The 20th century was justly hailed by many as a century of physics- a science which has considerably broadened our knowledge of the surrounding world and set the scene for the scientific and technological revolution. But at the turn of the millennia the focus of attention of the international scientific community began to shift to disciplines investigating life on this planet, and the Earth in general, including biology, medicine, protection of the environment, fundamental studies of materials (especially the development of new structural materials), etc. This change of research priorities, so to speak, does in no way deprive physics of its pride of place among the natural sciences. Specialists in various fields are inspired by the prospects and potential of using particle accelerators, nuclear reactors, large telescopes and high-capacity presses. One such "high-capacity" unit is the IBR-2 pulsed fast neutron reactor located at the Dubna Joint Institute for Nuclear Research (JINR).

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By Viktor AKSENOV, Dr. Sc. (Phys. & Math.), Head of Department of Neutron Studies of Condensed Media, Joint Institute for Nuclear Research (Dubna), head of IBR-2 reactor project

WHAT LIES IN STORE?

The year 1960 saw the launching in Dubna of the world's first pulsed nuclear reactor- IBR. The successful operation of the unit and its modifications stimulated in the mid- 1960s the development of similar projects in the United States, Italy and Japan and also in the Soviet Union. These efforts, however, produced but one practical result- IBR-2, a reactor which was also developed by Dubna experts (put into operation on February 10, 1984). This achievement was promoted to a large extent by a unique store of experience in operating such units under the general supervision of what was called the Ministry of Medium-Scale Engineering (now the Ministry of Atomic Energy of the Russian Federation- MINATOM) and of scientists from Dubna and from the Physics and Power Engineering Institute named after A. Leipunsky (Obninsk).

The IBR-2 reactor was designed and built by experts of the R&D Institute of Power Engineering (Chief Designer-Academician N. Dollezhal). The design was developed by experts of the State Specialized Design Institute and the field elements were produced at the All-Union Research Institute of Inorganic Materials and the MAYAK plant. Specialists from other institutes and R&D centers were attracted for dealing with some specific problems.

The first research director of the project was Academician D. Blokhin-tsev- Corresponding Member of the USSR Academy of Sciences and the first Director of Dubna Center. Later on he was joined on the project by Academician I. Frank, a Nobel Prize winner. On the basis of the new reactor, and for the development of other IBRs, a Neutron Physics Lab was set up, which is now bearing the scientist's name.

Research experiments on IBR-2 were started right after it was put into operation. The pulsed-mode reactor is generating a record flux of fast neutrons of 10 ¹⁷ n/cm ² /s; and thermal flux of 10 ¹⁶ n / cm ² / s.

The main feature of IBR-2 are periodic changes of the power levels obtained with the help of a movable steel neutron reflector. It consists of two sections rorating at the rates of 1,500 and 300 rev/min. When both these mirrors channel the neutron flux into the core (containing 90 kg of plutonium dioxide), a powerful energy surge of 1,500 Mwt is produced. This is about as much energy as is produced by sections of industrial nuclear power stations. And the basic difference consists in the fact that in the IBR-2 reactor this amount of energy is concentrated within a very small volume (221) and is generated 5 times per second, with the mean power being only 2 Mwt, something which provides for a safe and relatively inexpensive operation of the unit. Its development cost the Soviet Union around 20 mln dollars, as compared with the annual cost of no less than 1 mln dollars of running the unit and its further improvements. This, however, is 10 to 50 times less than the cost of similar projects in other countries.

Since IBR reactors were the first pulsed sources of neutrons for physics studies, the basic methods of time-of-flight structural neutronography and spectroscopy when the energy of neutron was measured in the experiment by its time of flight from the source to the sample, were also developed at Dubna.

During the current century IBR-2 will undergo modernization and with this aim in view a 10-year agreement was concluded at the start of the year 2000 between the Dubna Institute and the MINATOM on the replacement of the basic equipment involved.

Current plans even provide for stopping the reactor in 2007, and by the year of 2010 the Dubna Center will have a practically new unit-IBR-2, with improved parameters and safety standards. According to calculations its lifespan will be no less than 20 or 25 years.

In our work on this project we shall be oriented at the unique properties of neutrons in their interactions with matter, bearing in mind the areas of modern science which can potentially benefit from that. At the present stage we have in mind what we call the condensed state physics (crystalline structures and excited states of matter, magnetism and strongly correlated electron systems, non-crystalline materials and fluids), biology and pharmacology (structure and physical chemistry of macromolecules, the functioning of concrete biological systems, geophysics, textural analysis of minerals and geological rock), and, finally, studies of materials (nanostructures in polymers, structural materials). In order to have a general idea about the future trends of IBR-2 research, let us take a look at some concrete research programs carried out on this reactor which, however, require further development.

HIGH-TEMPERATURE SUPERCONDUCTIVITY

One striking example of the potential of using neutrons for structural studies of materials is what we call the decoding of the crystal structure of cuprate high- temperature semiconductors (HTSC). Their discovery in 1986 was a major and long- expected landmark in physics which had not only scientific, but also political repercus-

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Crystal structure of high-temperature superconductors on the basis of mercury. These compounds possess a record temperature of transition into superconductor state-134 К at atmospheric pressure and 152 К at a pressure of 11 GPa.

sions. Suffice it to say that the discovery brought a Nobel Prize in physics to the Swiss scientists K. Muller and D. Bednorz.

All of the economically advanced countries launched federal scientific and technical programs for studying the nature of HTSC and their technical applications*. The problem turned out to be very complicated and has remained on the agenda for the 21st century. A wealth of experimental material has been accumulated providing the basis for several theoretical models of HTSC, and the immediate task consists of choosing the most appropriate ones among them.

Using the method of neutron scattering has had a major role to play in this process. A crystal structure of medium complexity, with elements of very different atomic numbers and magnetic properties, provide for a range of uses of neutrons in the studies of the structure and dynamics of such compounds. It was with the help of neutron diffraction that scientists have been able to determine the position of light oxygen against the background of heavy elements like yttrium, mercury and barium, thus making the first step towards understanding the mechanism of the new phenomenon.

A lot of useful data on the problem has been obtained in studies on the IBR-2 reactor of the structure of mercury-containing HTSCs (Hg-HTSC) with a general formula- HgBa ₂ Ca _n-1 ,  . These compounds, discovered in 1993 by a team of Prof. E. Antipov at the Moscow State University, have a high temperature of transition into the semiconductor stage and a relatively simple crystal structure without any distortions due to the un-matching inter-atom spaces and heterogeneous distribution of cations. With the help of these structures scientists can best delve into the physics of superconductor processes.

Experiments at the Dubna Center on the interconnections of the temperature of superconductor transition and the specifics of the Hg-HTSC have confirmed the special role in the microscopic structure of superconductivity in these compounds of the anti-ferromagnetic exchanges between copper and oxygen in the geometric plane of CuO ₂ for spins (S= 1/2) at the lattice points of copper. It was also demonstrated that in Hg-HTSC the copper-oxygen plane has an angle of close to 180o of the Cu-O-Cu links which provides for the maximum anti-ferromagnetic exchange.

Apart from the usual experiments, the IBR-2 reactor can be used for diffraction experiments at high pressures (up to 200 kbar). It was established, among other things, that under the effect of external pressure the length of copper-oxygen links is reduced leading to more intense anti-ferromagnetic exchange. In fact, this makes the difference between the oxide-copper and other superconductors.

The experimental data thus obtained can be explained within the framework of the twin-zone Habbard model according to which in the oxide-copper HTSCs a specific coupling mechanism is implemented, depending on the anti-ferromagnetic exchange whose energy reaches a record level due to the peculiarities of the electron structure of these compounds.

* See: Yu. Osipyan, "High-Temperature Superconductivity: a Closer Look", Science in Russia, No. 1, 2001.- Ed.

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Process of aggregation in a model system-in a solution of csq fullerene in pyridine and water. Shown separately is a nanocapsule consisting of a fullerene cluster in a pyridine shell.

MEDICAL PROMISE OFFULLERENES

Fullerenes - one of the modifications of carbon, like diamonds and graphite, were discovered in 1985 by R. Kerl, H. Croto and R. Smolli (1996 Nobel Prize for chemistry) and named in honor of the American architect B. Fuller, are large cluster- molecules in the form of sphere-like lattices carbon hexagons and pentagons*. The most "popular" of them - fullerene C ₆₀ - has a frame in the form of a football in which carbon atoms (60 in all) are located at the junctures of the hexagon and pentagon corners.

Materials on the basis of fullerenes possess some unusual physical and chemical properties, which are being investigated by thousands of experts around the world. Medicine can very well be one of the most promising fields of applications of fullerenes because of their unusual biological activity. Scientists hope to be able to use them as a basis for a whole new class of medicinal preparations with a range of unusual therapeutical properties.

The biophysical, biological and medical studies of fullerenes, however, were hampered by them being practically insoluble in water under the ordinary conditions. It was only several years ago that Prof. G. Andrievsky with his staff at the Institute of Therapy of the Medical Academy of the Ukraine suggested a method of producing finely dispersed water solutions of fullerenes (C ₆₀ and C ₇₀ ). Today it is possible to produce water solutions of C ₆₀ (FWS) with concentrations of more than 1.4 mg/ml and containing no stabilizers or additives. Such solutions remain stable within a range of temperatures from 5 to 60C with "shelf-life" of no less than 18 months. Biological tests of FWS-solutions in different model systems in vitro have proved that: they possess antiviral activity (inhibition of AIDS and flue viruses of man), they have no carcinogenic or mutagenic properties, they do not suppress the processes of cell breathing, do not affect the system of blood coagulation, anti- oxidants, etc. Experts of the Russian Oncological Center named after N. Blokhin of the Russian Medical Academy have achieved the first positive results - so far with mice and rats - of the application of new fullerene-based medicinal preparations.

For understanding the processes of fullerene dissolution in water and of the mechanisms of FWS action in biological systems experts have been staging different physical and chemical experiments. Of key importance here has been the problem of the FWS structure and composition. In clarifying this matter an important role can belong to studies with the use of low-angle neutron scatter which were recently initiated in cooperation with a team of Prof. G. Andrievsky on the IBR-2 reactor. In these studies we proceed from the unique capacity of neutrons to easily "distinguish" the isotopes of, for example, hydrogen and deuterium. This so-called isotope contrast method (when hydrogen in a system is replaced fully or in part with deuterium and the resulting neutron scatter is studied) is very effective in the present-day structural molecular biology research.

Being used as a working model now is a model in which FWS is a molecular- colloidal system consisting of some hydrated (with H ₂ O attached) mole-

* See: "Fullerenes", Science in Russia, No. 6, 2000.- Ed.

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Diagram of lamellar fine film of CO-polymer PS-PBMA. On the left-image seen under atomic power microscope; right-model of the inner structure of an islet obtained on the basis of reflectometric measurement on the IBR-2 reactor.

Typical "spectrum-map" of neutron scatter intensity depending on the pulse of incident (Pi) and reflected (Pf) neutrons. Such "maps" make it possible to determine the parameters of surface structures and the morphology of interlayer boundaries.

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Distribution of "working time" on the IBR-2 reactor for researchers from various countries.

cules of C ₆₀ built in into the open-work water structure, and of their spherical fractional clusters of 7 nm and more in size. A structural unit of such clusters is a spherical aggregation of 13 molecules of C ₆₀ which are 3 - 4 nm in size which, like crystallohydrates, contains firmly bound molecules of water. This gives rise to two problems: studies of water structure in the complex of C ₆₀ & nH ₂ O and of the processes of clusters formation and aggregation cynetics.

In recent experiments on low-angle neutron scatter we were able to establish the picture of formation of fullerene clusters formation in a C ₆₀ solution in pyridine (colorless liquid found, for example, in coal tar) and in water. It turned out that the former are covered with a pyridine shell which makes fullerenes chemically inert-something like an inert nanocapsule. Consequently, the process of their aggregation consists in the unification of the latter. The next stage of our studies will involve investigation of purely water solutions of fullerenes.

NANOSTRUCTURES IN POLYMERS

In recent time all kinds of polymer films have found applications in biotechnology, electronics and other fields. This makes it necessary to have materials of this kind with dissimilar surfaces and morphology of boundaries between layers in multilayer nanostructures. Widely used in such studies of solids and liquids, including polymers, is neutron reflectometry-measurement of neutron beam parameters after its complete reflection from a surface or boundary between layers.

Some 40 such devices are already in operation in different countries; two, with some unique parameters, have been installed at our IBR-2 reactor. In recent studies, conducted in conjunction with our colleagues from other Russian centers, France and Germany, we have established that: the effects of non-mirror scatter have a key role to play in determining the structures of surfaces and layers. We studied lamellar fine film of a co-polymer of poly-sterenepolybutilmetacrelate (PS-PBMA) whose surface is covered with tubercles 40 nm high and 1 -5 mem in diameter. Such objects can be seen only with the help of the most powerful atomic power microscope*. And even then one cannot examine their inner structure (which can be done only by neutron physics methods).

CENTER FOR COLLECTIVE USE

Since IBR-2 is one of the world's best sources of neutrons for condensed matter studies, the allocation of time for its "users" is done on a competitive basis. And the "owner" of the reactor-the Neutron Physics Lab named after I. Frank (JINR) can use it for only some 25 percent of that time.

We have been using the reactor for a range of programs in conjunction with the leading research centers of this country, above all the RAS Institute of Nuclear Physics in St. Petersburg, the "Kurchatov Institute" Research Center, the Physics and Power Engineering Institute named after A. Leipunsky, the Institute of Physics of Metals of the RAS Urals Branch and the RAS Institute of Nuclear Research. This is in addition to some 150 - 200 experiments which are annually conducted here by scientists from 25 to 30 foreign countries.

We are laying special attention to the training of young specialists for IBR-2 experiments and our Department is used as a "base" for the Chair of Neutronography of the Moscow State University (named after Lomonosov). Enjoying considerable popularity is our School for Using Neutron Scatter and Synchrotron Emissions for the training of senior under-graduates, post-graduates and young researchers from different Russian universities and research centers. We are doing this work in conjunction with the "Kurchatov Institute" Russian Research Center and the RAS Institute of Crystallography for 4 to 6 months every year with the training being conducted in Dubna and in Moscow.

In a few years' time these young researchers will become members of what we call the Russian "neutron community" which has at its disposal some of the most advanced research equipment for studies of the condensed state of matter with the help of neutron scattering. Lying in store for them is a host of challenging tasks in the physics of condensed state of matter, biology, chemistry, geophysics and materials studies.

Illustrations provided by the author.

* See: V. Bykov, "Microscope... Examines Atoms", Science in Russia, No. 4, 2000.- Ed.

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