I live in this atom, with my other electron brothers

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2 Hello, my name is Electron, John Electron. I am going to tell you how my work is in an electron microscope. I live in this atom, with my other electron brothers The filament crowns the column of the electron microscope W This atom, along with other millions of atoms generate this tungsten filament (W)

3 - When a voltage is applied to the filament edges, we feel an uncontrollable need of jumping rapidly from atom to atom. After such a lot of exercise, we get hot, the filament becomes incandescent and my brothers and I arise to the surface, creating a kind of electronic cloud covering it all.

4 Being a good electron as I am, I have negative charge. This makes us react this way when meeting electric fields: We feel attracted by the positive charges... And the negative loads obligue us to run away

5 And this is how we generate an electron beam to volts WEHNELT Electrons are attracted by a high positive voltage, but before that, they are reduced with a negative potential (wehnelt electrode) which limits the quantity of electrons passing through, in order to control the beam s intensity. The anode high voltage throws them down the column like a slingshot at high speed. We call this microscope s part the electron gun. ANODE V The higher the anode s voltage of extraction, the faster we fly Electron beam

6 - Now we have to direct or focus the electron beam, in a manner similar to the way we modify the light trajectory in an optic microscope To change the electrons trajectory we use some electromagnetic cylindrical lenses, with a winding of copper inside of them. The electromagnetic field created by the external voltage force the electrons entering at high speed to turn to the lens centre, like a corkscrew. Varying the current flow we can control the degree of convergence of the electrons, that is, the focal length of the lens.

7 To allow the electron beam travelling through the column without interruptions, we need a high level of vacuum inside, since the air's gaseous molecules would disperse the electrons the same way the smoke disperses the light. In order to clear the path of air molecules we use three pump types: first a rotatory, then a diffusion and finally an ion pump. Depending on the microscope s model, the required vacuum can be from until 10-7Pa, that is, an equivalent pressure of 0, atmospheres.

8 When the electron beam has been tuned by the lenses, it finds in its way the specimen that is going to be observed, sufficiently thin to be penetrated by a significant number of electrons. Electron Gun Condenser Lens 0,0001 mm Sample Objective Lens Magnified Image At this point we can assimilate the image formation mechanism on screen with a slide-projector. The electron beam would be the light and the sample the slide. Depending of the potency supplied to the projector lens, we obtain a more or less magnified sample image on screen or in a digital camera. Fluorescent screen Projector Lens

9 - When working in the scanning electron microscope, we do not go through the specimen, which is thicker, on the contrary we hit it and extract out electrons of their most external atoms ELECTRON BEAM SECONDARY ELECTRONS DETECTOR SIGNAL SPECIMEN This secondary electrons pulled out of the specimen by the electron beam are attracted by a detector polarized by a positive potential and converted in an electronic signal.

10 In order to create the image, the electron beam becomes an electronic paintbrush, scanning the sample line to line in the surface which is going to be observed. ELECTRONIC BEAM SCANNING COILS SINCRO DETECTOR SAMPLE The deflecting coils moving the beam are synchronized with the detector of secondary electrons to form an image in a video screen and in real time that will faithfully represent the sample's surface, since the intensity of the signal collected by the detector is proportional to the topographic relief.

11 And that is all for now, friends. I hope to have clarified your doubts, but if you have any question, not doubt in writing to me. Goodbye! (C) 2009 Adolfo Martínez Orellana & Isabel Martínez Orellana