope in 1931, and after more than 50 years, he was awarded the Nobel Prize for Physics for his invention.
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Well, the main thing is that he lived to receive it.
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I am still waiting.
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I would still like to know what an electron actually is.
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An electron is the negatively charged particle of an atom orbiting around the nucleus.
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It can be released by heat or an electric field.
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Electrons are 2,000 times lighter than the smallest atom.
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Thus, they can easily be stopped or diverted when hitting materials.
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Now I see.
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That's why there must be a vacuum in the microscope.
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Great!
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The column of the electron microscope consists of basically the same parts as the optical one.
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However, the source of light is replaced by the so-called electron guns and the glass lenses by electromagnetic ones.
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The electron beam is produced by an electron gun, in which, for example, a tungsten filament can be placed as the electron source.
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The beam is produced by heaving the filament up to 2700 degrees Celsius and connecting it to high voltage.
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The higher the voltage, the higher the energy of the electrons.
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The electrons accelerated by 300 kilovolts almost reach the speed of light.
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However, to reach a high resolution, the accelerating voltage and series of lenses must be immensely stable.
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stable.
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The power cabinet contains a number of sources whose output voltage or current fluctuate not more than one millionth of the output value.
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I probably understand, but don't you have another example?
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The one with Tinnis was excellent.
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I do.
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Try to imagine that the allowed voltage fluctuation in a common socket has the height of Mount Everest.
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Then, the allowed voltage fluctuation of a 200 kilovolt high voltage source has the height of only 7 centimeters.
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Such stability needs very efficient and sophisticated electronic circuits.
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You see how good you are with examples?
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Let's go on.
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Since electrons move freely only in a vacuum, there must be a vacuum in the entire column.
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To achieve this, vacuum pumps are used.
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Various levels of vacuum are needed.
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The highest is around the electron gun.
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The difference between common air pressure and residual pressure in the microscope is about 10 orders of magnitude.
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So it means that the probability that an electron will hit an air molecule under this pressure when passing through the column is zero?
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Yes, almost zero.
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However, dozens of millions to billions of electrons hit the samples per second.
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Poor lizard.
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Electromagnetic lenses focus the electron beam on the examined sample in an optimal way.
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During its entire journey, the electron beam goes through a number of apertures with various diameters.
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The smallest ones can be just a few thousandths of a millimeter.
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These apertures stop electrons undesirable for creating the image.
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The electron beam hits the observed object and it either scans its surface step by step as if it were reading an inscription on the wall with a torch at night or it goes through the sample and shows its inner structure.
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I see and according to the way the electron beam hits the sample we distinguish two basic types of electron microscopes scanning and and transmission.
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Exactly.
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The sample for the transmission electron microscope is very thin, several hundred nanometers, and it is placed on a grid.
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If it wasn't thin, electrons would be stopped and no image would be created in the transmission microscope.
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There are various holders available for the examination of samples in the transmission microscope.
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depending on the application a customer would like to use.
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Here we can see a holder where more samples can be placed at the same time.
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On the other hand, the sample examined in the scanning microscope can be bigger, even dozens of centimeters.
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Therefore, the scanning microscope is always used where information about the surface of the sample is required.
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Yes, the only requirement is that the sample must withstand a vacuum and the irradiation with electrons.
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A holder is not necessary.
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Samples are placed in a small table which is placed under the electron column.
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As soon as the electron beam hits the sample or scans through it, various detectors placed in the microscope create the final image.
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And can I manipulate the sample placed this way in the microscope?
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You are right that it is not enough to move most of the examined objects along a horizontal axis.
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Information can be gained from various depths of the sample and we can observe them if we slightly turn the sample.
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The sample holder in the transmission microscope is inserted through a vacuum interlock in the goniometer.
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This enables not only movement along the X and Y axes, but also its inclination around one or both axes.
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Also, the rotation or movement along the z-axis in parallel to the electron beam.
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Phew!
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That sentence is a little complicated.
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It would be enough to say, yes, it's possible.
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We'd better move on.
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What influences the quality of the picture?
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The image quality in the scanning microscope depends on the orientation and distance of the sample from detectors and the final lens.
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The stage on which the sample is placed makes it possible to move it vertically along the x and y axes, up and down in the direction of the z-axis, and with a possible inclination and rotation.
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These movements are performed with step motors and are controlled by the computer.
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Great, but show me some pictures already.
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Biologists use electron microscopes to examine the structure of cells, bacteria, viruses, and colloidal particles.
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Scientists who are concerned with material characteristics want to observe inhomogenities and faults in metals, crystals, and ceramic materials.
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In geological fields, the electron microscope allowed the detailed studies of rocks, minerals, and fossils, and to understand the origin of our planet and its valuable mineral resources.
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electron microscopes not only display, but also analyze, measure, and modify in 2D, 3D, and even 4D.
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Wait, I know it here.
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This is Satek in Brno, Central European Institute of Technology.
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Yes, there is a Kryos electron microscope here with a special function.
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It can work with biological samples.
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They would soon be dehydrated by the vacuum in the common electron microscope and thus would be destroyed for the research.
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However, FEI has developed a system where each sample is permanently frozen to at least the temperature of liquid nitrogen, from its production to its examination in the Kryos microscope.
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I understand.
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Thus, the damage is prevented.
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Thanks to the transmission electron microscope Titan Kryos, created by the direct electron electron detector Falcon 2, we can determine the structures of virus, or protein complexes, with very high resolution.
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The structure with very high relationship is very important to our work, because it can be explained to us, for example, the mechanism of infection, in case of virus, or the mechanism of action of protein complexes that are involved in the important biological behavior of the unit.
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The risk of the microscope Kryos is its external stability, which can be done a long-term measurement, the capabilities of cannot-optctic mode, without any protection of ve Pla Premium service of случае.
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Therefore we can discuss more qualityberson more information in the meantime of the battery to perform contract and effectively use sound performance in the video는 which increases our LingQ Gold strive by gig security.
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