SUPERMICROSCOPE

From Big Medical Encyclopedia

SUPERMICROSCOPE — the high-voltage, vacuum device in which the enlarged image of an object is received by means of a cathode rays. It is intended for a research and photography of objects at big increases. E.m. have high resolving power. AA. m find broad application in science, the equipment, biology and medicine.

By the principle of action distinguish translucent (transmission), scanning, (raster) and combined E. m. The last can work in translucent, scanning or in two modes at the same time.

The domestic industry started release of the translucent supermicroscopes in the late forties 20 century. Need of creation E. the m was caused by low resolving power of light microscopes. For increase in resolving power more short-wave source of radiation was required. The solution became possible only using as the lighter of a bunch of electrons. Wavelength of a cathode rays, accelerated in electric field with potential difference 50 000 in, makes 0,005 nanometers. In a crust. time on translucent E. the m reached permission for films of gold of 0,01 nanometers.

The scheme of a supermicroscope of the translucent type: 1 — the electronic gun; 2 — condenser lenses; 3 — a lens; 4 — projective lenses; 5 — a tube with glory-holes through which it is possible to observe the image; 6 — a high-voltage cable; 7 — a vacuum system; 8 — the operating console; 9 — the stand; 10 — the high-voltage feeding device; 11 — the power supply of electromagnetic lenses.

The schematic diagram translucent E. the m a little in what differs from the scheme of a light microscope (see). The path of rays and basic elements of a design of both microscopes are similar. Despite a big variety of the released supermicroscopes, all of them are constructed according to one scheme. A basic element of a design translucent E. the m is the column of a microscope consisting of a source of electrons (the electronic gun), a set of electromagnetic lenses, a subject little table with an objektoderzhatel, the luminescent screen and the photochart recorder (see the scheme). All structural elements of a column of a microscope are collected hermetically. The system of vacuum pumps in a column creates a high vacuum for free passing of electrons and protection of a sample against destruction.

The cathode rays is formed in the gun of a microscope constructed by the principle of a three-electrode lamp (the cathode, the anode, a managing electrode). As a result of thermoissue from the warmed V-shaped tungsten cathode electrons are released, to-rye disperse to high energy in electric field with potential difference from not however ky tens to several hundred kilovolts. Through an opening in the anode the cathode rays directs in a gleam of electromagnetic lenses.

Along with tungsten thermoissue cathodes in a supermicroscope use the rod and autoissue cathodes providing considerably the big density of a bunch of electrons. However vacuum not below 10^-7 is necessary for mm of mercury for their work., what creates additional constructive and operational difficulties.

Other basic element of a design of a column of a microscope — the electromagnetic lens representing the coil with a large number of rounds of a thin copper wire placed in an armor from soft iron. During the passing through a winding of a lens of electric current in it the electromagnetic field, power lines is formed to-rogo concentrate in an internal ring rupture of an armor. For strengthening of magnetic field the polar tip allowing to receive the powerful, symmetric field at current requirements in a winding of a lens is placed in the area of a gap. A lack of electromagnetic lenses are various aberrations influencing resolving power of a microscope. The greatest value has the astigmatism caused by asymmetry of magnetic field of a lens. Apply mechanical and electric stigmator to its elimination.

The problem of dual condenser lenses, as well as the condenser of a light microscope, consists in change of illumination of an object due to change of flux density of electrons. Diaphragm of a condenser lens to dia. 40 — 80 microns the muchka of electrons chooses the central, most homogeneous part. An objective lens — the most short-focus lens with powerful magnetic field. Its task consists in focusing and primary increase in an angle of the movement of the electrons which passed through an object. Resolving power of a microscope in many respects depends on quality of production and uniformity of material of a polar tip of an objective lens. In intermediate and projective lenses there is a further increase in an angle of the movement of electrons.

Special requirements are imposed to quality of production of a subject little table and objektoderzhatel since they shall not only move and incline a sample in the set directions at big increase, but also if necessary subject it to stretching, heating or cooling.

Quite difficult electronic and mechanical device is the photoregistering part of a microscope, edges allows to carry out automatic exposure, replacement of the finished shooting photographic material, to make on it record of the necessary modes of a mikroskopirovaniye.

Unlike a light microscope an object of a research in translucent E. the m fastens on the thin grids made of non-magnetic material (copper, palladium, platinum, gold). On grids the film substrate from a collodion, polyvinyl formal or carbon thickness several tens of nanometers fastens, then the material subjected to microscopic examination is applied. Interaction of the falling electrons with atoms of a sample leads to change of the direction of their movement, a deviation on insignificant corners, to reflection or full absorption. Only those electrons take part in formation of the image on the luminescent screen or a photographic material, to-rye were rejected by substance of a sample on insignificant corners and could pass through an aperture diaphragm of an objective lens. Picture contrast depends on existence in a sample of the heavy atoms which are strongly influencing the direction of the movement of electrons. For strengthening of contrast biol. the objects constructed generally of light elements apply various methods of contrasting (see the Submicroscopy).

In translucent E. the m provided an opportunity to receive the darkfield image of a sample during the lighting by its inclined bunch of electrons. In this case through an aperture diaphragm there pass the electrons disseminated by a sample. The dark field method increases picture contrast at high-res of details of a sample. In translucent E. the m is provided also the mode of microdiffraction of the minimum crystals. Transition from svetlopolny to the darkfield mode and microdiffraction does not demand considerable changes in the scheme of a microscope.

In scanning E. the m a cathode rays forms the high-voltage gun. By means of dual condenser lenses receive a posterior pyramid of electrons (the electronic probe). By means of the rejecting coils the electronic probe is deployed on a surface of a sample, causing radiation. System of scanning in scanning E. the m reminds system, with the help a cut receive the television image. Interaction of an electron beam with a sample leads to emergence of the scattered electrons which lost a part of energy at interaction with atoms of a sample. For creation of the volume image in scanning E. m electrons gather the special detector, amplify and move on the generator of development. The quantity of the reflected and secondary electrons in each separate point depends on a relief and chemical structure of a sample, brightness and picture contrast of an object on a kinescope respectively changes. Resolving power scanning E. the m reaches 3 nanometers, increase — 300 000. The high vacuum provides obligatory dehydration in a column of the scanning supermicroscope biol. it is model by means of organic solvents or their lyophilizing from the frozen state.

Combined E. the m can be created on the basis of translucent or scanning E. m. Using the combined supermicroscope, it is possible to study at the same time a sample in translucent p scanning the modes. In combined E. the m, as well as in scanning, provided an opportunity for X-ray, power dispersion analysis of chemical composition of substance of an object, and also for the optiko-structural machine analysis of images.

For increase in efficiency of use of all types E. m are created the systems allowing to digitalize the electronic and microscopic image with after-treatment of this information on the Optiko-strukturny COMPUTER the machine analysis allows to make a statistical analysis of the image directly from a microscope, passing the traditional negative print method.

Bibliography: Stoyanova I. G. and Anasknn I. F. Physical bases of methods of the translucent submicroscopy, M., 1972; Suvorov A. L. Microscopy in science and technology, M., 1981; Finean Dzh. Biological ultrastructures, the lane with English, M., 1970; Shimmel G. A technique of a submicroscopy, the lane with it. M, 1972. See also bibliogr. to St. Submicroscopy.

V. S. Tyurin.

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