Then a charged particle flies in a circular orbit through a magnetic field, perpendicular to the orbit plane, it starts emitting electromagnetic waves-photons. This emission is known as bremsstrahlung, or synchronous.

Of particular interest here are particles flying at relativistic velocities (comparable to speed of light). In this case nearly all of the emitted photons move in the orbit plane, and the higher the energy the lower is their scatter. And that means that an annular accelerator of charged particles, placed in a perpendicularly oriented magnetic field, can be the source of synchrotron emission. The parameters and potentialities of such units have been discussed by the Director of the (Kurchatov Institute) Russian Research Center, Corresponding Member of RAS, Alexander Rumyantsev.

Schematically, the picture is as follows: the electrons (and it is they and positrons that can ensure maximum brightness of radiation), emit-

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ted by an electron gun, are first speeded up in a linear accelerator and then in an intermediate annular booster synchrotron after which they finally attain the required energy level in the buildup ring.

For the energy of the emitted quanta to approach 100 KeV, the electrons (positrons), should be at the level of several billions electron volts. Synchrotron radiation possesses one very important feature - it has a continuous spectrum which covers energy bands from the optical to hard X-rays and even gamma radiation. This was established back in 1946 by two Russian scientists - Academicians Lev Artsimovich and Isaac Pomeranchuk.

Ring accelerators of charged particles began to be built in the world's leading research centers in the early 1960s. In this country the first sources of colliding synchronous electron-positron beams were built under the direction of Academician Gersh Budker at the Novosibirsk Institute of Nuclear Physics (now bearing his name). In 1999 the Kurchatov Institute of High Energy Physics put into operation a whole set of such accelerators located in a special wing which is 80 m long and 70 m wide. Electrons from an injector pass into a linear accelerator where they attain the energy of 100 MeV and are then accelerated to 450 MeV in a smaller accumulator or buildup ring.

The latter, called SIBIR-1, is a separate specialized source of radiation in its own right. The beams of photons it emits in the energy band of vacuum ultraviolet radiation are channelled into a smaller experimental hall where different instruments are located. There the electrons are input into the big buildup ring, SIBIR-2, where they are accelerated to 2.5 GeV. This makes them emit synchrotron radiation corresponding to the hard X-ray band. A beam of such, quanta is channelled into a bigger hall with an array of more than 20 experimental units. The entire complex is located behind a thick concrete biological shield which ensures staff safety.

Now, a brief comparison of the buildup rings' parameters - SIBIR-1 and SIBIR-2. Electron energy - 0.45 and 2.5 GeV; length of orbit - 8.7 and 124.13 m; critical quanta energy - 0.21 and 7.1 KeV; number of channels with deflecting magnets - 8 and 24; electron beam lifetime in the buildup ring - 4 and 10 hours.

Designing and building of a giant research complex of this kind required the combined efforts of large teams of scientists from various institutes over a number of years. The work was started back in the early 1980s under the guidance of Academicians Alexander Alexandrov and Yuri Kagan, with the main burden of translating the project into life being shouldered by Academician Spartak Belyaev.

Considerable assistance in the building of the synchrotron was rendered by the Budker Institute of Nuclear Physics (in Siberia) which designed both the big and the small buildup rings (hence the names of SIBIR-1 and SIBIR-2). Active support for the project was also provided by the Russian Academy of Sciences and the RF Ministry for Atomic Power Engineering, and, at the final stage - by the RF Ministry for Industry, Science and Technologies. A research program "Synchronous Radiation, Radiation Applications", headed by Academician Alexander Andreev, fulfilled the coordinating role. Functioning now on the basis of the Kurchatov Institute is a center "for collective uses".

In his review RAS Corresponding Member Alexander Rumyantsev dwelled in detail on the parameters of synchrotron radiation itself and its potentialities in the studies of condensed media.

In the first place, this radiation has a very broad continuous spectrum which covers radiowaves, SH frequencies, the infrared band, visible light, ultraviolet, X-rays and gamma rays and superhigh energy photons. Second, it possesses very high brightness. In the X-ray band, for example, it is 7 to 10 orders brighter than X-ray tubes. What is more, by using built-in devices producing alternating magnetic fields along the linear flight path of electrons (positrons) it is possible to increase the brightness by several orders of magnitude.

Summing up, the unique parameters of synchrotron sources, above all the great intensity of their electromagnetic emission, have helped to improve considerably the experimental possibilities in the traditional areas of the studies of matter in the optical and X- ray bands. Studying the human hair, for example, (thickness of 20 mcm) with the help of a beam, focused down to 1 mcm, it is possible to "look through" several tens of spots and determine with the help of X-ray fluorescent analysis not only the elementary composition, but also the location of some identical layers of the material. Clearly registered in such an experiment are concentric circumferences in which one can see potassium, copper, zinc and sulphur. The same method can also be applied, although indirectly, for studies of human metabolism.

And one can also think of yet another example from the area of what we call microtechnologies. A layer of X-rays sensitive material is applied upon some base, or substrate (usually this is polymethylmetacrylat), and irradiated with a beam of synchrotron radiation through a special "mask" produced by the method of microscopy. Since X-ray quanta possess high energy, and the beam is of high intensity, the radiation penetrates to a con-

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siderable depth - down to fractions of a millimeter. Then the irradiated layer is scoured or etched away and, after metallization, we obtain casting forms for the making of various mechanical parts and components whose configuration is determined by the shape of the mask. These can be gears, tubes, shafts and complex chambers, etc. which are used in the manufacture of electric motors, pumps, and lever mechanisms - all within one millimeter in size.

Apart from such "applied" uses, the development of what we call synchrotron methods of studies of matter has promoted the progress of some new areas of basic research, including those using condensed media under high pressure. It is in such conditions that super-hard materials are synthesized, geophysical problems are solved and the physics of explosions is investigated. As compared with the late 1940s when it was possible to attain pressures of several thousand atmospheres, by the late 1970s experts crossed the one million margin. In the conditions of this kind matter radically alters its parameters. This includes even the electronic configuration of an element, which is a kind of a starting point for the development of solid-state physics in general.

In recent time it has become possible to study in experimental ways the structure and parameters of matter in the conditions which exist in the core of our planet (pressure of some 3 Mbars and temperature of about 5,000 ⁰ K). It has also been possible to model processes which occur on the border between the core and the mantle and study not only the compressibility of the main material of the Earth's core - iron - at 3 Mbars (so far this has been the limit achieved), but also to reproduce the actual situation there caused by a combination of hydrostatic pressure and multiaxial compression.

Speaking of matter studies in principle, the most promising are what we call the combined studies of the structural, magnetic and electron characteristics. This view is born out by a cycle of studies carried out over the past two years and focusing on the electron and structural transitions in solidified oxygen. Studies of X-ray diffraction, electric resistance and magnetic susceptibility have led to the discovery of the new physical phenomenon of superconductivity in the molecular crystal of oxygen at 1.2 Mbars and the temperature of 0.5 ⁰ K.

Experiments of this kind can be conducted at the synchrotron of the Kurchatov Institute. Its experts are also making plans for new basic studies in physics, chemistry, biology, micromechanics, ecology and medicine.

A. Yu. Rumyantsev, Synchrotron at the Russian Research Center (Kurchatov Institute). - Vestnik RAN, Vol. 70, No. 8, 2000.

Prepared by Arkady MALTSEV

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